Declaring and Creating Arrays

Array objects occupy space in memory. Like other objects, arrays are created with keyword new. To create an array object, the programmer specifies 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 variable c:

 int c[] = new int[ 12 ];

This expression can be used to create the array shown in Fig. 7.1. This task also can be performed in two steps as follows:

 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 variable name c 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. When an array is created, each element of the array receives a default valuezero for the numeric primitive-type elements, false for boolean elements and null for references (any nonprimitive type). As we will soon see, we can provide specific, nondefault initial element values when we create an array.

Common Programming Error 7.2

In an array declaration, specifying the number of elements in the square brackets of the declaration (e.g., int c[ 12 ];) is a syntax error.

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

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

In this case, the class name String applies to each variable in the declaration. For readability, we prefer to declare only one variable per declaration, as in:

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

Good Programming Practice 7.1

For readability, declare only one variable per declaration. Keep each declaration on a separate line, and include a comment describing the variable being declared.

When an array is declared, the type of the array and the square brackets can be combined at the beginning of the declaration to indicate that all the identifiers in the declaration are array variables. For example, the declaration

 double[] array1, array2;

indicates that array1 and array2 are "array of double" variables. The preceding declaration is equivalent to:

 double array1[];
 double array2[];

or

 double[] array1;
 double[] array2;

The preceding pairs of declarations are equivalentwhen only one variable is declared in each declaration, the square brackets can be placed either after the type or after the array variable name.

Common Programming Error 7.3

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 correctplacing 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 incorrectthe 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 type. Similarly, in an array of a reference type, every element is a reference to an object of the array's declared 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.