Java.util.Arraylist.indexOf() in Java

The indexOf() method of ArrayList returns the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element.

Syntax :

public int IndexOf(Object o)
obj : The element to search for.

// Java code to demonstrate the working of
// indexOf in ArrayList

// for ArrayList functions
import java.util.ArrayList;

public class IndexOfEx {
  public static void main(String[] args) {

  // creating an Empty Integer ArrayList
  ArrayList<Integer> arr = new ArrayList<Integer>(5);

  // using add() to initialize values
  arr.add(1);
  arr.add(2);
  arr.add(3);
  arr.add(4);

  // printing initial value
  System.out.print("The initial values in ArrayList are : ");
  for (Integer value : arr) {
  System.out.print(value);
  System.out.print(" ");
  }

  // using indexOf() to find index of 3
  int pos =arr.indexOf(3);

  // prints 2
  System.out.println("\nThe element 3 is at index : " + pos);
  }

}

Output:

The initial values in ArrayList are : 1 2 3 4 
The element 3 is at index : 2

Practical Application : The index functions are mostly useful to determine last or first occurrence of events, for example last occurrence of 6 in a throw of a die, or 1st occurrence of any letter in a name etc.

One more example:

// Java code to demonstrate the application of
// index functions in ArrayList

// for ArrayList functions
import java.util.ArrayList;

public class AppliIndex {
  public static void main(String[] args) {

  // creating an Empty Integer ArrayList
  ArrayList<Integer> arr = new ArrayList<Integer>(10);

  // using add() to initialize dice values
  arr.add(1);
  arr.add(2);
  arr.add(4);
  arr.add(6);
  arr.add(5);
  arr.add(2);
  arr.add(6);
  arr.add(1);
  arr.add(6);
  arr.add(4);

  // using IndexOf() to find first index of 6
  int pos1 =arr.indexOf(6);

  // using lastIndexOf() to find last index of 6
  int pos2 =arr.lastIndexOf(6);

  // to balance 0 based indexing
  pos1 = pos1+1;
  pos2 = pos2+1;

  // printing first index of 6
  System.out.println("The first occurrence of 6 is  : " + pos1);

  // printing last index of 6
  System.out.println("The last occurrence of 6 is  : " + pos2);

  }

}

Output:

The first occurrence of 6 is  : 4
The last occurrence of 6 is  : 9

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