E.3. Declaring and Creating Arrays

Array objects occupy space in memory. Like other objects, arrays are created with keyword new. To create an array object, you specify the type of the array elements and the number of elements as part of an array-creation expression that uses keyword new. Such an expression returns a reference that can be stored in an array variable. The following declaration and array-creation expression create an array object containing 12 int elements and store the array’s reference in array variable c:

int[] c = new int[ 12 ];

This expression can be used to create the array shown in Fig. E.1. When an array is created, each element of the array receives a default value—zero for the numeric primitive-type elements, false for boolean elements and null for references. As you’ll soon see, you can provide nondefault initial element values when you create an array.

Creating the array in Fig. E.1 can also be performed in two steps as follows:

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int[] c; // declare the array variable
c = new int[ 12 ]; // create the array; assign to array variable

In the declaration, the square brackets following the type indicate that c is a variable that will refer to an array (i.e., the variable will store an array reference). In the assignment statement, the array variable c receives the reference to a new array of 12 int elements.

A program can create several arrays in a single declaration. The following declaration reserves 100 elements for b and 27 elements for x:

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String[] b = new String[ 100 ], x = new String[ 27 ];

When the type of the array and the square brackets are combined at the beginning of the declaration, all the identifiers in the declaration are array variables. In this case, variables b and x refer to String arrays. For readability, we prefer to declare only one variable per declaration. The preceding declaration is equivalent to:

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String[] b = new String[ 100 ]; // create array b
String[] x = new String[ 27 ]; // create array x

When only one variable is declared in each declaration, the square brackets can be placed either after the type or after the array variable name, as in:

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String b[] = new String[ 100 ]; // create array b
String x[] = new String[ 27 ]; // create array x

Image Common Programming Error E.2

Declaring multiple array variables in a single declaration can lead to subtle errors. Consider the declaration int[] a, b, c;. If a, b and c should be declared as array variables, then this declaration is correct—placing square brackets directly following the type indicates that all the identifiers in the declaration are array variables. However, if only a is intended to be an array variable, and b and c are intended to be individual int variables, then this declaration is incorrect—the declaration int a[], b, c; would achieve the desired result.

A program can declare arrays of any type. Every element of a primitive-type array contains a value of the array’s declared element type. Similarly, in an array of a reference type, every element is a reference to an object of the array’s declared element type. For example, every element of an int array is an int value, and every element of a String array is a reference to a String object.

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