Array

Arrays

---

This section was updated to reflect features and conventions of the upcoming Java SE 8 release. You can download the current JDK 8 snapshot from java.net.

---

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. You have seen an example of arrays already, in the main method of the "Hello World!" application. This section discusses arrays in greater detail.

An array of ten elements

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 would 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. You will learn about the various looping constructs (for, while, and do-while) in the Control Flow section.

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:

byte[] anArrayOfBytes;

short[] anArrayOfShorts;

long[] anArrayOfLongs;

float[] anArrayOfFloats;

double[] anArrayOfDoubles;

boolean[] anArrayOfBooleans;

char[] anArrayOfChars;

String[] anArrayOfStrings;

You can also place the square brackets after the array's name:

// this form is discouraged

float anArrayOfFloats[];

However, convention discourages this form; the brackets identify the array type and should appear with the type designation.

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. This is unlike arrays in C or Fortran. A consequence of this is that the rows are allowed to vary in length, as shown in the following MultiDimArrayDemo program:

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 the System.arraycopy() method 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

Array Manipulations

Arrays are a powerful and useful concept used in programming. Java SE provides methods to perform some of the most common manipulations related to arrays. For instance, the ArrayCopyDemo example uses the arraycopy() method of the System class instead of manually iterating through the elements of the source array and placing each one into the destination array. This is performed behind the scenes, leaving the developer with the need to use just one line of code to call the method.
For your convenience, Java SE provides a number of methods for performing array manipulations (common tasks, such as copying, sorting and searching arrays) in the java.util.Arrays class. For instance, the previous example can be modified to use the CopyOfRange() method of the java.util.Arrays class, as you can see in the ArrayCopyOfDemo example. The difference is that it does not require you to create the destination array before calling the method, because the destination array is returned by the method:

class ArrayCopyOfDemo {

    public static void main(String[] args) {

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

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

        char[] copyTo = java.util.Arrays.copyOfRange(copyFrom, 2, 9);

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

    }

}

As you can see, the output from this program is the same (caffein), although it requires even less lines of code.
Some other useful operations provided by methods in the java.util.Arrays class, are:

• Searching an array for a specific value to get the index at which it is placed (the binarySearch() method).
• Comparing two arrays to determine if they are equal or not (the equals() method).
• Filling an array to place a specific value at each index (the fill() method).
• Sorting an array into ascending order. This can be done either sequentially, using the sort() method, or concurrently, using the parallelSort() method introduced in Java SE 8. Parallel sorting of large arrays on multi-processor systems is faster than sequential array sorting.

reference :

http://docs.oracle.com/javase/tutorial/java/nutsandbolts/arrays.html