Section 4.7. Arrays | Learning Java

4.7. 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. Java supports arrays of all primitive 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 index operator, []. Unlike other languages, however, arrays in Java are true, first-class objects. 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 type for us whenever we declare a new type of array. 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 later.
Java lets us use the [] operator to access array elements so that arrays look as we expect. We could implement our own classes that act like arrays, but we would have to settle for having methods such as get( ) and set( ) 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 with a specified length with the [] notation or initialize it directly from a structured list of values.

4.7.1. Array Types

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

The following are equivalent:

     int [] arrayOfInts;  // preferred     int arrayOfInts [];  // C-style

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 when declaring an array type variable. The size is strictly a function of the array object itself, not the reference to it.

An array of reference types can be created in the same way:

     String [] someStrings;     Button [] someButtons;

4.7.2. Array Creation and Initialization

The new operator is used to create an instance of an 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];

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, a newly allocated array of objects actually contains only reference variables, each with the value null.^[*] That's not to say that there is no memory associated with an empty array; memory is needed to hold those references (the empty "slots" in the array). Figure 4-4 illustrates the names array of the previous example.

^[*] The analog in C or C++ is 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 Java language specification.

Figure 4-4. A Java array

names is a variable of type String[] (i.e., a string array). This particular String[] object contains 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:

     int [] primes = { 1, 2, 3, 5, 7, 7+4 };    // e.g., 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. Note that we did not use the new keyword or the array type here. The type of the array was inferred from the assignment.

We can use the {} syntax with an array of objects. In this case, each expression 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 };

The following are equivalent:

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

4.7.3. 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. (Don't worry, the compiler tells you when you accidentally use parentheses as if it were a method; everyone does now and then.)

Array access in Java is just like array access in other languages; 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 ) );

Remember that we can also use the new enhanced for loop to iterate over array values. Here we'll use it to print all the values we just assigned:

     for( int i : keyPad )         System.out.println( 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 and handle it yourself, if you really expect it, or ignore it, as we've 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. The arraycopy( ) method is then 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 is garbage-collected on the next pass.

4.7.4. Anonymous Arrays

Often it is convenient to create "throw-away" arrays, arrays that are used in one place and never referenced anywhere else. Such arrays don't need 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. Provided Cat and Dog are subclasses of Animal, here's how to call setPets( ) using an anonymous array:

     Dog pokey = new Dog ("gray");     Cat boojum = new Cat ("grey");     Cat simon = new Cat ("orange");     setPets ( new Animal [] { pokey, boojum, simon });

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 and the array type to create the array object.

Prior to Java 5.0, anonymous arrays were used frequently as a substitute for variable-length argument lists to methods, which are discussed in Chapter 5. With the introduction of variable-length argument lists in Java, the usefulness of anonymous arrays may diminish.

4.7.5. 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, makes each element into an 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 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;     ...

We can specify a partial index of a multidimensional array to get a subarray of array type objects 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, in Java, is of type ChessPiece[]. For example, we can populate our chess board one row at a time:

     ChessPiece [] homeRow =  {         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] = homeRow;

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 trailing, 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 either) 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. That is, multidimensional arrays don't have to be rectangular. Here's a defective (but perfectly legal to Java) checkerboard:

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

And 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;     }

4.7.6. Inside Arrays

We 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, but we need to talk more about the object-oriented aspects of Java before answering them. That's the subject of the next chapter. For now, take it on faith that arrays fit into the class hierarchy.