In this article, we will discuss how to find and count duplicate values in a Map or HashMap

Find & Count duplicate values in a HashMap :

We will discuss 2 different approaches –

Using Java 8 Stream
Before Java 8

1. Using Java 8 Stream :

Iterate through values of Map entries using Stream and then collect it to another Map (either HashMap or LinkedHashMap or TreeMap) with,
- Key as Function Identity
- Value as Duplicate Count of Map Values
Finally, print duplicate count more than 2 to the console

GetDuplicateCountOfMapValuesUsingJava8Stream.java

package in.bench.resources.java.map;

import java.util.HashMap;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;

public class GetDuplicateCountOfMapValuesUsingJava8Stream {

	public static void main(String[] args) {

		// 1. create HashMap object
		Map<String, Double> playerPercentile = new HashMap<>();


		// 1.1 add map entries
		playerPercentile.put("Morgan", 99.67);
		playerPercentile.put("Stewart", 90.98);
		playerPercentile.put("Mike", 93.65);
		playerPercentile.put("Cook", 91.67);
		playerPercentile.put("Stokes", 92.35);
		playerPercentile.put("Broad", 99.67);
		playerPercentile.put("Pietersen", 92.35);
		playerPercentile.put("Hussain", 94.89);
		playerPercentile.put("Collingwood", 92.35);
		playerPercentile.put("Flintoff", 93.65);
		playerPercentile.put("Strauss", 91.67);
		playerPercentile.put("Butler", 99.67);


		// 1.2 print original Map entries
		System.out.println("1. Original Map Entries :- \n");


		// 1.3 print Map entries to console
		playerPercentile.forEach((key, value) -> System.out.println(
				"Percentile : "  + value  + "\t"  + "Player : " + key
				));



		// 2. Player's percentile and its duplicate count
		Map<Double, Long> playersPercentileCount = playerPercentile
				.values()
				.stream()
				.collect(
						Collectors.groupingBy(Function.identity(), 
								HashMap::new, 
								Collectors.counting())
						);


		// 2.1 print original Map entries
		System.out.println("\n\n2. Player's percentile & duplicate count :- \n");


		// 2.2 print Map entries to console
		playersPercentileCount.forEach((key, value) -> System.out.println(
				"Percentile : " + key  + "\t"  + "Count : "  + value
				));



		// 3. print duplicate count of more than 2
		System.out.println("\n\n3. Percentile count more than 2 :- \n");
		playersPercentileCount
		.entrySet()
		.stream()
		.filter(entry -> entry.getValue() > 2)
		.forEach(System.out::println);
	}
}

Output :

1. Original Map Entries :- 

Percentile : 99.67	Player : Morgan
Percentile : 93.65	Player : Mike
Percentile : 93.65	Player : Flintoff
Percentile : 92.35	Player : Stokes
Percentile : 92.35	Player : Collingwood
Percentile : 99.67	Player : Broad
Percentile : 91.67	Player : Cook
Percentile : 94.89	Player : Hussain
Percentile : 92.35	Player : Pietersen
Percentile : 90.98	Player : Stewart
Percentile : 91.67	Player : Strauss
Percentile : 99.67	Player : Butler


2. Player's percentile & duplicate count :- 

Percentile : 92.35	Count : 3
Percentile : 91.67	Count : 2
Percentile : 94.89	Count : 1
Percentile : 90.98	Count : 1
Percentile : 99.67	Count : 3
Percentile : 93.65	Count : 2


3. Percentile count more than 2 :- 

92.35=3
99.67=3

1.1 Duplicate Count of more than 2 :

If the requirement is to print duplicate count of Map values more than 2 in a single line then above example’s code can be reduced to single expression as illustrated below,

playerPercentile
		.values()
		.stream()
		.collect(
				Collectors.groupingBy(Function.identity(), 
						HashMap::new, 
						Collectors.counting())
				)
		.entrySet()
		.stream()
		.filter(entry -> entry.getValue() > 2)
		.forEach(System.out::println);

2. Before Java 8 :

Iterate through values of Map entries using enhanced for-loop and then collect it to another Map with,
- Key as Duplicate Value
- Value as Duplicate Count of Map Values
Finally, print duplicate count more than 2 using enhanced for-loop & if-condition to the console

GetDuplicateCountOfMapValues.java

package in.bench.resources.java.map;

import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;

public class GetDuplicateCountOfMapValues {

	public static void main(String[] args) {

		// 1. create HashMap object
		Map<String, Double> playerPercentile = new HashMap<>();


		// 1.1 add map entries
		playerPercentile.put("Morgan", 99.67);
		playerPercentile.put("Stewart", 90.98);
		playerPercentile.put("Mike", 93.65);
		playerPercentile.put("Cook", 91.67);
		playerPercentile.put("Stokes", 92.35);
		playerPercentile.put("Broad", 99.67);
		playerPercentile.put("Pietersen", 92.35);
		playerPercentile.put("Hussain", 94.89);
		playerPercentile.put("Collingwood", 92.35);
		playerPercentile.put("Flintoff", 93.65);
		playerPercentile.put("Strauss", 91.67);
		playerPercentile.put("Butler", 99.67);


		// 1.2 print original Map entries
		System.out.println("1. Original Map Entries :- \n");


		// 1.3 print Map entries to console
		playerPercentile.forEach((key, value) -> System.out.println(
				"Percentile : "  + value  + "\t"  + "Player : " + key
				));



		// 2. Player's percentile and its duplicate count
		Map<Double, Integer> playersPercentileCount = new HashMap<>();


		// 2.1 iterate through Map entries and get duplicate values count
		for(Map.Entry<String, Double> entry : playerPercentile.entrySet()) {

			if(playersPercentileCount.containsKey(entry.getValue())) {
				playersPercentileCount.put(
						entry.getValue(), (playersPercentileCount.get(entry.getValue()) + 1)
						);
			}
			else {
				playersPercentileCount.put(entry.getValue(), 1);
			}
		}


		// 2.1 print original Map entries
		System.out.println("\n\n2. Player's percentile & duplicate count :- \n");


		// 2.2 print Map entries to console
		playersPercentileCount.forEach((key, value) -> System.out.println(
				"Percentile : " + key  + "\t"  + "Count : "  + value
				));



		// 3. print duplicate count of more than 2
		System.out.println("\n\n3. Percentile count more than 2 :- \n");


		// 3.1 print to console
		for(Entry<Double, Integer> entry : playersPercentileCount.entrySet()) {

			if(entry.getValue() > 2) {
				System.out.println(entry);
			}
		}
	}
}

Output :

1. Original Map Entries :- 

Percentile : 99.67	Player : Morgan
Percentile : 93.65	Player : Mike
Percentile : 93.65	Player : Flintoff
Percentile : 92.35	Player : Stokes
Percentile : 92.35	Player : Collingwood
Percentile : 99.67	Player : Broad
Percentile : 91.67	Player : Cook
Percentile : 94.89	Player : Hussain
Percentile : 92.35	Player : Pietersen
Percentile : 90.98	Player : Stewart
Percentile : 91.67	Player : Strauss
Percentile : 99.67	Player : Butler


2. Player's percentile & duplicate count :- 

Percentile : 92.35	Count : 3
Percentile : 91.67	Count : 2
Percentile : 94.89	Count : 1
Percentile : 90.98	Count : 1
Percentile : 99.67	Count : 3
Percentile : 93.65	Count : 2


3. Percentile count more than 2 :- 

92.35=3
99.67=3

Java 8 – How to find duplicate and its count in a Stream or List ?
Java 8 – How to remove duplicates from ArrayList ?
Java 8 – How to remove duplicates from LinkedList ?
Java 8 – How to find duplicate and its count in an Arrays ?
Java 8 – How to remove duplicate from Arrays ?
Java 8 – Various ways to remove duplicate elements from Arrays
Java 8 – How to find and count duplicate values in a Map or HashMap ?

References :

Happy Coding !!
Happy Learning !!

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Java 8 – How to find and count duplicate values in a Map or HashMap ?

Find & Count duplicate values in a HashMap :

1. Using Java 8 Stream :

GetDuplicateCountOfMapValuesUsingJava8Stream.java

Output :

1.1 Duplicate Count of more than 2 :

2. Before Java 8 :

GetDuplicateCountOfMapValues.java

Output :

Related Articles :

References :

Related

Find & Count duplicate values in a HashMap :

1. Using Java 8 Stream :

GetDuplicateCountOfMapValuesUsingJava8Stream.java

Output :

1.1 Duplicate Count of more than 2 :

2. Before Java 8 :

GetDuplicateCountOfMapValues.java

Output :

Related Articles :

References :

Related

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