Array Initialisation

When an array is created, every element is by default is initialied with default values

Array initialisation representation in memory
Array initialisation representation in memory 
import java.lang.*;

class Test {
    public static void main(String[] args) {
        int[] a = new int[3];

        System.out.println(a); // [I@379619aa
        System.out.println(a[0]); // 0
    }
}

When printing any reference variable, internally, toString() method will be called, which is implemented by default to return the string in the following form: classname@hashcode_in_dexadecimal_form

import java.lang.*;
class Test {
    public static void main(String[] args) {
        int[][] a = new int[2][3];

        System.out.println(a); // [[I@581619ea
        System.out.println(a[0]); // [I@721639ee
        System.out.println(a[0][0]); // 0
    }
}

In the example above, the value of System.out.println(a[0]); is a reference to another array, since this is a 2d array, and as discussed before, this is the behavior of arrays of array style. 

import java.lang.*;
class Test {
    public static void main(String[] args) {
        int[][] a = new int[2][];

        System.out.println(a); // [[I@383619ea
        System.out.println(a[0]); // null
        System.out.println(a[0][0]); // Runtime Exception: Nullpointer Exception
    }
}

In the example above, only the base was specified, meaning the size of the array (a[0]) being referenced is unknown, hence it is null. Since it is null, the value of a[0][0] throws a Nullpointer exception because the array does not exist.

When trying to perform any operation on null, there will be a null pointer runtime exception. 

import java.lang.*;

class Test {
    public static void main(String[] args) {
        int[] a = new int[6];
        a[0] = 10;
        a[1] = 20;
        a[2] = 30;
        a[3] = 40;
        a[4] = 50;
        a[5] = 60;
        a[6] = 70; // INVALID: RuntimeException - ArrayIndexOutOfBoundsException
        a[-6] = 80; // INVALID: RuntimeException - ArrayIndexOutOfBoundsException
        a[2.5] = 90; // INVALID: Compile time error - Possible loss of precision, found double, required int
    }
}

Once an array is created, every array element is by default initialised with default values. These default values can be overidden by custom values The code snippet above shows an array overiding default values with custom values.

When trying to access a array element with out of range index (either positive or negative int value) then a RuntimeException will be thrown with the ArrayIndexOutOfBoundsException.