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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