Java Data Type Tutorial - Java Data Type

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We can refer to each individual element of the array using an element's index enclosed in brackets.

The index for the first element is 0, the second element 1, the third element 2, and so on.

This is called zero-based indexing. The index for the last element of an array is the length of the array minus 1.

If we have an array of length 5, the indexes of the array elements would be 0, 1, 2, 3, and 4.

It is a runtime error to refer to a non-existing element of an array.

For example, using myID[5] in the code will throw an exception, because myID has a length of 5 and myID[5] refers to the sixth element, which is non-existent.

We can assign values to elements of an array as follows:

myID[0]   = 10;    // Assign  10  to the   first element of  myID 
myID[1]   = 20;    // Assign  20  to the   second  element of  myID 
myID[2]   = 30;    // Assign  30  to the   third  element of  myID 
myID[3]   = 40;    // Assign  40  to the   fourth  element of  myID 
myID[4]   = 50;    // Assign  50  to the   fifth element of  myID

The following statement assigns the value of the third element of the myID array to an int variable temp:

int temp  = myID[2]; // Assigns 30  to temp

Length of an Array

An array object has a public final instance variable named length, which contains the number of elements in the array.

int[] myID = new int[5]; // Create an  array of  length 5 
int len   = myID.length;      // 5  will be  assigned to len

length is the property of the array object. Until we create the array object, we cannot use its length property.

Typically, elements of arrays are accessed using loops.

for (int i = 0 ; i < myID.length; i++)   {
    myID[i] = (i + 1)  *  10;
}

Initializing Array Elements

Array elements are always initialized.

Array elements of primitive data type are initialized to the default value for their data types.

For example, the numeric array elements are initialized to zero, boolean elements to false, and the reference type elements to null.

The following code illustrates the array initialization:

intArray[0], intArray[1] and intArray[2] are initialized to zero by default.

int[]  intArray = new int[3];

bArray[0] and bArray[1] are initialized to false.

boolean[] bArray  = new boolean[2];

An example of a reference type array. strArray[0] and strArray[1] are initialized to null.

String[]  strArray = new String[2]

All 100 elements of the person array are initialized to null.

Person[]  person = new Person[100];

The following illustrates the array initialization for an instance variable and some local variables.

public class Main {
  private boolean[] bArray = new boolean[3]; // An instance variable
/* www  .j  a  v a  2  s  . c  o  m*/
  public Main() {
    for (int i = 0; i < bArray.length; i++) {
      System.out.println("bArray[" + i + "]:" + bArray[i]);
    }
  }

  public static void main(String[] args) {
    int[] myID = new int[3]; // A local array variable
    for (int i = 0; i < myID.length; i++) {
      System.out.println("myID[" + i + "]:" + myID[i]);
    }
    String[] name = new String[3]; // A local array variable
    for (int i = 0; i < name.length; i++) {
      System.out.println("name[" + i + "]:" + name[i]);
    }
  }
}

The code above generates the following result.

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