Find duplicate elements from the given a list using Java 8 Stream

       In this post, we need to have a look one of the important coding interview questions asked for java 8 streams. Given list contains either String or Integers. Refer all coding questions here

1) Print duplicate elements using frequency and toSet methods    . 

      In the below code used Collections.frequency to filter the data with occurance value greater than 1 and toSet method used to remove the duplicated in output.

import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.Set;
import java.util.stream.Collectors;

public class Java8StreamEx {

    public static void main(String[] args) {

	 List<Integer> integerList = Arrays.asList(2, 3, 3, 5, 6, 6, 6);

	 Set<Integer> list = integerList.stream().filter(
			        s -> Collections.frequency(integerList, s) > 1)

                                  .collect(Collectors.toSet());
		
	 list.stream().forEach(s -> System.out.println("Duplicate element - "+s));
     }
}

Output:- Duplicate element - 3

              Duplicate element - 6

2) Using frequency and toMap method(map used to store element with occurance)

Used frequency and toMap method to store duplicates into the Map with occurance as a value.

import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;

public class Java8StreamEx {

    public static void main(String[] args) {

	 List<Integer> integerList = Arrays.asList(2, 3, 3, 5, 6, 6, 6);

		// store duplicate into the map
	  Map<Integer, Long> map = 

                         integerList.stream().filter(
	                    s -> Collections.frequency(integerList, s) > 1).collect(
			    Collectors.toMap(Function.identity(), v -> 1L, Long::sum));

		map.entrySet().stream().forEach(s -> System.out.println(s));
	}
}

Output: - 3=2

               6=3

3) Using groupingBy and counting methods - group the similar elements and to find the occarance used counting.

To write the code to find the duplicate using groupingBy,

import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;

public class Java8StreamEx {

     public static void main(String[] args) {

          List<String> listOfString = Arrays.asList("A", "B", "C", "A", "A", "D", "B");

	  Map<String, Long> mapOfStrings = 
			listOfString.stream().filter(
			    s -> Collections.frequency(listOfString, s) > 1)
			    .collect(Collectors.groupingBy(Function.identity(), 
			        Collectors.counting()));

	   mapOfStrings.entrySet().stream().forEach(s -> System.out.println(s));
    }
}

Output:-A=3

             B=2

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