What does the method indexOf(obj o) do in java?

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

Example

import java.util.ArrayList;

public class ArrayListDemo {
   public static void main(String[] args) {
      ArrayList<String> arrlist = new ArrayList<String>(5);
      arrlist.add("G");
      arrlist.add("E");
      arrlist.add("F");
      arrlist.add("M");
      System.out.println("Size of list: " + arrlist.size());
      for (String value : arrlist) {
         System.out.println("Value = " + value);
      }
      int retval=arrlist.indexOf("E");
      System.out.println("The element E is at index " + retval);
   }
}

output

Size of list: 4
Value = G
Value = E
Value = F
Value = M
The element E is at index 1