Computer programs often manage many objects of the same type, e.g., a bank's accounting program must manage hundreds of customer accounts. It is inconvenient and usually impossible to declare distinctly named variables for each of the customer accounts; instead, one constructs a new form of object---a data structure---to collectively hold and name the customer accounts.
The most popular form of data structure is the array, and this chapter introduces standard uses of arrays. After studying this chapter, the reader should be able to
Say that the six scores are saved in these variables,
double score0; double score1; double score2; double score3; double score4; double score5;and say that you must write a method that locates and prints the highest score of the six. How will you do this? Alas, the method you write almost certainly will use a sequence of conditional statements, like this:
double high_score = score0;
if ( score1 > high_score ) { high_score = score1; }
if ( score2 > high_score ) { high_score = score2; }
if ( score3 > high_score ) { high_score = score3; }
if ( score4 > high_score ) { high_score = score4; }
if ( score5 > high_score ) { high_score = score5; }
System.out.println(high_score);
This unpleasant approach
becomes even more unpleasant if there
are more even scores to compare or if a more difficult task, such as
ordering the scores from highest to lowest, must be performed.
Some other approach is required.
Can we employ a loop to examine each of score0 through score6? But the six variables have distinct names, and a loop has no way of changing from name to name. Ideally, we wish to use ``subscripts'' or ``indexes,'' so that we may refer to score0 as score0, score1 as score1, and so on. The notation would be exploited by this for-loop,
double high_score = score0;
for ( int i = 1; i <= 5; i = i + 1 )
{ if ( scorei > high_score )
{ high_score = scorei; }
}
System.out.println(high_score);
which would examine all six variables.
Java uses array variables, for indexing like this. An array variable names a collection of individual variables, each of which possesses the same data type. For example, we can declare and initialize an array variable that holds six doubles by stating the following:
double[] score = new double[6];The name of this variable is score, and its declared data type is double[] (read this as ``double array''). The data type indicates that score is the name of a collection of doubles, and the doubles named by the array variable are score[0], score[1], ..., score[5].
The initialization statement's right-hand side, new double[6], constructs a new form of object that holds six elements, each of which is a double variable.
It is traditional to draw an array like this:
The diagram shows that a newly constructed array that holds numbers starts with zeros in all the elements.
When we wish to refer to one of the elements in the array named score, we use an index (also known as subscript) to identify the element. The elements are indexed as score[0], score[1], and so on, up to score[5]. For example, we can print the number held in element 3 of score by writing,
System.out.println(score[3]);Java requires that an array's indexes must be integers starting with zero.
It is helpful to think of variable score as the name of a ``hotel'' that has six ``rooms,'' where the rooms are labelled 0 through 5. By stating score[3], we specify the precise address of one of the hotel's rooms, where we locate the room's ``occupant.''
Of course, each of the elements of an array is itself a variable that can be assigned. For example, say that the leading judge gives the score, 5.9, to a skater. We insert the number into the array with this assignment:
score[0] = 5.9;If the skater's next score is 4.4, then we might write,
score[1] = 4.4;Here is a picture that shows what we have accomplished:
We can insert values into all the array's elements in this fashion.
But most importantly, the index contained within the square brackets may be a variable or even an integer-valued arithmetic expression. For example,
int i = 3; System.out.println(score[i]); System.out.println(score[i + 1]);locates and prints the doubles held in elements 3 and 4 of score. By using variables and expressions as indexes, we can write intelligent loops, such the one that locates and prints a skater's highest score:
double high_score = score[0];
for ( int i = 1; i <= 5; i = i + 1 )
// invariant: high_score holds the highest score in the range,
// score[0] ..upto.. score[i-1]
{ if ( score[i] > high_score )
{ high_score = score[i]; }
}
System.out.println(high_score);
By changing the value of i at each iteration,
the loop's body
examines all the array's elements, solving the problem we faced at the
beginning of this section.
As noted above, we can use integer-valued arithmetic expressions as indexes. For example, perhaps variable i remembers an array index; if we wish to exchange the number in score[i] with that of its predecessor element, we can write
int temp = score[i]; score[i - 1] = score[i]; score[i] = temp;The phrase, score[i - 1], refers to the element that immediately precedes element score[i]. For example, for the array pictured earlier and when i holds 2, the result of executing the above assignments produces this array:
If variable i held 0 (or a negative number), then score[i - 1] would be a nonsensical reference, and the execution would halt with a run-time exception, called an ArrayIndexOutOfBoundsException.
The above example showed how an array might hold a set of numbers. But arrays can hold characters, booleans, strings, and indeed, any form of object whatsoever. For example, the words of a sentence might be stored into an array like this:
String[] word = new String[3];
word[0] = "Hello";
word[1] = "to";
word{2] = "you";
Array word is declared so that it keeps strings in its elements.
Second, if we have written a class, say, class BankAccount, then we can declare an array to hold objects constructed from the class:
BankAccount[] r = new BankAccount[10]; r[3] = new BankAccount(); BankAccount x = new BankAccount(); r[0] = x;The previous sequence of statements constructs two BankAccount objects and assigns them to array r.
Because they can hold numbers, booleans, characters, and objects, arrays are heavily used in computer programming to model sets or collections. The collection of skating scores seen at the beginning of this section is one simple example, but there are many others:
int r = new int[4];What do each of these loops print? (Hint: It will be helpful to draw a picture of array r, like the ones seen in this section, and update the picture while you trace the execution of each loop.)
for ( int i = 0; i < 4; i = i + 1 )
{ System.out.println(r[i]); }
int i = 1;
r[i] = 10;
r[i + 2] = r[i] + 2;
for ( int i = 0; i < 4; i = i + 1 )
{ System.out.println(r[i]); }
for ( int i = 3; i >= 0; i = i - 1 )
{ r[i] = i * 2; }
for ( int i = 0; i < 4; i = i + 1 )
{ System.out.println(r[i]); }
r[0] = 10;
for ( int i = 1; i != 4; i = i + 1 )
{ r[i] = r[i - 1] * 2; }
for ( int i = 0; i < 4; i = i + 1 )
{ System.out.println(r[i]); }
Arrays are particularly useful for collecting input data that arrive in random order. A good example is vote counting: Perhaps you must write a program that tallies the votes of a four-candidate election. (For simplicity, we will say that the candidates' ``names'' are Candidate 0, Candidate 1, Candidate 2, and Candidate 3. ) Votes arrive one at a time, where a vote for Candidate i is denoted by the number, i. For example, two votes for Candidate 3 followed by one vote for Candidate 0 would appear:
3 3 0and so on.
Vote counting will go smoothly with an array that holds the tallies for the four candidates: We construct an array whose elements are integers,
int[] votes = new int[4];where votes[0] holds Candidate 0's votes, and so on. When a vote arrives, it must be added to the appropriate element:
int v = ...read the next vote from the input... votes[v] = votes[v] + 1;The algorithm for vote counting follows the ``input processing pattern'' from Chapter 7:
boolean processing = true;
while ( processing )
{ int v = ...read the next vote from the input...
if ( v is a legal vote, that is, in the range, 0..3 )
{ votes[v] = votes[v] + 1; }
else { processing = false; }
}
Once all the votes are tallied, they must be printed. There is
a standard way of printing the contents of an array like votes:
for ( int i = 0; i != votes.length; i = i + 1 )
// invariant: values of votes[0]..votes[i-1] have been printed
{ System.out.println( ... votes[i] ... ); }
A for-loop works with a loop-counter variable, i, to
print all the array's elements. Notice the phrase,
votes.length, which is new and appears in the loop's termination
test:
The phrase is a built-in Java convenience that
denotes the length of array votes (here, 4).
It is always better to use votes.length, rather than 4,
in the coding, because the former need not be changed if array
votes is changed in a later version of the
program.
Figure 1 presents the resulting vote counting program.
FIGURE 1: vote counting================================================
import javax.swing.*;
/** VoteCount tallies the votes for election candidates.
* input: a sequence of votes, terminated by a -1
* output: the listing of the candidates and their tallied votes */
public class VoteCount
{ public static void main(String[] args)
{ int num_candidates = 4; // how many candidates
int[] votes = new int[num_candidates]; // holds the votes;
// recall that each element is initialized to 0
// collect the votes:
boolean processing = true;
while ( processing )
// invariant: all votes read have been tallied in array votes
{ int v = new Integer(JOptionPane.showInputDialog
("Vote for (0,1,2,3):")).intValue();
if ( v >= 0 && v < votes.length ) // is it a legal vote?
{ votes[v] = votes[v] + 1; }
else { processing = false; } // quit if there is an illegal vote
}
// print the totals:
for ( int i = 0; i != votes.length; i = i + 1 )
// totals for votes[0]..votes[i-1] have been printed
{ System.out.println("Candidate" + i + " has " + votes[i] + " votes"); }
}
}
ENDFIGURE===========================================================
For example, if the election's votes arrived in the order,
3, 3, 0, 2, 3, followed by a terminating number, e.g., -1,
the program prints
Perhaps the key statement in the program is the conditional statement,
if ( v >= 0 && v < votes.length )
{ votes[v] = votes[v] + 1; }
else { ... }
The conditional adds the vote to the array only if the vote falls within
the range 0..3. If an improperly valued v, say, 7,
was used with votes[v] = votes[v] + 1, then execution would halt
with this exception,
java.lang.ArrayIndexOutOfBoundsException: 7
at Test.main(VoteCount.java:...)
because there is no element, votes[7].
It is always best to include conditional statements that help a program
defend itself against possible invalid array indexes.
a bed is readis encoded into this sequence of integers:
1 0 2 5 4 0 9 19 0 18 5 1 4This simple substitution code is all too easy for outsiders to decipher.
Here is a slightly more challenging substitution code: Starting with a ``seed'' integer, k, we encode a blank space by k. Call this value code(' '). Next, we encode the alphabet in this pattern, where each character's code is the twice as large as its precedessor's, plus one:
code(' ') = k;
code('a') = (code(' ') * 2) + 1
code('b') = (code('a') * 2) + 1
...
code('z') = (code('y') * 2) + 1
For example, with a starting seed of 7, a bed is read encodes to
15 7 31 255 127 7 4095 4194303 7 2097151 255 15 127
The encoding program will work most efficiently if it first calculates the integer codes for all the letters and saves them in a translation table---an array. Then, the letters in the input words are quickly encoded by consulting the table for the codes.
We build the table as follows. First we declare the array:
int[] code = new int[27]; // this is the translation table:
// code[0] holds the code for ' ',
// code[1] holds the code for 'a',
// code[2] holds the code for 'b', and so on
Next, we systematically compute the codes to store in array code:
the value of
code[i] is defined in terms of its predecessor,
code[i - 1], when i is positive:
code[i] = (code[i - 1] * 2) + 1;The arithmetic expression, i - 1, can be used, because Java allows integer-valued expressions as indexes.
We now write this loop to compute the codes:
int seed = ... ;
code[0] = seed;
for ( int i = 1; i != code.length; i = i + 1 )
{ code[i] = (code[i - 1] * 2) + 1; }
We are now ready to read a string and translate its characters one by one into integer codes: Java treats characters like they are integers, which makes it easy to check if a character, c, is a lower-case letter (c >= 'a' && c <= 'z' does this) and to convert the character into the correct index for array code ((c - 'a') + 1 does this):
String input_line = JOptionPane.showInputDialog("type sentence to encode: ");
for ( int j = 0; j != input_line.length(); j = j + 1 )
{ char c = input_line.charAt(j);
if ( c == ' ' )
{ System.out.println(code[0]); }
else if ( c >= 'a' && c <= 'z' )
{ int index = (c - 'a') + 1;
System.out.println(code[index]);
}
else { System.out.println("error: bad input character"); }
}
(Recall that S.length() returns
the length of string S
and that
S.charAt(j) extracts the jth character from
S.)
When we build a translation table, we compute each possible translation exactly once, and we save and reuse the answers when we read the inputs. For this reason, translation tables are most useful when it is expensive to compute translations and there are many inputs to translate.
powers_of_two[i] = 2^ifor all values of i in the range, 0 to 9.
summation(0) = 0 summation(n) = n + summation(n-1), if n > 0``defines'' the following translation table:
int[] summation = new int[...];
summation[0] = 0;
for ( int n = 1; n != summation.length; n = n + 1 )
{ summation[n] = n + summation[n-1]; }
Write applications that build tables (arrays of 20 elements)
for the following recursive definitions;
make the applications print the contents of each table, in reverse order.
Now that we have some experience with arrays, we should learn about their internal structure. The story starts innocently enough: A Java array variable is declared like any Java variable, e.g.,
int[] r;This declares variable r with data type int[] (``int array''). But variable r is meant to hold the address of an array object; it is not itself the array. We use assignment to place an address in r's cell:
r = new int[6];As noted earlier, the phrase, new int[6], constructs an array object of six integer elements. We can do the two previous two steps in a single initialization statement:
int[] r = new int[6];As a result of the initialization to r, computer storage looks like this:
The address, a1, of the array object, new int[6], is saved in variable r. Notice that the run-time data type of the object at address a1 is int[6]---the data type includes the arrays's length and its elements' data type.
Many programmers fail to understand the difference between an array variable and an array object and try to duplicate an array like this:
int[] s = r;This statement duplicates the address of the array object and not the object itself! When we examine computer storage, we see that variable s holds the same address held by r---the two variables share the same array object:
This means assignments to s's elements alter r's elements as well: For example, s[0] = 3 will make r[0] == 3 as well.
Once constructed, an array object's length cannot change. The length of an array, like r, can be obtained by the phrase, r.length. Note that r.length lacks a parentheses set, (). For whatever reason, the Java designers made length a ``public field'' of an array object, rather than a ``public method.''
The array named r is called one dimensional because one index (subscript) is required to identify a specific element in the array. Some programming languages permit construction of a two-dimensional array (also known as a matrix or grid), where two indexes are required to identify an element. Two dimensional arrays are studied later in this Chapter.
We can construct arrays of integers, doubles, booleans, strings, and indeed, of any legal data type. Here are some examples:
The last statement of the last example is crucial to understanding Java arrays: When you construct an array whose elements hold objects, the array's elements are initialized to null values---there is only a ``container'' but no objects in it. Therefore, you must explicitly construct new objects and assign them to the elements. For the last example, we might write a loop that constructs and inserts objects into array account:
BankAccont[] account = new BankAccount[100];
for ( int i = 0; i != account.length; i = i + 1 )
{ account[i] = new BankAccount( ... ); } // see Figure 11, Chapter 6
An array can be partially filled with objects, just like an hotel can have some occupied rooms and some vacancies. We can test if an element is occupied by comparing the element's value to null. For example, here is a loop that prints the balances of all accounts in array account, skipping over those elements that are empty:
for ( int i = 0; i != account.length; i = i + 1 )
{ if ( account[i] != null )
{ System.out.println( "Balance of account " + i
+ " is " + account[i].balanceOf() );
}
}
As noted in the previous section, integer-valued arithmetic expressions are used to index an array's elements. For example, these statements make short work of building a lookup table of powers of two:
int[] r = new int[6];
r[0] = 1;
for ( int i = 1; i < r.length; i = i + 1 )
{ r[i] = r[i - 1] * 2; }
Now, r[j] holds the value of 2j:
A numeric or boolean array can be constructed and initialized with a set-like notation, which looks like this:
int[] r = {1, 2, 4, 8, 16, 32};
Because they are objects, array objects can be parameters to methods and can be results from methods. Here is an example: Method reverse accepts (the address of) an array object as its argument and returns as its result a newly constructed array object whose elements are arranged in reverse order of those in its argument:
public double[] reverse(double[] r)
{ double[] answer = new double[r.length];
for ( int i = 0; i != r.length; i = i+1 )
{ answer[(r.length - 1) - i] = r[i]; }
return answer;
}
When this method is invoked, for example,
double[] numbers d = {2.3, -4.6, 8, 3.14};
double[] e = reverse(d);
it is the address of the
four-element array named by d that is
bound to the formal parameter, r.
Inside the method, a second array is
constructed, and the numbers held in the array named d are copied
into the new array object.
When the method finishes, it returns
the address of the second array---variables d and e
hold the addresses of distinct array objects.
The main method that comes with every Java application uses an array parameter, args:
public static void main(String[] args)
{ ... args[0] ... args[1] ... }
When an application is started, whatever program arguments supplied
in the application's start-up command are packaged
into an array object, and the address of the array is bound to args.
This explains why the leading program argument is referred to as
args[0], the next argument is args[1], and so on.
The number of program arguments supplied is of course
args.length.
/** maxElement returns the largest integer in its array parameter. * @param r - an array of 1 or more integers * @return the largest integer in the array */ public int maxElement(int[] r)
/** add adds the elements of two arrays, element-wise * @param r1 - an array * @param r2 - an array. Note: the two arrays' lengths must be the same * @return a newly constructed array, s, such that s[i] = r1[i] + r2[i], * for all i; return null, if r1 and r2 have different lengths. */ public double[] add (double[] r1, double[] r2)
BankAccount[] bank = new BankAccount[100];
Recall once more, class BankAccount, from Figure 11, Chapter 6. When we construct a new BankAccount(N), we construct an object with an initial balance of N that can receive deposits and withdrawals. For example,
BankAccount x = new BankAccount(70); ... x.withdraw(50) ...constructs an account named x with an initial balance of 70 and performs a withdrawal of 50 upon it.
A bank has hundreds, if not thousands, of customers, so a program that maintains the bank's accounts must construct many BankAccount objects. An array is a good structure for saving the objects. Here is one simple modelling: Say that a bank is able to maintain at most 100 accounts. Each account is given an identification number in the range of 0 to 99. The program that does accounting for the bank will use an array like this:
BankAccount[] bank = new BankAccount[100];This constructs an array that has 100 elements, such that each element holds the initial value, null. (There are no accounts yet, just a structure to hold the accounts.)
Now, say that a customer opens an account at the bank with an initial desposit of 200, and the customer wants her account to have the identification number, 75. The programming statements that enact this request might read,
BankAccount new_account = new BankAccount(200); bank[75] = new_account;Or, more directly stated,
bank[75] = new BankAccount(200);Here is a diagram of the array and the newly constructed object whose address is saved within the array:
If the customer wishes to withdraw 60 from the account, this invocation,
bank[75].withdraw(60)will preform the action. Note that bank[75] names the bank account object whose withdraw method is invoked. This example emphasizes that the array, bank, holds objects that are indexed by integers.
Next, say that the customer closes her account, so that it is no longer needed by the bank. The bank ``erases'' the account by stating,
bank[75] = null;Since null means ``no value,'' this means another customer might open an account and choose 75 for its identification number.
As just noted, an array can be partially filled with objects; we can test if an array element is occupied by comparing the element's value to null. For example, here is a loop that prints the balances of all accounts in array bank, skipping over those elements that are empty:
for ( int i = 0; i != bank.length; i = i + 1 )
{ if ( bank[i] != null )
{ System.out.println( "Balance of account " + i
+ " is " + bank[i].getBalance() );
}
}
With these basic ideas in mind, we can write a simple bank-accounting application that lets customers open bank accounts, make deposits and withdrawals, check the balances, and close the accounts when they are no longer needed---we use an array to hold the accounts, and we use the array's indexes as the identification numbers for the accounts.
Of course, it is overly simplistic to use simple integers as identification numbers for bank accounts---we should use a more general notion of a key to identify an account. The key might be a word, or a sequence of numerals and letters, or even an object itself. The case study in the next section develops a more realistic approach to maintaining databases of objects where objects are identified by keys.
public class Counter
{ private int c;
public Counter(int v) { c = v; }
public void increment() { c = c + 1; }
public int getCount() { return c; }
}
Say that we change the declaration of votes
in the vote-counting application in Figure 1 to be
Counter[] votes = new Counter[num_candidates];Revise the vote-counting application to operate with this array.
/** Bank models a collection of bank accounts */
public class Bank
{ BankAccount[] bank; // the array that holds the account
int max_account; // the maximum account number
/** Constructor Bank initialize the bank
* @param how_many - the maximum number of bank accounts */
public Bank(int how_many)
{ max_account = how_many;
bank = new BankAccount[how_many];
}
/** addNewAccount adds a new account to the bank
* @param id_number - the account's identification number; must be in
* the range, 0..maximum_account_number - 1
* @param account - the new bank account object
* @return true, if the account is succesfully added;
* return false, if the id_number is illegal */
public boolean addNewAccount(int id_number, BankAccount account)
{ boolean result = false;
if ( id_number >= 0 && id_number < max_account
&& bank[id_number] == null ) // is id_number legal?
{ bank[id_number] = account;
result = true;
}
return result;
}
/** getAccount finds a bank account
* @param id_number - the identification number of the desired account
* @return the bank account whose identification is id_number;
* if there is no account with the id_number, return null */
public BankAccount getAccount(int id_number)
{ ... }
/** deleteAccount removes a bank account
* @param id_number - the identification number of the account to be removed
* @return true, if the deletion was successful;
* return false, if no account has the id_number */
public boolean deleteAccount(int id_number)
{ ... }
}
We might use the class as follows:
Bank b = new Bank(500); b.addNewAccount(155, new BankAccount(100)); ... BankAccount a = b.getAccount(155); ... a.deposit(200) ...Write the two missing methods for class Bank.
A large collection of information, such as a company's sales records or its customer accounts or its payroll information, is called a database. An important programming challenge is determining the proper structure for a database.
In simplest terms, a database is a ``container'' into which objects are inserted, located, and removed; the objects that are stored in a database are called records. An important feature about a record is that it is uniquely identified by its key, which is held within the record itself. Here are some examples of records:
How can we model and build a general-purpose database in Java? Here are some crucial concepts:
The picture suggests that the database will operate the same, regardless of whether books, bank accounts, and so on, are saved. As long as the records---whatever they are---hold keys, the database can do its insertions, lookups, and deletions, by manipulating the records' keys and not the records themselves. This is strongly reminiscent of arrays, which can hold a variety of objects without manipulating the objects themselves.
So, how does a database manipulate a key? Regardless of whether keys are numbers or strings or pairs of items, keys are manipulated by comparing them for equality. Consider a lookup operation: The database receives a key object, and the database searches its collection of records, asking each record to tell its key, so that each key can be compared for equality to the desired key. When an equality is found true, the corresponding record is returned. This algorithm operates the same whether integers, strings, or whatever else is used for keys.
In summary,
We are now ready to design and build a
database subassembly in Java. We will build a subassembly---not an
entire program---such that the subassembly can be inserted as the
model into a complete application.
We follow the usual stages for design and construction:
Regardless of whether a database holds bank accounts, tax records, or payroll information, its behaviors are the same: a database must be able to insert, locate, and delete records based on the records' keys. We plan to write a class Database so that an application can construct a database object by stating,
Database db = new Database(...);Then, the application might insert a record---call it r0---into db with a method invocation like this:
db.insert(r0);As stated earlier, each record possesses its own key. Say that record r0 holds object k0 as its key. To retrieve record r0 from the database, we use a command like this:
Record r = db.find(k0);This places the address of record r0 into variable r for later use. We can delete the record from the database by stating:
db.delete(k0);Notice that variable r still holds the address of the record, but the record no longer lives in the database.
The above behaviors imply nothing about the techniques that the database uses to store and retrieve records; these activities are internal to class Database and are best left unknown to the database's users.
The previous examples suggest there are at least three components to the database's design: the Database itself, the Records that are inserted into it, and the Keys that are kept within records and are used to do insertions, lookups, and deletions. The class diagram in Figure 2 lists these components and their dependencies.
FIGURE 2: architecture for a database===================================There is a new notation in the Figure's class diagram: The annotation, 1 --> *, on the arrow emphasizes that one Database collaborates with (or collects) multiple Records, suggesting that an array will be useful in the coding of class Database. As noted earlier, whatever a Record or Key might be, the methods getKey and equals are required. (The format of the equals method will be explained momentarily.)ENDFIGURE=============================================================
To keep its design as general as possible, we will not commit class Database to saving any particular form of Record---the only requirement that a database will make of a record is that a record can be asked for its key. Similarly, the only requirement a database will make of a key is that the key can be compared to another key for an equality check.
Since class Database must hold multiple records, its primary attribute will be an array of records, and the database will have at least the three methods listed in Figure 2.
The specification for Record is kept as minimal as possible: whatever a record object might be, it has a function, getKey, that returns the key that uniquely identifies the record. Similarly, it is unimportant whether a key is a number or a string or whatever else; therefore, we require only that a key possesses a method, equals, that checks the equality of itself to another key.
Table 3 presents the specifications that summarize our assumptions about databases, records, and keys.
TABLE 3: specifications for database building=======================
| Database | a container for data items, called Records |
| Attribute | |
| private Record[] base | Holds the records inserted into the database. |
| Methods | |
| insert(Record r): boolean | Attempts to insert the record, r, into the database. Returns true if the record is successfully added, false otherwise. |
| find(Key k): Record | Attempts to locate the record whose key has value k. If successful, the address of the record is returned, otherwise, null is returned. |
| delete(Key k): boolean | Deletes the record whose key has value k. If successful, true is returned; if no record has key k, false is returned. |
| Record | a data item that can be stored in a database |
| Methods | |
| getKey(): Key | Returns the key that uniquely identifies the record. |
| Key | an identification, or ``key,'' value |
| Methods | |
| equals(Key m): boolean | Compares itself to another key, m, for equality. If this key and m are same key value, then true is returned; if m is a different key value, then false is returned. |
ENDTABLE===========================================================Because we have provided partial (incomplete) specifications for Record and Key, many different classes might implement the two specifications. For example, we might write class Book to implement a Record so that we can build a database of books, or we might write class BankAccount to implement a database of bank accounts. Different classes of keys might also be written, if only because books use different keys than do bank accounts.
Key's specification deserves a close look: the specification is written as if keys are objects (and not mere ints). For this reason, given two Key objects, K1 and K2, we must write K1.equals(K2) to ask if the two keys have the same value. (This is similar to writing S1.equals(s2) when comparing two strings, S1 and S2, for equality.) We exploit this generality in the next section.
The specifications for Record and Key make it possible to write a complete coding for class Database without knowing any details about the codings for the records and keys. Let's consider the implementation of class Database.
The database's primary attribute is an array that will hold the inserted records. class Database must contain this field declaration:
private Record[] base;The constructor method for the class will initialize the field to an array:
base = new Record[HOW_MANY_RECORDS];where all the array's elements have value null, because the array is empty. Records will be inserted into the database one by one. To do an insert(Record r), follow this algorithm:
/** findLocation is a helper method that searches base for a record * whose key is k. If found, the array index of the record within * base is returned, else -1 is returned. */ private int findLocation(Key k)Then, Step 1 of the algorithm is merely,
if ( findLocation(r.getKey()) == -1 )because r.getKey() extracts the key held within record r, and a result of -1 from findLocation means that no record with the same key is already present.
Step 2 of the algorithm is clearly a searching loop, and we use the techniques from Chapter 7 to write this loop, which searches for the first empty element in base where a new record can be inserted:
boolean found_empty_place = false;
int i = 0;
while ( !found_empty_place && i != base.length )
// so far, all of base[0]..base[i-1] are occupied
{ if ( base[i] == null ) // is this element empty?
{ found_empty_place = true; }
else { i = i + 1; }
}
When this loop completes, i holds the index of the first empty element in base, meaning that Step 3 is just base[i] = r, unless array base is completely filled with records and there is no available space. What should we do in the latter situation?
Because Java arrays are objects, it is possible to construct a new array object that is larger than the current array and copy all the elements from the current array to the new array. Here is a standard technique for doing so:
// This constructs a new array twice as large as base:
Record[] temp = new Record[base.length * 2];
// Copy elements in array named by base into temp:
for ( int j = 0; j != base.length; j = j + 1 )
{ temp[j] = base[j]; }
// Change base to hold address of temp:
base = temp;
The last assignment, base = temp, copies the address of the
larger array into array variable base, meaning that
base once again holds the address of an array of records.
BeginFootnote: If you have studied the Java libraries, perhaps you discovered class Vector, which behaves like an array but automatically expands to a greater length when full. The technique that a Java Vector uses to expand is exactly the one presented above. EndFootnote.
Figure 4 displays the completed version of insert.
Next, we consider how to delete an element from the database: The algorithm for method, delete(Key k), would go,
int index = findLocation(k);
if ( index != -1 )
{ base[index] = null; }
See Figure 4 for the completed method.
We can write the lookup method so that it merely asks findLocation to find the desired record in the array. Again, see Figure 4.
To finish, we must write the findLocation method, which finds the record in array base whose key is k. The algorithm is a standard searching loop, but there is a small complication, because array base might have null values appearing in arbitrary places, due to deletions of previously inserted records:
private int locationOf(Key k)
{ int result = -1; // recall that -1 means ``not found''
boolean found = false;
int i = 0;
while ( !found && i != base.length )
{ if ( base[i] != null // is this element occupied?
&& base[i].getKey().equals(k) ) // is it the desired record?
{ found = true;
result = i;
}
else { i = i + 1; }
}
return result; // return array index of the record found
}
Note the conditional statement in the loop's body:
if ( base[i] != null // is this array element occupied?
&& base[i].getKey().equals(k) ) // is it the desired record?
{ ... } // we found the record at array element, i
else { i = i + 1; } // the record is not yet found; try i + 1 next
The test expression first asks if there is a record stored in element,
base[i], and if the answer is true, then the element's key
(namely, base[i].getKey()) is compared for equality to
the desired key, k.
The completed Database class appears in Figure 4. In addition to attribute base, we define the variable, NOT_FOUND, as a memorable name for the -1 answer used to denote when a search for a record failed.
FIGURE 4: class Database============================================
/** Database implements a database of records */
public class Database
{ private Record[] base; // the collection of records
private int NOT_FOUND = -1; // int used to denote when a record not found
/** Constructor Database initializes the database
* @param initial_size - the size of the database */
public Database(int initial_size)
{ if ( initial_size > 0 )
{ base = new Record[initial_size]; }
else { base = new Record[1]; }
}
/** findLocation is a helper method that searches base for a record
* whose key is k. If found, the index of the record is returned,
* else NOT_FOUND is returned. */
private int findLocation(Key k)
{ int result = NOT_FOUND;
boolean found = false;
int i = 0;
while ( !found && i != base.length )
{ if ( base[i] != null && base[i].getKey().equals(k) )
{ found = true;
result = i;
}
else { i = i + 1; }
}
return result;
}
/** find locates a record in the database based on a key
* @param key - the key of the desired record
* @return (the address of) the desired record;
* return null if record not found. */
public Record find(Key k)
{ Record answer = null;
int index = findLocation(k);
if ( index != NOT_FOUND )
{ answer = base[index]; }
return answer;
}
...
ENDFIGURE======================================================
FIGURECONT 4: class Database (concl.)=============================
/** insert inserts a new record into the database.
* @param r - the record
* @return true, if record added; return false if record not added because
* another record with the same key already exists in the database */
public boolean insert(Record r)
{ boolean success = false;
if ( findLocation(r.getKey()) == NOT_FOUND ) // r not already in base?
{ // find an empty element in base for insertion of r:
boolean found_empty_place = false;
int i = 0;
while ( !found_empty_place && i != base.length )
// so far, all of base[0]..base[i-1] are occupied
{ if ( base[i] == null ) // is this element empty?
{ found_empty_place = true; }
else { i = i + 1; }
}
if ( found_empty_place )
{ base[i] = r; }
else { // array is full! So, create a new one to hold more records:
Record[] temp = new Record[base.length * 2];
for ( int j = 0; j != base.length; j = j + 1 )
{ temp[j] = base[j]; } // copy base into temp
temp[base.length] = r; // insert r in first free element
base = temp; // change base to hold address of temp
}
success = true;
}
return success;
}
/** delete removes a record in the database based on a key
* @param key - the record's key (identification)
* @return true, if record is found and deleted; return false otherwise */
public boolean delete(Key k)
{ boolean result = false;
int index = findLocation(k);
if ( index != NOT_FOUND )
{ base[index] = null;
result = true;
}
return result;
}
}
ENDFIGURE===============================================================
The coding presents several lessons:
FIGURE 5: BankAccount Record and AccountKey==============================
/** Record models a bank account with an identification key */
public class Record
{ private int balance; // the account's balance
private Key id; // the identification key
/** Constructor Record initializes the account
* @param initial_amount - the starting account balance, a nonnegative.
* @param id - the account's identification key */
public Record(int initial_amount, Key id)
{ balance = initial_amount;
key = id;
}
/** deposit adds money to the account.
* @param amount - the amount of money to be added, a nonnegative int */
public void deposit(int amount)
{ balance = balance + amount; }
/** getBalance reports the current account balance
* @return the balance */
public int getBalance() { return balance; }
/** getKey returns the account's key
* @return the key */
public int getKey() { return key; }
}
/** Key models an integer key */
public class Key
{ private int k; // the integer key
/** Constructor Key constructs the Key
* @param i - the integer that uniquely defines the key */
public Key(int i) { k = i; }
/** equals compares this Key to another for equality
* @param c - the other key
* @return true, if this key equals k's; return false, otherwise */
public boolean equals(Key c)
{ return ( k == c.getInt() ); }
/** getInt returns the integer value held within this key */
public int getInt() { return k; }
}
ENDFIGURE==============================================================
The Record in Figure 5 has additional methods that let us do deposits and check balances of a bank account, but the all-important getKey method is present, meaning that the record can be used with class Database of Figure 4.
In order to conform to the requirements demanded by class Database, the integer key must be embedded within a class Key. This means the integer is saved as a private field within class Key and that the equals method must be written so that it asks another key for its integer attribute, by means of an extra method, getInt.
Here is how we might use the classes in Figure 5 in combination with Figure 4. Perhaps we are modelling a bank, and we require this database:
Database bank = new Database(1000);When a customer opens a new account, we might ask the customer to select an integer key for the account and make an initial deposit:
int i = ...some integer selected by the customer...;
int start_balance = ...some initial deposit by the customer...;
Key k1 = new Key(i);
boolean success = bank.insert( new Record(start_balance, k1) );
System.out.println("account inserted = " + success);
The fourth statement both constructs the new account and inserts it into
the database.
Later, if the account must be fetched so that its balance can be checked, we can find it and print its balance like this:
Record r = bank.find(k1); // recall that k1 is the account's key
if ( r != null ) // did we successfully fetch the account?
{ System.out.println(r.getBalance()); }
To show that the database can be used in a completely different application, we find in Figure 6 a new coding of record and key, this time for library books. Now, class Record holds attributes for a book's title, author, publication date, and catalog number; the catalog number serves as the book's key.
FIGURE 6: Book Record and CatalogNumber Key=============================
/** Record models a Library Book */
public class Record
{ // the names of the fields describe their contents:
private Key catalog_number;
private String title;
private String author;
private int publication_date;
/** Constructor Record constructs the book.
* @param num - the book's catalog number
* @param a - the book's author
* @param t - the book's title */
public Record(Key num, String a, String t, int date)
{ catalog_number = num;
title = t;
author = a;
publication_date = date;
}
/** getkey returns the key that identifies the record
* @return the key */
public Key getKey() { return catalog_number; }
/** getTitle returns the book's title
* @return the title */
public String getTitle() { return title; }
/** getAuthor returns the book's author
* @return the author */
public String getAuthor() { return author; }
/** getDate returns the book's publication date
* @return the date */
public int getDate() { return publication_date; }
}
ENDFIGURE======================================================
FIGURECONT 6: CatalogNumber Key (concl.)=============================
/** Key models a Library-of-Congress-style id number,
* consisting of a letter code concatenated to a decimal number */
public class Key
{ private String letter_code; // the letter code, e.g., "QA"
private double number_code; // the number code, e.g., 76.884
/** Constructor Key constructs a catalog number
* @param letters - the letter code, e.g., "QA"
* @param num - the decimal number code, e.g., 76.884 */
public Key(String letters, double num)
{ letter_code = letters;
number_code = num;
}
/** equals returns whether the catalog number held within this object
* is identical to the catalog number held within c
* @param c - the other catalog number
* @return true, if this catalog number equals c; return false, otherwise */
public boolean equals(Key c)
{ String s = c.getLetterCode();
double d = c.getNumberCode();
return ( s.equals(letter_code) && d == number_code );
}
/** getLetterCode returns the letter code part of this catalog number
* @return the letter code, e.g., "QA" */
public String getLetterCode() { return letter_code; }
/** getNumberCode returns the number code part of this catalog number
* @return the number code, e.g., "76.884" */
public double getNumberCode() { return number_code; }
}
ENDFIGURE=================================================================
The structure of the catalog number is more complex: Its class Key holds a string and a double, because we are using the U.S. Library of Congress coding for catalog numbers, which requires a string and a fractional number. The class's equals method compares the strings and fractional numbers of two keys.
Here is a short code fragment that constructs a database for a library and inserts a book into it:
Database library = new Database(50000);
Record book = new Book( new Key("QA", 76.8), "Charles Dickens",
"Great Expectations", 1860 );
library.insert(book);
// We might locate the book this way:
Key lookup_key = new Key("QA", 76.8);
book = library.find(lookup_key);
// We can delete the book, if necessary:
boolean deleted = library.delete(lookup_key);
As noted by the statement, Key lookup_key = new Key("QA", 76.8),
we can manufacture keys as needed to perform lookups and deletions.
It is a bit unfortunate that the bank account record in Figure 5 was named class Record and that the book record in Figure 6 was also named class Record; more descriptive names, like class BankAccount and class Book would be far more appropriate and would let us include both classes in the same application if necessary. (Perhaps a database must store both bank accounts and books together, or perhaps one single application must construct one database for books and another for bank accounts.)
Of course, we were forced to use the name, class Record, for both records because of the coding for class Database demanded it. The Java language lets us repair this naming problem with a new construction, called a Java interface. We will return to the database example in the next chapter and show how to use a Java interface with class Database to resolve this difficulty.
In almost every game, playing pieces must be placed onto or into a container or playing board---cards are collected into a container called a deck. Unlike the database developed in the previous section, a card deck begins completely filled with cards. The deck's primary behavior is to surrender a card from itself whenever asked, until the deck is finally empty.
Card games often use other forms of containers. For example, each player might have its own personal container for cards, called a hand (of cards). A hand is initialized so that it is empty and cards can be added to it, one by one.
For this small case study, we will design a subassembly consisting only of ordinary playing cards and a deck to hold them. The architecture for the two classes is simple:
The specification for a card deck is presented in Table 7.
TABLE 7: specification for class CardDeck================
| class CardDeck | models a deck of cards |
| Attribute | |
| private Card[] deck | container for the cards left in the deck |
| Methods | |
| newCard(): Card | return a card from the deck; if the deck is empty, return null |
| moreCards(): boolean | return true if more cards remain in the deck; return false, otherwise |
ENDTABLE============================================================Because it is a container for cards, class Deck requires an attribute that is an array. The class's methods include one that returns a card and one that replies whether or not there are more cards to return.
CardDeck collaborates with class Card, which we consider next. As noted earlier, a playing card has two crucial attributes: its suit and its count, as seen in Table 8.
TABLE 8: specification of class Card=======================
| class Card | models a playing card |
| Attributes | |
| private String suit | the card's suit, e.g., spades, hearts, diamonds, clubs |
| private int count | the card's count, e.g., ace, 2, 3, ..., king |
ENDTABLE===========================================================Of course, class Card will be coded to have accessor methods that return a card's suit and count.
There is a technical issue that we should resolve before we write the two classes: When people use playing cards, they perfer to use the names of suits and counts, like ``hearts,'' ``clubs,'' ``ace,'' and ``queen.'' These are values, just like integers, and we would like to use such values when we construct the playing cards, e.g., ``new Card(queen, hearts).''
We define values like ``queen'' and ``hearts'' in Java by declaring a public static final variable for each such value, and place the public static final variables within class Card. We see this in Figure 9, which presents class Card.
FIGURE 9: playing card==================================================
/** Card models a playing card */
public class Card
{ // definitions that one can use to describe the value of a card:
public static final String SPADES = "spades";
public static final String HEARTS = "hearts";
public static final String DIAMONDS = "diamonds";
public static final String CLUBS = "clubs";
public static final int ACE = 1;
public static final int JACK = 11;
public static final int QUEEN = 12;
public static final int KING = 13;
public static final int SIZE_OF_ONE_SUIT = 13; // how many cards in one suit
// These are the card's attributes:
private String suit;
private int count;
/** Constructor Card sets the suit and count.
* @param s - the suit
* @param c - the count */
public Card(String s, int c)
{ suit = s;
count = c;
}
/** getSuit returns the card's suit. */
public String getSuit()
{ return suit; }
/** getCount returns the card's count. */
public int getCount()
{ return count; }
}
ENDFIGURE==============================================================
The public static final variables declared in class Card are public names that can be used by the other components of an application; the names are used as if they are new values. For example, other components can refer to the values, Card.ACE, Card.DIAMOND, and so on. Now, we can say:
Card c = new Card(Card.HEARTS, Card.QUEEN)to construct a queen-of-hearts object. And, we can ask,
if ( c.getCount() == Card.QUEEN ) { ... }
But remember that the public static final variables are
merely names for integers
and strings. For example, since Card.QUEEN is the name of
an integer, we can state,
if ( c.getCount() >= Card.QUEEN ) { ... }
because integers can be compared by greater-than.
As always, the keyword, public, means that the variable can be referenced by other classes; the keyword, final, states that the variable name can not be changed by an assignment; the value is forever constant. Finally, the keyword, static, ensures that the variable is not copied as a field into any Card objects that are constructed from class Card.
It is traditional to declare public static final variables with names that are all upper-case letters.
We have used public static final variables already when in previous chapters we used predefined Java-library values like Color.red (to paint on a graphics window) and Calendar.DAY_OF_MONTH (to get the date from a Gregorian calendar object).
The remainder of class Card is simple---it contains a constructor method, which initializes a card object's suit and count, and two accessor methods, which return the suit and count.
Next, we consider class CardDeck. Its attributes are
private int card_count; // how many cards remain in the deck
private Card[] deck = new Card[4 * Card.SIZE_OF_ONE_SUIT];
// invariant: elements deck[0]..deck[card_count - 1] hold cards
Array deck is constructed to hold the four suits's worth of
cards, where the quantity of cards in a suit is the static
variable defined in class Card.
The class's constructor method must fill array deck with a complete collection of cards. As Figure 8 shows, a helper method, createSuit knows how to generate one complete suit of cards and insert it into the array; therefore, the constructor can be written as
createSuit(Card.SPADES); createSuit(Card.HEARTS); createSuit(Card.CLUBS); createSuit(Card.DIAMONDS);The helper method is written so that it inserts the cards into the array in ascending order. This is not the ideal state for the deck for playing a typical card game---the deck's cards are expected to be randomly mixed, or ``shuffled.''
Rather than write a method that randomly mixes the elements in the array, we can write the newCard method so that when it is asked to remove a card from the array it randomly calculates an array index from which the card is extracted. The algorithm might go like this:
int index = (int)(Math.random() * card_count);The computation, Math.random() * card_count, generates a nonnegative fractional number that must be less than card_count (why?), and the cast, (int), truncates the factional part, leaving an integer in the range, 0 to card_count - 1.
Step 3 is performed with a simple loop:
for ( int i = index + 1; i != card_count; i = i + 1 )
// so far, cards from index+1 to i-1 have been shifted left
// in the array by one position
{ deck[i - 1] = deck[i]; }
The completed version of the method is in Figure 10.
FIGURE 10: class CardDeck========================================
/** CardDeck models a deck of cards. */
public class CardDeck
{
private int card_count; // how many cards remain in the deck
private Card[] deck = new Card[4 * Card.SIZE_OF_ONE_SUIT];
// invariant: elements deck[0]..deck[card_count - 1] hold cards
/** Constructor CardDeck creates a new card deck with all its cards */
public CardDeck()
{ createSuit(Card.SPADES);
createSuit(Card.HEARTS);
createSuit(Card.CLUBS);
createSuit(Card.DIAMONDS);
}
/** newCard gets a new card from the deck.
* @return a card not used before, or return null, if no cards are left */
public Card newCard()
{ Card next_card = null;
if ( card_count == 0 )
{ System.out.println("CardDeck error: no more cards"); }
else { int index = (int)(Math.random() * card_count); // randomly choose
next_card = deck[index];
// once card is extracted from deck, shift other cards to fill gap:
for ( int i = index+1; i != card_count; i = i + 1 )
// so far, cards from index+1 to i-1 have been shifted left
// in the array by one position
{ deck[i - 1] = deck[i]; }
card_count = card_count - 1;
}
return next_card;
}
/** moreCards states whether the deck has more cards to give.
* @return whether the deck is nonempty */
public boolean moreCards()
{ return (card_count > 0); }
/** createSuit creates a suit of cards for a new card deck. */
private void createSuit(String which_suit)
{ for ( int i = 1; i <= Card.SIZE_OF_ONE_SUIT; i = i + 1 )
{ deck[card_count] = new Card(which_suit, i);
card_count = card_count + 1;
}
}
}
ENDFIGURE==========================================================
| class Dealer | models a dealer of cards |
| Responsibilities (methods) | |
| dealTo(Player p) | gives cards, one by one, to player p, until p no longer wants a card |
| class Player | models a player of a card game |
| Responsibilities (methods) | |
| wantsACard(): boolean | replies as to whether another card is desired |
| receiveCard(Card c) | accepts card c and adds it to its hand |
| showCards(): Card[] | returns an array of the cards that it holds |
Write classes for these two specifications. (Write class Player so that a player wants cards until it has exactly five cards.) Next, write a controller that creates a dealer object and a player object. The controller tells the dealer to deal to the player. Then, the controller asks the player to reveal its cards.
The Candidates' names are listed along the top of the matrix (for simplicity, we call them Candidate 0, ..., Candidate 3), labelling the columns, and the regions are listed along the left (they are Regions 0, 1, and 2), labelling the matrix's rows---We say that the matrix has three rows and four columns.
Thus, a vote in Region 1 for Candidate 3 would be recorded in the middle row within its rightmost element:
Other votes are recorded this manner. When voting is completed, we can see which candidate received the most votes overall by adding each of the four columns.
In programming, we use a two-dimensional array to model a matrix like the one just seen. The above matrix is coded as a two-dimensional array in Java by stating
int[][] election = new int[3][4];The data type of variable election is int[][] (``int array array''), indicating that individual integers in the collection named election are uniquely identified by means of two indexes. For example,
election[1][3]identifies the integer element that holds the votes in Region 1 for Candidate 3.
The right-hand side of the initialization, new int[3][4], constructs the array object that holds the collection of twelve integers, organized into 3 rows of 4 elements, each---three rows and four columns. It is helpful to visualize the collection as a matrix, like the ones just displayed. As usual for Java, the array's elements are initialized with zeros.
Let's do some small exercises with array election: To add one more vote from Region 1 for Candidate 3, we write
election[1][3] = election[1][3] + 1;The phrase, election[1][3], indicates the specific element in the matrix.
To give every candidate in Region 2 exactly 100 votes, we would say
for ( int i = 0; i != 4; i = i + 1 )
{ election[2][i] = 100; }
Here, we use election[2][i] to indicate a
cell within Row 2 of the matrix---the value of i determines
the specific cell in the row.
Similarly, to give Candidate 3 an additional 200 votes in every region, we would say
for ( int i = 0; i != 3; i = i + 1 )
{ election[i][3] = election[i][3] + 200; }
To print each candidate's grand total of votes, we write a nested for-loop that totals each column of the matrix:
for ( int j = 0; j != 4; j = j + 1 )
{ int votes = 0;
for ( int i = 0; i != 3; i = i + 1 )
{ votes = votes + election[i][j]; }
System.out.println("Candidate " + j + " has " votes + " votes");
}
The previous for loop displays the standard
pattern for examining each and
every element of a matrix. Yet another example is printing the
total votes cast in each region of the election, which requires that
we total each
row of the matrix:
for ( int i = 0; i != 3; i = i + 1 )
{ int total = 0;
for ( int j = 0; j != 4; j = j + 1 )
{ total = total + election[i][j]; }
System.out.println(total + " votes were cast in Region " + i);
}
In the above example, the order of the loops is reversed, because
rows are traversed, rather than columns.
for ( int i = 0; i != 4; i = i + 1 )
{ for ( int j = 0; j != 4; j = j + 1 )
{ System.out.print( r[i][j] + " " ); }
System.out.println();
}
write for-loops that initialize r so that the following
patterns are printed:
1 0 0 0 1 2 0 0 1 2 3 0 1 2 3 4
1 2 3 4 0 3 4 5 0 0 5 6 0 0 0 7
1 0 1 0 0 1 0 1 1 0 1 0 0 1 0 1
int[][] election = new int[3][4];Here is its layout in computer storage:
In Java, a two-dimensional array is built in terms of multiple one-dimensional array objects. The diagram shows that the object at address a1 is a one dimensional array that holds the addresses of the matrix's rows.
For example, when we write, election[1][3], this is computed to a1[1][3], that is, the array object at a1 is the one that will be indexed. Since element 1 within object a1 is a3, the indexing is computed to a3[3], which identifies element 3 in the array object at address a3.
Fortunately, this detailed address lookup is performed automatically and is hidden from the programmer---it is indeed safe to pretend that election is the name of a matrix of integer elements. But sometimes the knowledge of the matrix's underlying structure is exposed in the code that traverses the array, as in these for-loops, which print the total votes for each region:
for ( int i = 0; i != 3; i = i + 1 )
{ int[] region = election[i]; // region holds the address of a row
int total = 0;
for ( int j = 0; j != 4; j = j + 1 )
{ total = total + region[j]; }
System.out.println(total + " votes were cast in Region " + i);
}
This exposes that each row of matrix election is itself
a (one-dimensional) array.
Alas, we cannot treat a matrix's columns in a similar way.
Another consequence of the storage layout is that the ``length'' of a matrix is the number of rows it possesses. For the above example,
election.lengthcomputes to 3. To learn the number of columns in a matrix, we ask for the length of one of the matrix's rows. For example,
election[0].lengthcomputes to 4.
Finally, some care must be taken when asking for the number of columns of a two-dimensional array. Consider this odd example:
double[][] ragged = new double[4][]; double[0] = new double[2]; double[2] = new double[1]; double[3] = new double[0];The first statement constructs an array variable, ragged, that will have four rows and an undetermined number of columns; it looks like this in storage:
The following three statements construct one-dimensional array objects and assign them to ragged's elements, giving us
This is an example of a ``ragged array''---a two-dimensional array whose rows have different lengths. For example, an election where different candidates are eligible in different regions of a country might be modelled by a ragged array.
Notice that ragged[0].length equals 2, whereas ragged[2].length equals 1, whereas ragged[1].length generates an exception (run-time error), because there is no array of any length at the element. The array at element ragged[3] truly has length 0, meaning that there are no elements at all that can be indexed in that row.
The user instructs the puzzle to move a piece by typing a number (in this configuration, only 1 or 4 would be allowed), and the game responds by moving the requested piece:
The user may request similar moves for as long as she wishes.
FIGURE 11: architecture of model for slide puzzle========================ENDFIGURE==============================================================
The SlidePuzzleBoard's sole responsibility is to move a puzzle piece when asked, e.g., move(4) asks the board to move the piece whose face is labelled by 4.
A PuzzlePiece has only the attribute of the number on its face; it has no responsibilities. For this reason, its coding is simple and appears in Figure 12.
FIGURE 12: class PuzzlePiece=============================================
/** PuzzlePiece defines a slide-puzzle playing piece */
public class PuzzlePiece
{ private int face_value; // the value written on the piece's face
/** Constructor PuzzlePiece creates a piece
* @param value - the value that appears on the face of the piece */
public PuzzlePiece(int value)
{ face_value = value; }
/** valueOf returns the face value of the piece */
public int valueOf()
{ return face_value; }
}
ENDFIGURE================================================================
Next, we consider writing class SlidePuzzleBoard. The class's primary attribute is the array that holds the (addresses of the) puzzle pieces:
private PuzzlePiece[][] board; // the array that holds the pieces // one position on the board must be an empty space: private int empty_row; private int empty_col; // representation invariant: board[empty_row][empty_col] == nullExactly one element within array board must be empty (hold null), and it is convenient to declare two fields, empty_row and empty_col, to remember the coordinates of the empty space.
Method move(w) must move the piece labelled by integer w into the empty space. For the move to succeed, the piece labelled by w must be adjacent to the empty space. This means the algorithm for move(int w) must perform the appropriate checks:
/** found returns whether the piece at position row, col is labeled w */ private boolean found(int w, int row, int col)Then, Step 1 can be coded to ask the helper function to check the four positions surrounding the empty space whether piece w is there. See Figure 13 for the completed coding of method move.
The board's constructor method creates the board array and fills it with newly created puzzle pieces. Finally, we add a method that returns the contents of the puzzle board. (This will be useful for painting the board on the display) The contents method returns as its result the value of the board. It is best that contents not return the address of its array board. (If it did, the client that received the address could alter the contents of the array!) Instead, a new two-dimensional array is created, and the addresses of the playing pieces are copied into the new array.
FIGURE 13: class SlidePuzzleBoard========================================
/** SlidePuzzleBoard models a slide puzzle. */
public class SlidePuzzleBoard
{ private int size; // the board's size
private PuzzlePiece[][] board; // the array that holds the pieces
// one position on the board must be an empty space:
private int empty_row;
private int empty_col;
// representation invariant: board[empty_row][empty_col] == null
/** Constructor SlidePuzzleBoard constructs the initial puzzle, which has
* the pieces arranged in descending numerical order.
* @param s - the size of the puzzle, a positive integer (e.g., s==4 means
* the puzzle is 4 x 4 and will have pieces numbered 15, 14, ..., 1) */
public SlidePuzzleBoard(int s)
{ size = s;
board = new PuzzlePiece[size][size];
// create the individual pieces and place on the board in reverse order:
for ( int num = 1; num != size * size; num = num + 1 )
{ PuzzlePiece p = new PuzzlePiece(num);
int row = num / size;
int col = num % size;
// set p in a ``reversed position'' on the board:
board[size - 1 - row][size - 1 - col] = p;
}
// remember the location on the board where initially there is no piece:
empty_row = size - 1;
empty_col = size - 1;
}
/** contents returns the current state of the puzzle
* @return a matrix that contains the addresses of the pieces */
public PuzzlePiece[][] contents()
{ PuzzlePiece[][] answer = new PuzzlePiece[size][size];
for ( int i = 0; i != size; i = i + 1 )
{ for ( int j = 0; j != size; j = j + 1 )
{ answer[i][j] = board[i][j]; }
}
return answer;
}
...
ENDFIGURE============================================================
FIGURECONT 13: class SlidePuzzleBoard (concl.)===========================
/** move moves a piece into the blank space, provided it is a legal move.
* @param w - the face value of the piece that one wishes to move
* @return true, if the piece labelled w was moved into the empty space;
* return false if the piece cannot be moved into the empty space */
public boolean move(int w)
{ int NOT_FOUND = -1;
int row = NOT_FOUND; // row and col will remember where piece w lives
int col = NOT_FOUND;
// try to find w adjacent to the empty space on the board:
if ( found(w, empty_row - 1, empty_col) )
{ row = empty_row - 1;
col = empty_col;
}
else if ( found(w, empty_row + 1, empty_col) )
{ row = empty_row + 1;
col = empty_col;
}
else if ( found(w, empty_row, empty_col - 1) )
{ row = empty_row;
col = empty_col - 1;
}
else if ( found(w, empty_row, empty_col + 1) )
{ row = empty_row;
col = empty_col + 1;
}
if ( row != NOT_FOUND )
{ // move the piece into the empty space:
board[empty_row][empty_col] = board[row][col];
// mark the new empty space on board:
empty_row = row;
empty_col = col;
board[empty_row][empty_col] = null;
}
return row != NOT_FOUND;
}
/** found returns whether the piece at position row, col is labeled v */
private boolean found(int v, int row, int col)
{ boolean answer = false;
if ( row >= 0 && row < size && col >= 0 && col < size )
{ answer = ( board[row][col].valueOf() == v ); }
return answer;
}
}
ENDFIGURE================================================================
The model subassembly neatly fits into a standard model-view-controller architecture, which we see in Figure 14.
FIGURE 14: class diagram for slide-puzzle program=======================ENDFIGURE==========================================================
FIGURE 15: controller for slide puzzle program==========================
import javax.swing.*;
/** PuzzleController controls the moves of a slide puzzle */
public class PuzzleController
{ private SlidePuzzleBoard board; // model
private PuzzleWriter writer; // output view
/** Constructor PuzzleController initializes the controller
* @param b - the model, the puzzle board
* @param w - the output view */
public PuzzleController(SlidePuzzleBoard b, PuzzleWriter w)
{ board = b;
writer = w;
}
/** play lets the user play the puzzle */
public void play()
{ while ( true )
{ writer.displayPuzzle();
int i = new Integer
(JOptionPane.showInputDialog("Your move:")).intValue();
boolean good_outcome = board.move(i);
if ( !good_outcome )
{ writer.printError("Bad move--puzzle remains the same."); }
}
}
}
/** SlidePuzzle implements a 4 x 4 slide puzzle. Input to the program
* is a sequence of integers between 1..15. The program never terminates. */
public class SlidePuzzle
{ public static void main(String[] args)
{ int size = 4; // a 4 x 4 slide puzzle
SlidePuzzleBoard board = new SlidePuzzleBoard(size);
PuzzleWriter writer = new PuzzleWriter(board, size);
PuzzleController controller = new PuzzleController(board, writer);
controller.play();
}
}
ENDFIGURE================================================================
The output-view class, PuzzleWriter, has the responsibility of painting the contents of the puzzle board in a graphics window. Because the controller, class PuzzleController, sends its repaint requests via method, displayPuzzle, the displayPuzzle method merely asks the view to repaint itself. The hard work of painting is done by paintComponent, which must display the puzzle pieces as a grid. A nested for-loop invokes the helper method, paintPiece, which paints each of the pieces, one by one. Figure 16 shows the coding.
FIGURE 16: output-view class for puzzle game===========================
import java.awt.*; import javax.swing.*;
/** PuzzleWriter displays the contents of a slide puzzle */
public class PuzzleWriter extends JPanel
{ private SlidePuzzleBoard board; // the board that is displayed
private int size; // the board's size
private int piece_size = 30; // the size of one playing piece, in pixels
private int panel_width; // the panel's width and height
private int panel_height;
/** Constructor PuzzleWriter builds the graphics window.
* @param b - the slide puzzle that is displayed
* @param s - the size of the slide puzzle, e.g., 4 means 4 x 4 */
public PuzzleWriter(SlidePuzzleBoard b, int s)
{ board = b;
size = s;
panel_width = piece_size * size + 100;
panel_height = piece_size * size + 100;
JFrame my_frame = new JFrame();
my_frame.getContentPane().add(this);
my_frame.setTitle("Slide Puzzle");
my_frame.setSize(panel_width, panel_height);
my_frame.setVisible(true);
}
/** paintPiece draws piece p at position i,j in the window */
private void paintPiece(Graphics g, PuzzlePiece p, int i, int j)
{ int initial_offset = piece_size;
int x_pos = initial_offset + (piece_size * j);
int y_pos = initial_offset + (piece_size * i);
if ( p != null )
{ g.setColor(Color.white);
g.fillRect(x_pos, y_pos, piece_size, piece_size);
g.setColor(Color.black);
g.drawRect(x_pos, y_pos, piece_size, piece_size);
g.drawString(p.valueOf() + "", x_pos + 10, y_pos + 20);
}
else { g.setColor(Color.black);
g.fillRect(x_pos, y_pos, piece_size, piece_size);
}
}
...
ENDFIGURE======================================================
FIGURECONT 16: output-view class for puzzle game (concl.)================
/** paintComponent displays the puzzle in the frame. */
public void paintComponent(Graphics g)
{ g.setColor(Color.yellow);
g.fillRect(0, 0, panel_width, panel_height);
PuzzlePiece[][] r = board.contents();
for ( int i = 0; i != size; i= i+1 )
{ for ( int j = 0; j != size; j = j+1 )
{ paintPiece(g, r[i][j], i, j); }
}
}
/** displayPuzzle displays the current state of the slide puzzle. */
public void displayPuzzle()
{ this.repaint(); }
/** printError displays an error message.
* @param s - the error message */
public void printError(String s)
{ JOptionPane.showMessageDialog(null, "PuzzleWriter error: " + s ); }
}
ENDFIGURE================================================================
PuzzlePiece[][] r = board.contents();
for ( int i = 0; i != r.length; i = i+1 )
{ for ( int j = 0; j != r[i].length; j = j+1 )
{ if ( r[i][j] == null )
{ System.out.print("X "); }
else { System.out.print( r[i][j].valueOf() + " " ); }
}
System.out.println();
}
Next, use the object's move method to ask the board to
move several numbers.
Display the board resulting from each move.
SlidePuzzleBoard board = new SlidePuzzleBoard(3); PuzzleWriter writer = new PuzzleWriter(board, 3); writer.displayPuzzle();where you use this ``dummy'' class:
public class SlidePuzzleBoard
{ private int size;
public SlidePuzzleBoard(int s) { size = s; }
public PuzzlePiece[][] contents()
{ PuzzlePiece[][] answer = new PuzzlePiece[size][size];
int k = 0;
for ( int i = 0; i != size; i= i+1 )
{ for ( int j = 0; j != size; j = j+1 )
{ answer[i][j] = new PuzzlePiece(k);
k = k + 1;
}
}
return answer;
}
}
Programs with arrays prove notoriously difficult to test thoroughly, because we often use an arithmetic expression as an array index, and the expression might compute to an unacceptable integer. Here is an example: We have this method, which attempts to exchange two adjacent array elements:
/** exchange swaps the values in elements r[i] and r[i-1] */
public void exchange(int[] r, int i)
{ int temp = r[i];
r[i] = r[i - 1];
r[i - 1] = temp;
}
We wish to verify that the method behaves properly for all possible
arguments. We have success for simple test cases, like this one,
int[] test0 = new int[10]; test0[3] = 3; exchange(test0, 4);But what other tests should we attempt? To answer this, we should list all the indexings of array r that appear in the method---they are r[i] and r[i - 1]---and we should predict the range of values that the index expressions, i and i - 1, might have. Remember that the values must fall in the range, 0 to r.length - 1. Now, do they?
A bit of thought lets us invent this test,
int[] test1 = new int[10]; test0[0] = 3; exchange(test0, 0);which generates an exception, because i - 1 has a value that is invalid for array r. Another test case,
int[] test1 = new int[10]; test0[9] = 3; exchange(test0, 10);shows that the attempted indexing, r[i], leads to an error.
The tests make clear that parameter i must be greater than zero and less than the length of the array. We modify the method to read,
public void exchange(int[] r, int i)
{ if ( i > 0 && i < r.length )
{ int temp = r[i];
r[i] = r[i - 1];
r[i - 1] = temp;
}
else { ... announce there is a problem ... }
}
There is, alas, one more test that exposes an error that
goes beyond index-value calculation:
int[] test2 = null; exchange(test2, 1);This invocation of exchange leads to an exception when the array argument is referenced. If we are uncertain that we can validate that all the method's invocations are with proper arrays, then we must add one more test:
public void exchange(int[] r, int i)
{ if ( r != null && i > 0 && i < r.length )
{ int temp = r[i];
r[i] = r[i - 1];
r[i - 1] = temp;
}
else { ... announce there is a problem ... }
}
Testing the values of array indexes becomes harder still when loops are used to examine an array's elements, because it is crucial that the loop starts and terminates appropriately. Consider again the first loop we saw in this chapter, which attempts to select the largest number in an array:
double high_score = score[0];
for ( int i = 1; i <= 5; i = i + 1 )
{ if ( score[i] > high_score )
{ high_score = score[i]; }
}
System.out.println(high_score);
We note that the only array indexing is score[i], and we
readily see that
the range of values denoted by index i is
0 to 5. But this range is sensible only if we are certain that
the length of array score is at least 6---indeed, it should
equal 6. It is better to use score.length<