Creating and Using Arrays - Java Tutorial 5.0 英文版

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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();
    }
}

The output from this program is:

0 1 2 3 4 5 6 7 8 9

This section covers these topics:

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

Declaring a Variable to Refer to an Array

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;

You can write an array declaration like this:

float anArrayOfFloats[]; //this form is discouraged

However, convention discourages this form because the brackets identify the array type, and so they should appear with the type designation, not with the array name.

As with declarations for variables of other types, the declaration for an array variable does not create an array and does not allocate any memory to contain array elements. The code must create the array explicitly and assign it to anArray.

Creating 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 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.

Array Initializers

You can use a shortcut syntax for creating and initializing an array. Here's an example:

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

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

Accessing an Array Element

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 refer to an array element, either to assign a value to it or to get its value, you append brackets to the array name. The value between the brackets indicates (with a variable or other expression) the index of the element to access.

Getting the Size of an Array

To get the size of an array, you write

arrayname.length

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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.

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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.

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