In this tutorial, we will see “How to find Duplicate Element in a Stream in Java 8?”. In order to find duplicates, we are going to use several techniques. Find Duplicate Elements in a Stream in Java 8...!!! Share on X
1. Find Duplicate Elements using ‘Set’
Set doesn’t allow duplicates and therefore return ‘false’ if the element is already present.
Example
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 | /** * Find Duplicate Elements in a Stream using Set * @author Deepak Verma * */ import java.util.*; import java.util.function.Predicate; import java.util.stream.*; public class Find_Duplicate_Element_In_Stream_Using_Set { public static void main(String[] args) { //Stream of Integers that may or may not contain Duplicate Elements Stream streamOfIntegers = Stream.of(2, 52, 5, 3, 21, 7, 99, 76, 21, 15, 34, 9, 21, 3, 81, 34, 1, 99); //Print the list of Duplicate Elements Preset in the above Stream System.out.println("Duplicate Elements preset in the Stream: "+ findDuplicateElementInStream(streamOfIntegers)); } public static Set findDuplicateElementInStream(Stream stream) { //Create Set to store the duplicate elements Set setOfElements = new HashSet(); //Set return false every time a newly added element is already present in the Set. And, then filter those duplicate elements Predicate filterDuplicateIntegers = x -> !setOfElements.add(x); //Apply the filter created above to the Original Stream and then Collect them as a new Set return stream.filter(filterDuplicateIntegers) .collect(Collectors.toSet()); } } |
1 | Duplicate Elements preset in the Stream: [34, 3, 99, 21] |
2. Find Duplicate Elements using ‘Collectors.groupingBy()’
Group the elements in a Map using Collectors.groupingBy and find elements if their occurrence is more than 1
Example
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 | /** * Find Duplicate Elements in a Stream using Collectors.groupingBy * @author Deepak Verma * */ import java.util.Map; import java.util.Set; import java.util.function.Function; import java.util.stream.Collectors; import java.util.stream.Stream; public class Find_Duplicate_Element_In_Stream_Using_Collectors_GroupingBy { public static void main(String[] args) { //Stream of Integers that may or may not contain Duplicate Elements Stream streamOfIntegers = Stream.of(2, 52, 5, 3, 21, 7, 99, 76, 21, 15, 34, 9, 21, 3, 81, 34, 1, 99); //Print the list of Duplicate Elements Preset in the above Stream System.out.println("Duplicate Elements preset in the Stream: "+ findDuplicateElementInStream(streamOfIntegers)); } public static Set findDuplicateElementInStream(Stream stream) { //return the set of duplicate elements return stream .collect(Collectors.groupingBy(Function.identity(),Collectors.counting())) //Create the map and group them with the frequency of their occurrence .entrySet().stream() //Convert the map to the stream .filter(m -> m.getValue() > 1) //Check if the occurrence of the element is more than 1 .map(Map.Entry::getKey).collect(Collectors.toSet()); //Find such elements and store them in a Set } } |
1 | Duplicate Elements preset in the Stream: [34, 99, 3, 21] |
3. Find Duplicate Elements using ‘Collections.frequency()’
Filter the elements from the List whose occurrence is more than 1 using Collections.frequency() and then count those elements
Example
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 | /** * Find Duplicate Elements in a Stream using Collections.frequency * @author Deepak Verma * */ import java.util.Arrays; import java.util.Collections; import java.util.List; import java.util.Set; import java.util.stream.Collectors; public class Find_Duplicate_Element_In_Stream_Using_Collections_Frequency { public static void main(String[] args) { //Create a list of elements var listOfIntegers = Arrays.asList(2, 52, 5, 3, 21, 7, 99, 76, 21, 15, 34, 9, 21, 3, 81, 34, 1, 99); //Print the list of Duplicate Elements System.out.println("Duplicate Elements preset in the Stream: "+ findDuplicateElementInStream(listOfIntegers)); } public static Set findDuplicateElementInStream(List list) { //return the set of duplicate elements return list.stream() //Convert the list into a Stream .filter(x -> Collections.frequency(list, x) > 1) //Filter the elements whose occurrence are greater than 1 and count those elements .collect(Collectors.toSet()); //Store those elements into a set } } |
1 | Duplicate Elements preset in the Stream: [34, 3, 99, 21] |
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