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.

Creating  an Array

Two steps procedure

1)  declares a reference to array of integers

type array_name [];                                           //type is the datatype of the array.
type []  array_name;   

int[] anArray;          or    int anArray[];         

2) create array referenced by this variable (allocate memory)

array_name = new type[size];      

    anArray = new int[5];     // all elements have 0 value

int[] anArray ={8,12,56,43,8};

Access the Elements of an Array 

You access an array element by referring to the index number [0 .. n).

public class Arr {
    public static void main(String arg[]) {
        int[] anArray ={8,12,56,43,8};
        System.out.println("Element [3]: "+anArray[3]); // the fourth  element in the array
    }
}

variable length - public data member, gives the number of elements in the array

anArray.length

public class Arr {
    public static void main(String arg[]) {
        int[] anArray ={8,12,56,43,8};
        System.out.println("Element [last]: "+anArray[anArray.length-1]); // the last element in the array
    }
}

Change an Array Element

public class Arr {
    public static void main(String arg[]) {
        int[] anArray ={8,12,56,43,8};
        System.out.println("Element [3]: "+anArray[3]); // the fourth  element in the array
        anArray[3]=84;
        System.out.println("new value - Element [3]: "+anArray[3]); // the fourth  element in the array
    }
}

Multi-dimensional Arrays

In additions to one-dimensional arrays, you can create multi-dimensional arrays. To declare two-dimensional arrays, you need to specify multiple square brackets after the array name.

Syntax to declare a two dimensional array
type array_name[][] = new type[rows][cols];

e.g.,
int multidim[][] = new int[3][6];   // crreate an array with 3*6 = 18 zero elements

In a two-dimensional array,

   1. You need to allocate memory for only the first dimension.
   2. You can allocate the remaining dimensions separately.
   3. When you allocate memory to the second dimension, you can also allocate different number to each dimension.

e.g.,

new int[3][];  // three lines       
multidim[0] = new int[2];         // first line - 2 elements
multidim[1] = new int[4];         // second line -4 elements
multidim[1] = new int[3];         // third line -3 elements

public class Arr2 {
    public static void main(String[] args) {     // creating an Array with 2+4+3=9 elements with value 0
        int multidim[][] = new int[3][];  // three lines
        multidim[0] = new int[2];         // first line - 2 elements
        multidim[1] = new int[4];         // second line -4 elements
        multidim[1] = new int[3];         // third line -3 elements
    }
}

initiation

public class Arr2_2 {
    public static void main(String[] args) {     // creating an Array with 2+4+3=9 elements with value 0
        int multidim[][] = { {2,54}, {4,3,1,5},{34,12,4}};  //  // first line - 2 elements, second - 4, third - 3
    }
}        

variable length   

multidim.length - number of lines in the array
multidim[0].length  - number of elements in the first line in the array
multidim[i].length -  number of elements in the line number i+1 in the array

Accessing Arrays

You need to access various elements of an array to assign, retrieve, and manipulate the values stored in the array.
Assigning values to the Elements of an Array

To access a specific array element:

   1. You need to specify the name of the array and the index number of the element.
   2. The index position of the first element in the array is 0.
   3. In multidimensional array you need to give an index for each dimension starting with index 0.
 

public class Arr3 {
    public static void main(String[] args) {     // creating an Array with 2+4+3=9 elements with value 0
        int multidim[][][] = { {{2,54}, {4,3,1,5}},{{34,12,4},{34,21},{1,9,5}}}; 
        System.out.println (multidim[0][0][1]);
    }
}   

ArrayIndexOutOfBoundsException

Traversing a one-dimensional array

    simple for loop

    for-each loop

for (<type>  <variable_name> : <collection>)
{
        // for body
}

The for-each loop is used to traverse arrays or other collections in Java. It is easier to use than a simple "for loop" because we don't need to increment the value and use subscript notation.

Traverse a bi-dimensional array 

    nested simple for loop

    for-each loop

The object's references are put in the array.  The constructors must be used to create the objects.

Java array having instances of different classes (base and derived classes)

arracopy() method -

public static void arraycopy(source_arr, sourcePos,
                            dest_arr, destPos, len)
Parameters : 
source_arr : array to be copied from
sourcePos : starting position in source array from where to copy
dest_arr : array to be copied in
destPos : starting position in destination array, where to copy in
len : total no. of components to be copied.

Attention: The method copies the references, so the references in both arrays point to the same objects in the system heap!