The indexOfSubList() method of java.util.Collections class is used to return the starting position of the first occurrence of the specified target list within the specified source list, or -1 if there is no such occurrence. More formally, returns the lowest index i such that source.subList(i, i+target.size()).equals(target), or -1 if there is no such index. (Returns -1 if target.size() > source.size().)

This implementation uses the “brute force” technique of scanning over the source list, looking for a match with the target at each location in turn.

Syntax:

public static int indexOfSubList(List source, List target)

Parameters: This method takes following argument as parameter

• source – the list in which to search for the first occurrence of target.
• target – the list to search for as a subList of source.

  Return Value: This method returns the starting position of the first occurrence of the specified target list within the specified source list, or -1 if there is no such occurrence.

  
  
  
  

  Below are the examples to illustrate the indexOfSubList() method

  Example 1:

  
  

  
  
  

  // Java program to demonstrate // indexOfSubList() method // for String value    import java.util.*;    public class GFG1 {     public static void main(String[] argv)         throws Exception     {         try {                // creating object of List<String>             List<String> arrlistsrc = new ArrayList<String>();             List<String> arrlisttarget = new ArrayList<String>();                // Adding element to arrlistsrc             arrlistsrc.add("A");             arrlistsrc.add("B");             arrlistsrc.add("C");             arrlistsrc.add("D");             arrlistsrc.add("E");                // Adding element to arrlisttarget             arrlisttarget.add("C");             arrlisttarget.add("D");             arrlisttarget.add("E");                // print the source list             System.out.println("Source list: " + arrlistsrc);                // print the target list             System.out.println("Target list: " + arrlisttarget);                // check target list in source list             int index = Collections                             .indexOfSubList(arrlistsrc,                                             arrlisttarget);                // print the index             System.out.println("Target list starts at index: "                                + index);         }            catch (IllegalArgumentException e) {             System.out.println("Exception thrown : " + e);         }     } }
  Output:

  Source list: [A, B, C, D, E]
  Target list: [C, D, E]
  Target list starts at index: 2
  

  Example 2:

  
  

  
  
  

  // Java program to demonstrate // indexOfSubList() method // for Integer value    import java.util.*;    public class GFG1 {     public static void main(String[] argv)         throws Exception     {         try {                // creating object of List<Integer>             List<Integer> arrlistsrc = new ArrayList<Integer>();             List<Integer> arrlisttarget = new ArrayList<Integer>();                // Adding element to arrlistsrc             arrlistsrc.add(20);             arrlistsrc.add(30);             arrlistsrc.add(40);             arrlistsrc.add(50);             arrlistsrc.add(60);                // Adding element to arrlisttarget             arrlisttarget.add(40);             arrlisttarget.add(50);                // print the source list             System.out.println("Source list: " + arrlistsrc);                // print the target list             System.out.println("Target list: " + arrlisttarget);                // check target list in source list             int index = Collections                             .indexOfSubList(arrlistsrc,                                             arrlisttarget);                // print the index             System.out.println("Target list starts at index: "                                + index);         }            catch (IllegalArgumentException e) {             System.out.println("Exception thrown : " + e);         }     } }
  Output:

  Source list: [20, 30, 40, 50, 60]
  Target list: [40, 50]
  Target list starts at index: 2
  

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