Java Tutorial 7: Variables-array

An array is a container object that holds a fixed number of values of a single type. The length of an array is established when the array is created. After creation, its length is fixed.

Each item in an array is called an element, and each element is accessed by its numerical index. As shown in the above illustration, numbering begins with 0. The 9th element, for example, would therefore be accessed at index 8.

The following program, ArrayDemo, creates an array of integers, puts some values in it, and prints each value to standard output.

class ArrayDemo {

    public static void main(String[] args) {

        // declares an array of integers

        int[] anArray;

        // allocates memory for 10 integers

        anArray = new int[10];

          

        // initialize first element

        anArray[0] = 100;

        // initialize second element

        anArray[1] = 200;

        // etc.

        anArray[2] = 300;

        anArray[3] = 400;

        anArray[4] = 500;

        anArray[5] = 600;

        anArray[6] = 700;

        anArray[7] = 800;

        anArray[8] = 900;

        anArray[9] = 1000;

        System.out.println("Element at index 0: "

                           + anArray[0]);

        System.out.println("Element at index 1: "

                           + anArray[1]);

        System.out.println("Element at index 2: "

                           + anArray[2]);

        System.out.println("Element at index 3: "

                           + anArray[3]);

        System.out.println("Element at index 4: "

                           + anArray[4]);

        System.out.println("Element at index 5: "

                           + anArray[5]);

        System.out.println("Element at index 6: "

                           + anArray[6]);

        System.out.println("Element at index 7: "

                           + anArray[7]);

        System.out.println("Element at index 8: "

                           + anArray[8]);

        System.out.println("Element at index 9: "

                           + anArray[9]);

    }

}

The output from this program is:

Element at index 0: 100

Element at index 1: 200

Element at index 2: 300

Element at index 3: 400

Element at index 4: 500

Element at index 5: 600

Element at index 6: 700

Element at index 7: 800

Element at index 8: 900

Element at index 9: 1000

In a real-world programming situation, you'd probably use one of the supported looping constructs to iterate through each element of the array, rather than write each line individually as shown above. However, this example clearly illustrates the array syntax.

Declaring a Variable to Refer to an Array

The above program declares anArray with the following line of code:

// declares an array of integers

int[] anArray;

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 as type[], where type is the data type of the contained elements; the square brackets are special symbols indicating that this variable holds an array. The size of the array is not part of its type (which is why the brackets are empty). An array's name can be anything you want, provided that it follows the rules and conventions as previously discussed in the naming section. As with variables of other types, the declaration does not actually create an array — it simply tells the compiler that this variable will hold an array of the specified type.

Similarly, you can declare arrays of other types:

double[] anArrayOfDoubles;

boolean[] anArrayOfBooleans;

String[] anArrayOfStrings;

Creating, Initializing, and Accessing an Array

One way to create an array is with the new operator. The next statement in the ArrayDemo program allocates an array with enough memory for ten integer elements and assigns the array to the anArray variable.

// create an array of integers

anArray = new int[10];

If this statement were missing, the compiler would print an error like the following, and compilation would fail:

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

The next few lines assign values to each element of the array:

anArray[0] = 100; // initialize first element

anArray[1] = 200; // initialize second element

anArray[2] = 300; // etc.

Each array element is accessed by its numerical index:

System.out.println("Element 1 at index 0: " + anArray[0]);

System.out.println("Element 2 at index 1: " + anArray[1]);

System.out.println("Element 3 at index 2: " + anArray[2]);

Alternatively, you can use the shortcut syntax to create and initialize an array:

int[] anArray = {

    100, 200, 300,

    400, 500, 600,

    700, 800, 900, 1000

};

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

You can also declare an array of arrays (also known as a multidimensional array) by using two or more sets of square brackets, such as String[][] names. Each element, therefore, must be accessed by a corresponding number of index values.

In the Java programming language, a multidimensional array is simply an array whose components are themselves arrays. A consequence of this is that the rows are allowed to vary in length, as shown in the following MultiDimArrayDemoprogram:

class MultiDimArrayDemo {

    public static void main(String[] args) {

        String[][] names = {

            {"Mr. ", "Mrs. ", "Ms. "},

            {"Smith", "Jones"}

        };

        // Mr. Smith

        System.out.println(names[0][0] + names[1][0]);

        // Ms. Jones

        System.out.println(names[0][2] + names[1][1]);

    }

}

The output from this program is:

Mr. Smith

Ms. Jones

Finally, you can use the built-in length property to determine the size of any array. The code

 System.out.println(anArray.length);

will print the array's size to standard output.

Copying Arrays

The System class has an arraycopy method that you can use to efficiently copy data from one array into another:

public static void arraycopy(Object src, int srcPos,

                             Object dest, int destPos, int length)

The two Object arguments specify the array to copy from and the array to copy to. The three int arguments specify the starting position in the source array, the starting position in the destination array, and the number of array elements to copy.

The following program, ArrayCopyDemo, declares an array of char elements, spelling the word "decaffeinated". It uses arraycopy to copy a subsequence of array components into a second array:

class ArrayCopyDemo {

    public static void main(String[] args) {

        char[] copyFrom = { 'd', 'e', 'c', 'a', 'f', 'f', 'e',

                                                    'i', 'n', 'a', 't', 'e', 'd' };

        char[] copyTo = new char[7];

        System.arraycopy(copyFrom, 2, copyTo, 0, 7);

        System.out.println(new String(copyTo));

    }

}

The output from this program is:

caffein

Summary of Variables

The Java programming language uses both "fields" and "variables" as part of its terminology. Instance variables (non-static fields) are unique to each instance of a class. Class variables (static fields) are fields declared with the static modifier; there is exactly one copy of a class variable, regardless of how many times the class has been instantiated. Local variables store temporary state inside a method. Parameters are variables that provide extra information to a method; both local variables and parameters are always classified as "variables" (not "fields"). When naming your fields or variables, there are rules and conventions that you  must follow.

The eight primitive data types are: int, double, boolean, byte, short, long, float, and char. The java.lang.String class represents character strings. A literal is the source code representation of a fixed value. An array is a container object that holds a fixed number of values of a single type. The length of an array is established when the array is created. After creation, its length is fixed.

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