• 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-

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