Java ArrayList indexOf() Method example

Java.util.ArrayList class method indexOf(Object o) is used to find out the index of a particular element in a list.

Method indexOf() Signature

public int indexOf(Object o)

This method returns -1 if the specified element is not present in the list.

ArrayList indexOf() Method example

In the following example we have an arraylist of strings and we have added few elements to the arraylist. Here we are using the indexOf() method to find the index of few specified elements in the ArrayList.

import java.util.ArrayList;
public class IndexOfExample {
  public static void main(String[] args) {
      ArrayList<String> al = new ArrayList<String>();
      al.add("AB");
      al.add("CD");
      al.add("EF");
      al.add("GH");
      al.add("IJ");
      al.add("KL");
      al.add("MN");

      System.out.println("Index of 'AB': "+al.indexOf("AB"));
      System.out.println("Index of 'KL': "+al.indexOf("KL"));
      System.out.println("Index of 'AA': "+al.indexOf("AA"));
      System.out.println("Index of 'EF': "+al.indexOf("EF"));
  }
}

Output:

Index of 'AB': 0
Index of 'KL': 5
Index of 'AA': -1
Index of 'EF': 2

As you can see in the output that the index of element ‘AA’ is returned as -1 because this element does not exist in the ArrayList.

ArrayList indexOf() Method example if duplicate elements are present

In the following example, the element 90 is present twice in ArrayList, at the index 0 and index 3. When we used the indexOf() method to find the index of 90, we got the index of first occurrence of 90, this is because this method returns the index of first occurrence of the specified element. If you want to find out the last occurrence of the specified element use lastIndexOf() method instead.

import java.util.ArrayList;
public class JavaExample {
   public static void main(String[] args) {
      ArrayList<Integer> al = new ArrayList<Integer>();
      al.add(90);
      al.add(15);
      al.add(20);
      al.add(90);

      System.out.println("Index of 90: "+al.indexOf(90));
      System.out.println("Index of 20: "+al.indexOf(20));
      System.out.println("Index of 15: "+al.indexOf(15));
   }
}

Output:

Reference:

http://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html#indexOf(java.lang.Object)