CHAPTER 10

Arrays

In the Java programming language arrays are objects (§4.3.1), are dynamically created, and may be assigned to variables of type Object (§4.3.2). All methods of class Object may be invoked on an array.

An array object contains a number of variables. The number of variables may be zero, in which case the array is said to be empty. The variables contained in an array have no names; instead they are referenced by array access expressions that use nonnegative integer index values. These variables are called the components of the array. If an array has n components, we say n is the length of the array; the components of the array are referenced using integer indices from 0 to n - 1, inclusive.

All the components of an array have the same type, called the component type of the array. If the component type of an array is T, then the type of the array itself is written T[].

The value of an array component of type float is always an element of the float value set (§4.2.3); similarly, the value of an array component of type double is always an element of the double value set. It is not permitted for the value of an array component of type float to be an element of the float-extended-exponent value set that is not also an element of the float value set, nor for the value of an array component of type double to be an element of the double-extended-exponent value set that is not also an element of the double value set.

The component type of an array may itself be an array type. The components of such an array may contain references to subarrays. If, starting from any array type, one considers its component type, and then (if that is also an array type) the component type of that type, and so on, eventually one must reach a component type that is not an array type; this is called the element type of the original array, and the components at this level of the data structure are called the elements of the original array.

There are some situations in which an element of an array can be an array: if the element type is Object or Cloneable or java.io.Serializable, then some or all of the elements may be arrays, because any array object can be assigned to any variable of these types.

10.1 Array Types

The element type of an array may be any type, whether primitive or reference. In particular:

• Arrays with an interface type as the component type are allowed. The elements of such an array may have as their value a null reference or instances of any type that implements the interface.
• Arrays with an abstract class type as the component type are allowed. The elements of such an array may have as their value a null reference or instances of any subclass of the abstract class that is not itself abstract.

10.2 Array Variables

Because an array's length is not part of its type, a single variable of array type may contain references to arrays of different lengths.

Here are examples of declarations of array variables that do not create arrays:

int[] ai;		// array of int
short[][] as;		// array of array of short
Object[]    ao,		// array of Object
	otherAo;	// array of Object
short	s,		// scalar short 
	aas[][];	// array of array of short

Exception ae[] = new Exception[3]; 
Object aao[][] = new Exception[2][3];
int[] factorial = { 1, 1, 2, 6, 24, 120, 720, 5040 };
char ac[] = { 'n', 'o', 't', ' ', 'a', ' ',
				 'S', 't', 'r', 'i', 'n', 'g' }; 
String[] aas = { "array", "of", "String", };

byte[] rowvector, colvector, matrix[];

byte rowvector[], colvector[], matrix[][];

If an array variable v has type A[], where A is a reference type, then v can hold a reference to an instance of any array type B[], provided B can be assigned to A. This may result in a run-time exception on a later assignment; see §10.10 for a discussion.

10.3 Array Creation

An array creation expression specifies the element type, the number of levels of nested arrays, and the length of the array for at least one of the levels of nesting. The array's length is available as a final instance variable length.

An array initializer creates an array and provides initial values for all its components.

10.4 Array Access

Arrays must be indexed by int values; short, byte, or char values may also be used as index values because they are subjected to unary numeric promotion (§5.6.1) and become int values. An attempt to access an array component with a long index value results in a compile-time error.

All array accesses are checked at run time; an attempt to use an index that is less than zero or greater than or equal to the length of the array causes an ArrayIndexOutOfBoundsException to be thrown.

10.5 Arrays: A Simple Example

class Gauss {
	public static void main(String[] args) {
		int[] ia = new int[101];
		for (int i = 0; i < ia.length; i++)
			ia[i] = i;
		int sum = 0;
		for (int i = 0; i < ia.length; i++)
			sum += ia[i];
		System.out.println(sum);
	}
}

5050

The example program fills the array with the integers from 0 to 100, sums these integers, and prints the result.

10.6 Array Initializers

ArrayInitializer:
	{ VariableInitializersopt ,opt }

VariableInitializers:
	VariableInitializer
	VariableInitializers , VariableInitializer

VariableInitializer:
	Expression
	ArrayInitializer

The length of the constructed array will equal the number of expressions.

The expressions in an array initializer are executed from left to right in the textual order they occur in the source code. The nth variable initializer specifies the value of the n-1st array component. Each expression must be assignment-compatible (§5.2) with the array's component type, or a compile-time error results.

If the component type is itself an array type, then the expression specifying a component may itself be an array initializer; that is, array initializers may be nested.

A trailing comma may appear after the last expression in an array initializer and is ignored.

As an example:

class Test {
	public static void main(String[] args) {
		int ia[][] = { {1, 2}, null };
		for (int i = 0; i < 2; i++)
			for (int j = 0; j < 2; j++)
				System.out.println(ia[i][j]);
	}
}

1
2

10.7 Array Members

• The public final field length, which contains the number of components of the array (length may be positive or zero)
• The public method clone, which overrides the method of the same name in class Object and throws no checked exceptions
• All the members inherited from class Object; the only method of Object that is not inherited is its clone method

An array thus has the same public fields and methods as the following class:

class A implements Cloneable, java.io.Serializable {
	public final int length = X;
	public Object clone() {
		try {
			return super.clone();
		} catch (CloneNotSupportedException e) {
			throw new InternalError(e.getMessage());
		}
	}
}

That arrays are cloneable is shown by the test program:

class Test {
	public static void main(String[] args) {
		int ia1[] = { 1, 2 };
		int ia2[] = (int[])ia1.clone();
		System.out.print((ia1 == ia2) + " ");
		ia1[1]++;
		System.out.println(ia2[1]);
	}
}

false 2

A clone of a multidimensional array is shallow, which is to say that it creates only a single new array. Subarrays are shared.

This is shown by the example program:

class Test {
	public static void main(String[] args) throws Throwable {
		int ia[][] = { { 1 , 2}, null };
		int ja[][] = (int[][])ia.clone();
		System.out.print((ia == ja) + " ");
		System.out.println(ia[0] == ja[0] && ia[1] == ja[1]);
	}
}

false true

10.8 Class Objects for Arrays

This is shown by the following example code:

class Test {
	public static void main(String[] args) {
		int[] ia = new int[3];
		System.out.println(ia.getClass());
		System.out.println(ia.getClass().getSuperclass());
	}
}

class [I
class java.lang.Object

10.9 An Array of Characters is Not a String

A String object is immutable, that is, its contents never change, while an array of char has mutable elements. The method toCharArray in class String returns an array of characters containing the same character sequence as a String. The class StringBuffer implements useful methods on mutable arrays of characters.

10.10 Array Store Exception

Thus, the example:

class Point { int x, y; }
class ColoredPoint extends Point { int color; }
class Test {
	public static void main(String[] args) {
		ColoredPoint[] cpa = new ColoredPoint[10];
		Point[] pa = cpa;
		System.out.println(pa[1] == null);
		try {
			pa[0] = new Point();
		} catch (ArrayStoreException e) {
			System.out.println(e);
		}
	}
}

true
java.lang.ArrayStoreException

A reference to this array pa, for example, testing whether pa[1] is null, will not result in a run-time type error. This is because the element of the array of type ColoredPoint[] is a ColoredPoint, and every ColoredPoint can stand in for a Point, since Point is the superclass of ColoredPoint.

On the other hand, an assignment to the array pa can result in a run-time error. At compile time, an assignment to an element of pa is checked to make sure that the value assigned is a Point. But since pa holds a reference to an array of ColoredPoint, the assignment is valid only if the type of the value assigned at run-time is, more specifically, a ColoredPoint.

The Java virtual machine checks for such a situation at run-time to ensure that the assignment is valid; if not, an ArrayStoreException is thrown. More formally: an assignment to an element of an array whose type is A[], where A is a reference type, is checked at run-time to ensure that the value assigned can be assigned to the actual element type of the array, where the actual element type may be any reference type that is assignable to A.