CopyOnWriteArrayList indexOf() method in Java

• The indexOf(Object o) method of CopyOnWriteArrayList returns the first occurrence of the element passed in the list. It returns -1 if the element is not present in the list.

  Syntax:

  public int indexOf(Object o)

  Parameters: The function accepts a parameter o whose first occurrence is to be returned.

  Return Value: The function returns the first occurrence of the element. It returns -1 if the element is not present in the list.

  Below programs illustrate the above function:

  Program 1:

  // Java Program to illustrate the CopyOnWriteArrayList
  // indexOf() method in Java
  import java.util.concurrent.CopyOnWriteArrayList;
  import java.util.*;
  
  public class GFG {
      public static void main(String[] args)
      {
  
          // create object of CopyOnWriteArrayList
          CopyOnWriteArrayList<Integer> ArrLis
              = new CopyOnWriteArrayList<Integer>();
  
          // Add elements
          ArrLis.add(32);
          ArrLis.add(67);
          ArrLis.add(67);
          ArrLis.add(100);
  
          // print CopyOnWriteArrayList
          System.out.println("CopyOnWriteArrayList: " + ArrLis);
  
          // first occurrence of 67
          System.out.println("indexOf value: " + ArrLis.indexOf(67));
      }
  }
  Output:

  CopyOnWriteArrayList: [32, 67, 67, 100]
  indexOf value: 1
  

  Program 2:

  // Java Program to illustrate the CopyOnWriteArrayList
  // indexOf() method in Java
  import java.util.concurrent.CopyOnWriteArrayList;
  import java.util.*;
  
  public class GFG {
      public static void main(String[] args)
      {
  
          // create object of CopyOnWriteArrayList
          CopyOnWriteArrayList<Integer> ArrLis
              = new CopyOnWriteArrayList<Integer>();
  
          // Add elements
          ArrLis.add(32);
          ArrLis.add(67);
          ArrLis.add(67);
          ArrLis.add(100);
  
          // print CopyOnWriteArrayList
          System.out.println("CopyOnWriteArrayList: "
                                           + ArrLis);
  
          // 200 is not present
          System.out.println("indexOf value: "
                           + ArrLis.indexOf(200));
      }
  }
  Output:

  CopyOnWriteArrayList: [32, 67, 67, 100]
  indexOf value: -1
  

• The indexOf(E e, int index) method of CopyOnWriteArrayList returns the first occurrence of the element passed in the list after position index. It returns -1 if the element is not present in the list.

  Syntax:

  public int indexOf(E e, int index)

  Parameters: The function accepts two parameters which are described below:

  • index: specifies the index from which the occurrence is to be searched.
  • e: specifies the element whose first occurrence from position index is to be returned.

  Return Value: The function returns the first occurrence of the element after position index. It returns -1 if the element is not present in the list.

  Exceptions: The function throws an IndexOutOfBoundsException if the specified index is negative.

  Below programs illustrate the above function:

  Program 1:

  // Java Program to illustrate the CopyOnWriteArrayList
  // indexOf() method in Java
  import java.util.concurrent.CopyOnWriteArrayList;
  import java.util.*;
  
  public class GFG {
      public static void main(String[] args)
      {
  
          // create object of CopyOnWriteArrayList
          CopyOnWriteArrayList<Integer> ArrLis
              = new CopyOnWriteArrayList<Integer>();
  
          // Add elements
          ArrLis.add(32);
          ArrLis.add(67);
          ArrLis.add(67);
          ArrLis.add(67);
  
          // print CopyOnWriteArrayList
          System.out.println("CopyOnWriteArrayList: "
                                             + ArrLis);
  
          // first occurrence of 67 from 2nd index
          System.out.println("indexOf value: "
                            + ArrLis.indexOf(67, 2));
      }
  }
  Output:

  CopyOnWriteArrayList: [32, 67, 67, 67]
  indexOf value: 2
  

  Program 2:

  // Java Program to illustrate the CopyOnWriteArrayList
  // indexOf() method in Java
  import java.util.concurrent.CopyOnWriteArrayList;
  import java.util.*;
  
  public class GFG {
      public static void main(String[] args)
      {
  
          // create object of CopyOnWriteArrayList
          CopyOnWriteArrayList<Integer> ArrLis
              = new CopyOnWriteArrayList<Integer>();
  
          // Add elements
          ArrLis.add(32);
          ArrLis.add(67);
          ArrLis.add(67);
          ArrLis.add(100);
  
          // print CopyOnWriteArrayList
          System.out.println("CopyOnWriteArrayList: "
                                           + ArrLis);
  
          // -1 is out of range, hence exception
          System.out.println("indexOf value: "
                        + ArrLis.indexOf(-1, 200));
      }
  }

  Output:

  Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: -1
      at java.util.concurrent.CopyOnWriteArrayList.indexOf(CopyOnWriteArrayList.java:198)
      at java.util.concurrent.CopyOnWriteArrayList.indexOf(CopyOnWriteArrayList.java:263)
      at GFG.main(GFG.java:24)

Reference: https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/CopyOnWriteArrayList.html#indexOf-E-int-