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Third Edition


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

An array type is written as the name of an element type followed by some number of empty pairs of square brackets []. The number of bracket pairs indicates the depth of array nesting. An array's length is not part of its type.

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

Array types are used in declarations and in cast expressions (§15.16).

10.2 Array Variables

A variable of array type holds a reference to an object. Declaring a variable of array type does not create an array object or allocate any space for array components. It creates only the variable itself, which can contain a reference to an array. However, the initializer part of a declarator (§8.3) may create an array, a reference to which then becomes the initial value of the variable.

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
Collection<?>[] ca;		// array of Collection of unknown type
short		s,		// scalar short 
		aas[][];	// array of array of short
Here are some examples of declarations of array variables that create array objects:

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", };

The [] may appear as part of the type at the beginning of the declaration, or as part of the declarator for a particular variable, or both, as in this example:

byte[] rowvector, colvector, matrix[];
This declaration is equivalent to:

byte rowvector[], colvector[], matrix[][];
Once an array object is created, its length never changes. To make an array variable refer to an array of different length, a reference to a different array must be assigned to the variable.

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 is created by an array creation expression (§15.10) or an array initializer (§10.6).

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. It is a compile-time error if the element type is not a reifiable type (§4.7)

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

10.4 Array Access

A component of an array is accessed by an array access expression (§15.13) that consists of an expression whose value is an array reference followed by an indexing expression enclosed by [ and ], as in A[i]. All arrays are 0-origin. An array with length n can be indexed by the integers 0 to n-1.

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 (§) 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

The 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 e : ia)
			sum += e;
		System.out.println(sum);
	}
}
that produces the output:

5050
declares a variable ia that has type array of int, that is, int[]. The variable ia is initialized to reference a newly created array object, created by an array creation expression (§15.10). The array creation expression specifies that the array should have 101 components. The length of the array is available using the field length, as shown.

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

10.6 Array Initializers

An array initializer may be specified in a declaration, or as part of an array creation expression (§15.10), creating an array and providing some initial values:


ArrayInitializer:
	{ VariableInitializersopt ,opt }

VariableInitializers:
	VariableInitializer
	VariableInitializers , VariableInitializer
	
The following is repeated from §8.3 to make the presentation here clearer:


VariableInitializer:
	Expression
	ArrayInitializer
	
An array initializer is written as a comma-separated list of expressions, enclosed by braces "{" and "}".

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. It is a compile-time error if the component type of the array being initialized is not reifiable (§4.7).

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[] ea : ia)
			for (int e: ea)
				System.out.println(e);
	}
}
prints:

1
2
before causing a NullPointerException in trying to index the second component of the array ia, which is a null reference.

10.7 Array Members

The members of an array type are all of the following:

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

class A<T> implements Cloneable, java.io.Serializable {
	public final int length = X;
	public T[] clone() {
		try {
			return (T[])super.clone(); // unchecked warning
		} catch (CloneNotSupportedException e) {
			throw new InternalError(e.getMessage());
		}
	}
}
Note that the cast in the example above would generate an unchecked warning (§5.1.9) if arrays were really implemented this way.

Every array implements the interfaces Cloneable and java.io.Serializable.

That arrays are cloneable is shown by the test program:

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

false 2
showing that the components of the arrays referenced by ia1 and ia2 are different variables. (In some early implementations of the Java programming language this example failed to compile because the compiler incorrectly believed that the clone method for an array could throw a CloneNotSupportedException.)

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[][] = ia.clone();
		System.out.print((ia == ja) + " ");
		System.out.println(ia[0] == ja[0] && ia[1] == ja[1]);
	}
}
which prints:

false true
showing that the int[] array that is ia[0] and the int[] array that is ja[0] are the same array.

10.8 Class Objects for Arrays

Every array has an associated Class object, shared with all other arrays with the same component type. The direct superclass of an array type is Object. Every array type implements the interfaces Cloneable and java.io.Serializable.

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());
	}
}
which prints:

class [I
class java.lang.Object
where the string "[I" is the run-time type signature for the class object "array with component type int".

10.9 An Array of Characters is Not a String

In the Java programming language, unlike C, an array of char is not a String, and neither a String nor an array of char is terminated by '\u0000' (the NUL character).

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

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.

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);
		}
	}
}
produces the output:

true
java.lang.ArrayStoreException
Here the variable pa has type Point[] and the variable cpa has as its value a reference to an object of type ColoredPoint[]. A ColoredPoint can be assigned to a Point; therefore, the value of cpa can be assigned to pa.

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.


Discussion

If the element type of an array were not reifiable (§4.7), the virtual machine could not perform the store check described in the preceding paragraph. This is why creation of arrays of non-reifiable types is forbidden. One may declare variables of array types whose element type is not reifiable, but any attempt to assign them a value will give rise to an unchecked warning (§5.1.9).



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