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Creating and Using Arrays

Here's a simple program, called ArrayDemo, that creates the array, puts some values in it, and displays the values.

public class ArrayDemo {
    public static void main(String[] args) {
        int[] anArray;	        // declare an array of integers

        anArray = new int[10];	// create an array of integers

        // assign a value to each array element and print 
        for (int i = 0; i < anArray.length; i++) {
            anArray[i] = i;
            System.out.print(anArray[i] + " ");
        }
        System.out.println();
    }
}

This section covers these topics:

• Declaring a Variable to Refer to an Array
• Creating an Array
• Accessing an Array Element
• Getting the Size of an Array
• Array Initializers

This line of code from the sample program declares an array variable:

int[] anArray;          // declare an array of integers

Like declarations for variables of other types, an array declaration has two components: the array's type and the array's name. An array's type is written type[], where type is the data type of the elements contained within the array, and [] indicates that this is an array. Remember that all of the elements within an array are of the same type. The sample program uses int[], so the array called anArray will be used to hold integer data. Here are declarations for arrays that hold other types of data:

float[] anArrayOfFloats;
boolean[] anArrayOfBooleans;
Object[] anArrayOfObjects;
String[] anArrayOfStrings;

As with declarations for variables of other types, the declaration for an array variable does not allocate any memory to contain the array elements. The sample program must assign a value to anArray before the name refers to an array.

You create an array explicitly using Java's new operator. The next statement in the sample program allocates an array with enough memory for ten integer elements and assigns the array to the variable anArray declared earlier.

anArray = new int[10];  // create an array of integers

In general, when creating an array, you use the new operator, plus the data type of the array elements, plus the number of elements desired enclosed within square brackets ('[' and ']').

new elementType[arraySize]

If the new statement were omitted from the sample program, the compiler would print an error like the following one and compilation would fail.

ArrayDemo.java:4: Variable anArray may not have been initialized.

Now that some memory has been allocated for the array, the program assign values to the array elements:

for (int i = 0; i < anArray.length; i++) {
    anArray[i] = i;
    System.out.print(anArray[i] + " ");

}

This part of the code shows that to reference an array element, either to assign a value to it, or to access the value, you append square brackets to the array name. The value between the square brackets indicates (either with a variable or some other expression) the index of the element to access. Note that in Java, array indices begin at 0 and end at the array length minus 1.

To get the size of an array, you write

arrayname.length

Be careful: Programmers new to the Java programming language are tempted to follow length with an empty set of parenthesis. This doesn't work because length is not a method. length is a property provided by the Java platform for all arrays.

The for loop in our sample program iterates over each element of anArray, assigning values to its elements. The for loop uses anArray.length to determine when to terminate the loop.

The Java programming language provides a shortcut syntax for creating and initializing an array. Here's an example of this syntax:

boolean[] answers = { true, false, true, true, false };

The length of the array is determined by the number of values provided between { and }.

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