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Exploring Java

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The Java Language
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4.6 Arrays

An array is a special type of object that can hold an ordered collection of elements. The type of the elements of the array is called the base type of the array; the number of elements it holds is a fixed attribute called its length. (For a collection with a variable length, see the discussion of Vector objects in Chapter 7, Basic Utility Classes.) Java supports arrays of all numeric and reference types.

The basic syntax of arrays looks much like that of C or C++. We create an array of a specified length and access the elements with the special index operator, []. Unlike other languages, however, arrays in Java are true, first-class objects, which means they are real objects within the Java language. An array is an instance of a special Java array class and has a corresponding type in the type system. This means that to use an array, as with any other object, we first declare a variable of the appropriate type and then use the new operator to create an instance of it.

Array objects differ from other objects in Java in three respects:

  • Java implicitly creates a special array class for us whenever we declare an array type variable. It's not strictly necessary to know about this process in order to use arrays, but it helps in understanding their structure and their relationship to other objects in Java.

  • Java lets us use the special [] operator to access array elements, so that arrays look as we expect. We could implement our own classes that act like arrays, but because Java doesn't have user-defined operator overloading, we would have to settle for having methods like get() and put() instead of using the special [] notation.

  • Java provides a corresponding special form of the new operator that lets us construct an instance of an array and specify its length with the [] notation.

Array Types

An array type variable is denoted by a base type followed by empty brackets []. Alternatively, Java accepts a C-style declaration, with the brackets placed after the array name. The following are equivalent:

int [] arrayOfInts; 
int arrayOfInts []; 

In each case, arrayOfInts is declared as an array of integers. The size of the array is not yet an issue, because we are declaring only the array type variable. We have not yet created an actual instance of the array class, with its associated storage. It's not even possible to specify the length of an array as part of its type.

An array of objects can be created in the same way:

String [] someStrings; 
Button someButtons []; 

Array Creation and Initialization

Having declared an array type variable, we can now use the new operator to create an instance of the array. After the new operator, we specify the base type of the array and its length, with a bracketed integer expression:

arrayOfInts = new int [42]; 
someStrings = new String [ number + 2 ]; 

We can, of course, combine the steps of declaring and allocating the array:

double [] someNumbers = new double [20]; 
Component widgets [] = new Component [12]; 

As in C, array indices start with zero. Thus, the first element of someNumbers [] is 0 and the last element is 19. After creation, the array elements are initialized to the default values for their type. For numeric types, this means the elements are initially zero:

int [] grades = new int [30]; 
grades[0] = 99; 
grades[1] = 72; 
// grades[2] == 0 

The elements of an array of objects are references to the objects, not actual instances of the objects. The default value of each element is therefore null, until we assign instances of appropriate objects:

String names [] = new String [4]; 
names [0] = new String(); 
names [1] = "Boofa"; 
names [2] = someObject.toString(); 
// names[3] == null 

This is an important distinction that can cause confusion. In many other languages, the act of creating an array is the same as allocating storage for its elements. In Java, an array of objects actually contains only reference variables and those variables, have the value null until they are assigned to real objects.[5] Figure 4.4 illustrates the names array of the previous example:

[5] The analog in C or C++ would be an array of pointers to objects. However, pointers in C or C++ are themselves two- or four-byte values. Allocating an array of pointers is, in actuality, allocating the storage for some number of those pointer objects. An array of references is conceptually similar, although references are not themselves objects. We can't manipulate references or parts of references other than by assignment, and their storage requirements (or lack thereof) are not part of the high-level language specification.

names is a variable of type String[] (i.e., a string array). The String[] object can be thought of as containing four String type variables. We have assigned String objects to the first three array elements. The fourth has the default value null.

Java supports the C-style curly braces {} construct for creating an array and initializing its elements when it is declared:

int [] primes = { 1, 2, 3, 5, 7, 7+4 };    // primes[2] == 3 

An array object of the proper type and length is implicitly created and the values of the comma-separated list of expressions are assigned to its elements.

We can use the {} syntax with an array of objects. In this case, each of the expressions must evaluate to an object that can be assigned to a variable of the base type of the array, or the value null. Here are some examples:

String [] verbs = { "run", "jump", someWord.toString() }; 
Button [] controls = { stopButton, new Button("Forwards"),  
    new Button("Backwards") }; 
// all types are subtypes of Object 
Object [] objects = { stopButton, "A word", null }; 

You should create and initialize arrays in whatever manner is appropriate for your application. The following are equivalent:

Button [] threeButtons = new Button [3]; 
Button [] threeButtons = { null, null, null }; 

Using Arrays

The size of an array object is available in the public variable length:

char [] alphabet = new char [26]; 
int alphaLen = alphabet.length;             // alphaLen == 26 
 
String [] musketeers = { "one", "two", "three" }; 
int num = musketeers.length;                // num == 3 

length is the only accessible field of an array; it is a variable, not a method.

Array access in Java is just like array access in C; you access an element by putting an integer-valued expression between brackets after the name of the array. The following example creates an array of Button objects called keyPad and then fills the array with Button objects:

Button [] keyPad = new Button [ 10 ]; 
for ( int i=0; i < keyPad.length; i++ )      keyPad[ i ] = new Button( Integer.toString( i ) );

Attempting to access an element that is outside the range of the array generates an ArrayIndexOutOfBoundsException. This is a type of RuntimeException, so you can either catch it and handle it yourself, or ignore it, as we already discussed:

String [] states = new String [50]; 
 
try { 
    states[0] = "California"; 
    states[1] = "Oregon"; 
    ... 
    states[50] = "McDonald's Land";  // Error--array out of bounds 
}  
catch ( ArrayIndexOutOfBoundsException err ) { 
    System.out.println( "Handled error: " + err.getMessage() ); 
} 

It's a common task to copy a range of elements from one array into another. Java supplies the arraycopy() method for this purpose; it's a utility method of the System class:

System.arraycopy( source, sourceStart, destination, 
                  destStart, length ); 

The following example doubles the size of the names array from an earlier example:

String [] tmpVar = new String [ 2 * names.length ]; 
System.arraycopy( names, 0, tmpVar, 0, names.length ); 
names = tmpVar; 

A new array, twice the size of names, is allocated and assigned to a temporary variable tmpVar. arraycopy() is used to copy the elements of names to the new array. Finally, the new array is assigned to names. If there are no remaining references to the old array object after names has been copied, it will be garbage collected on the next pass.

Anonymous Arrays

You often want to create "throw-away" arrays: arrays that are only used in one place, and never referenced anywhere else. Such arrays don't need to have a name, because you never need to refer to them again in that context. For example, you may want to create a collection of objects to pass as an argument to some method. It's easy enough to create a normal, named array--but if you don't actually work with the array (if you use the array only as a holder for some collection), you shouldn't have to. Java makes it easy to create "anonymous" (i.e., unnamed) arrays.

Let's say you need to call a method named setPets(), which takes an array of Animal objects as arguments. Cat and Dog are subclasses of Animal. Here's how to call setPets() using an anonymous array:

Dog pokey = new Dog ("gray");
Cat squiggles = new Cat ("black");
Cat jasmine = new Cat ("orange");
setPets ( new Animal [] { pokey, squiggles, jasmine });

The syntax looks just like the initialization of an array in a variable declaration. We implicitly define the size of the array and fill in its elements using the curly brace notation. However, since this is not a variable declaration we have to explicitly use the new operator to create the array object.

You can use anonymous arrays to simulate variable length argument lists (often called VARARGS), a feature of many programming languages that Java doesn't provide. The advantage of anonymous arrays over variable length argument lists is that it allows stricter type checking; the compiler always knows exactly what arguments are expected, and therefore can verify that method calls are correct.

Multidimensional Arrays

Java supports multidimensional arrays in the form of arrays of array type objects. You create a multidimensional array with C-like syntax, using multiple bracket pairs, one for each dimension. You also use this syntax to access elements at various positions within the array. Here's an example of a multidimensional array that represents a chess board:

ChessPiece [][] chessBoard; 
chessBoard = new ChessPiece [8][8]; 
chessBoard[0][0] = new ChessPiece( "Rook" ); 
chessBoard[1][0] = new ChessPiece( "Pawn" ); 
... 

Here chessBoard is declared as a variable of type ChessPiece[][] (i.e., an array of ChessPiece arrays). This declaration implicitly creates the type ChessPiece[] as well. The example illustrates the special form of the new operator used to create a multidimensional array. It creates an array of ChessPiece[] objects and then, in turn, creates each array of ChessPiece objects. We then index chessBoard to specify values for particular ChessPiece elements. (We'll neglect the color of the pieces here.)

Of course, you can create arrays of with more than two dimensions. Here's a slightly impractical example:

Color [][][] rgbCube = new Color [256][256][256]; 
rgbCube[0][0][0] = Color.black; 
rgbCube[255][255][0] = Color.yellow; 
... 

As in C, we can specify the initial index of a multidimensional array to get an array type object with fewer dimensions. In our example, the variable chessBoard is of type ChessPiece[][]. The expression chessBoard[0] is valid and refers to the first element of chessBoard, which is of type ChessPiece[]. For example, we can create a row for our chess board:

ChessPiece [] startRow =  { 
    new ChessPiece("Rook"), new ChessPiece("Knight"), 
    new ChessPiece("Bishop"), new ChessPiece("King"), 
    new ChessPiece("Queen"), new ChessPiece("Bishop"), 
    new ChessPiece("Knight"), new ChessPiece("Rook") 
}; 
 
chessBoard[0] = startRow; 

We don't necessarily have to specify the dimension sizes of a multidimensional array with a single new operation. The syntax of the new operator lets us leave the sizes of some dimensions unspecified. The size of at least the first dimension (the most significant dimension of the array) has to be specified, but the sizes of any number of the less significant array dimensions may be left undefined. We can assign appropriate array type values later.

We can create a checkerboard of boolean values (which is not quite sufficient for a real game of checkers) using this technique:

boolean [][] checkerBoard; 
checkerBoard = new boolean [8][]; 

Here, checkerBoard is declared and created, but its elements, the eight boolean[] objects of the next level, are left empty. Thus, for example, checkerBoard[0] is null until we explicitly create an array and assign it, as follows:

checkerBoard[0] = new boolean [8]; 
checkerBoard[1] = new boolean [8]; 
... 
checkerBoard[7] = new boolean [8]; 

The code of the previous two examples is equivalent to:

boolean [][] checkerBoard = new boolean [8][8]; 

One reason we might want to leave dimensions of an array unspecified is so that we can store arrays given to us by another method.

Note that since the length of the array is not part of its type, the arrays in the checkerboard do not necessarily have to be of the same length. Here's a defective (but perfectly legal) checkerboard:

checkerBoard[2] = new boolean [3]; 
checkerBoard[3] = new boolean [10]; 

Since Java implements multidimensional arrays as arrays of arrays, multidimensional arrays do not have to be rectangular. For example, here's how you could create and initialize a triangular array:

int []][] triangle = new int [5][]; 
for (int i = 0; i < triangle.length; i++) {  
    triangle[i] = new int [i + 1]; 
    for (int j = 0; j < i + 1; j++) 
        triangle[i][j] = i + j;    } 

Inside Arrays

I said earlier that arrays are instances of special array classes in the Java language. If arrays have classes, where do they fit into the class hierarchy and how are they related? These are good questions; however, we need to talk more about the object-oriented aspects of Java before I can answer them. For now, take it on faith that arrays fit into the class hierarchy; details are in Chapter 5, Objects in Java.


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