Chapter 10. Arrays

Table of Contents

10.1. Array Types
10.2. Array Variables
10.3. Array Creation
10.4. Array Access
10.5. Array Store Exception
10.6. Array Initializers
10.7. Array Members
10.8. Class Objects for Arrays
10.9. An Array of Characters is Not a String

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 non-negative 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

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

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:

  • Arrays with an interface type as the element type are allowed.

    An element of such an array may have as its value a null reference or an instance of any type that implements the interface.

  • Arrays with an abstract class type as the element type are allowed.

    An element of such an array may have as its value a null reference or an instance of any subclass of the abstract class that is not itself abstract.

The supertypes of an array type are specified in §4.10.3.

The supertype relation for array types is not the same as the superclass relation. The direct supertype of Integer[] is Number[] according to §4.10.3, but the direct superclass of Integer[] is Object according to the Class object for Integer[] (§10.8). This does not matter in practice, because Object is also a supertype of all array types.

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, §9.3, §14.4.1) may create an array, a reference to which then becomes the initial value of the variable.

Example 10.2-1. Declarations of Array Variables


int[]     ai;        // array of int
short[][] as;        // array of array of short
short     s,         // scalar short
          aas[][];   // array of array of short
Object[]  ao,        // array of Object
          otherAo;   // array of Object
Collection<?>[] ca;  // array of Collection of unknown type

The declarations above do not create array objects. The following are examples of declarations of array variables that do 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.

For example:

byte[] rowvector, colvector, matrix[];

This declaration is equivalent to:

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

In a variable declaration (§8.3, §8.4.1, §9.3, §14.14, §14.20) except for a variable arity parameter, the array type of a variable is denoted by the array type that appears at the beginning of the declaration, followed by any bracket pairs that follow the variable's Identifier in the declarator.

For example, the local variable declaration:

int a, b[], c[][];

is equivalent to the series of declarations:

int a;
int[] b;
int[][] c;

Brackets are allowed in declarators as a nod to the tradition of C and C++. The general rules for variable declaration, however, permit brackets to appear on both the type and in declarators, so that the local variable declaration:

float[][] f[][], g[][][], h[];  // Yechh!

is equivalent to the series of declarations:

float[][][][] f;
float[][][][][] g;
float[][][] h;

We do not recommend "mixed notation" in an array variable declaration, where brackets appear on both the type and in declarators.

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.

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

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 (§5.2). This may result in a run-time exception on a later assignment; see §10.5 for a discussion.

10.3. Array Creation

An array is created by an array creation expression (§15.10.1) 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.

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 (§15.10.3) 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.

Example 10.4-1. Array Access

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);
    }
}

This program produces the output:

5050

The program 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 (§15.10.1). 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 program fills the array with the integers from 0 to 100, sums these integers, and prints the result.


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 (§15.10.4).

10.5. Array Store Exception

For an array whose type is A[], where A is a reference type, an assignment to a component of the array is checked at run time to ensure that the value being assigned is assignable to the component.

If the type of the value being assigned is not assignment-compatible (§5.2) with the component type, an ArrayStoreException is thrown.

If the component type of an array were not reifiable (§4.7), the Java Virtual Machine could not perform the store check described in the preceding paragraph. This is why an array creation expression with a non-reifiable element type is forbidden (§15.10.1). One may declare a variable of an array type whose element type is non-reifiable, but assignment of the result of an array creation expression to the variable will necessarily cause an unchecked warning (§5.1.9).

Example 10.5-1. ArrayStoreException

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);
        }
    }
}

This program produces the output:

true
java.lang.ArrayStoreException: Point

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.


10.6. Array Initializers

An array initializer may be specified in a field declaration (§8.3, §9.3) or local variable declaration (§14.4), or as part of an array creation expression (§15.10.1), to create an array and provide some initial values.

ArrayInitializer:
VariableInitializerList:

The following production from §8.3 is shown here for convenience:

VariableInitializer:

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

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

Each variable initializer must be assignment-compatible (§5.2) with the array's component type, or a compile-time error occurs.

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

The length of the array to be constructed is equal to the number of variable initializers immediately enclosed by the braces of the array initializer. Space is allocated for a new array of that length. If there is insufficient space to allocate the array, evaluation of the array initializer completes abruptly by throwing an OutOfMemoryError. Otherwise, a one-dimensional array is created of the specified length, and each component of the array is initialized to its default value (§4.12.5).

The variable initializers immediately enclosed by the braces of the array initializer are then executed from left to right in the textual order they occur in the source code. The n'th variable initializer specifies the value of the n-1'th array component. If execution of a variable initializer completes abruptly, then execution of the array initializer completes abruptly for the same reason. If all the variable initializer expressions complete normally, the array initializer completes normally, with the value of the newly initialized array.

If the component type is an array type, then the variable initializer specifying a component may itself be an array initializer; that is, array initializers may be nested. In this case, execution of the nested array initializer constructs and initializes an array object by recursive application of the algorithm above, and assigns it to the component.

Example 10.6-1. Array Initializers

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);
            }
        }
    }
}

This program produces the output:

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:

  • 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. The return type of the clone method of an array type T[] is T[].

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

  • All the members inherited from class Object; the only method of Object that is not inherited is its clone method.

    See §9.6.4.4 for another situation where the difference between public and non-public methods of Object requires special care.

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();
        } catch (CloneNotSupportedException e) {
            throw new InternalError(e.getMessage());
        }
    }
}

Note that the cast to T[] in the code above would generate an unchecked warning (§5.1.9) if arrays were really implemented this way.

Example 10.7-1. Arrays Are Cloneable

class Test1 {
    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]);
    }
}

This program produces the output:

false 2

showing that the components of the arrays referenced by ia1 and ia2 are different variables.


Example 10.7-2. Shared Subarrays After A Clone

The fact that subarrays are shared when a multidimensional array is cloned is shown by this program:

class Test2 {
    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]);
    }
}

This program produces the output:

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.

Although an array type is not a class, the Class object of every array acts as if:

  • The direct superclass of every array type is Object.

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

Example 10.8-1. Class Object Of Array

class Test1 {
    public static void main(String[] args) {
        int[] ia = new int[3];
        System.out.println(ia.getClass());
        System.out.println(ia.getClass().getSuperclass());
        for (Class<?> c : ia.getClass().getInterfaces())
            System.out.println("Superinterface: " + c);
    }
}

This program produces the output:

class [I
class java.lang.Object
Superinterface: interface java.lang.Cloneable
Superinterface: interface java.io.Serializable

where the string "[I" is the run-time type signature for the Class object "array with component type int".


Example 10.8-2. Array Class Objects Are Shared

class Test2 {
    public static void main(String[] args) {
        int[] ia = new int[3];
        int[] ib = new int[6];
        System.out.println(ia == ib);
        System.out.println(ia.getClass() == ib.getClass());
    }
}

This program produces the output:

false
true

While ia and ib refer to different arrays, the result of the comparison of the Class objects demonstrates that all arrays whose components are of type int are instances of the same array type (namely 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.