Java 8 Streams - How to find the duplicate elements from a Collection - Multiple ways

Example 1: Using Collections.frequency()

To find frequency of a particular element we can use the frequency() method:
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.stream.Collectors;

public class DriverClass {
    public static void main(String[] args) {

        List<String> obj = Arrays.asList("ant", "1", "cat", "ant",
                "we", "java", "cat");
        obj.stream().filter(i -> Collections.
            frequency(obj, i) > 1).collect(Collectors.toSet())
                .forEach(System.out::println);

    }
}

Output:

ant
cat



Example 2: Using Collectors.groupingBy()

We can also use the Collectors.groupingBy() method to count the frequency of elements present in a stream. 
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.function.Function;
import java.util.stream.Collectors;

public class DriverClass {
    public static void main(String[] args) {
        List<String> obj = Arrays.asList("ant", "1", "cat", "ant",
                "we", "java", "cat");
        Set<String> result = obj.stream().
               collect(Collectors.groupingBy(Function.identity(),
                Collectors.counting())).entrySet().
                 stream().filter(m -> m.getValue() > 1).
                   map(Map.Entry::getKey)
                   .collect(Collectors.toSet());
        result.stream().forEach(System.out::println);

    }
}

Output:
ant
cat



Example 3: Using Set.add()

If the element is present in the Set already, then this Set.add() returns false.
import java.util.Arrays;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
import java.util.stream.Collectors;

public class DriverClass {
    public static void main(String[] args) {

        List<String> obj = Arrays.asList("ant", "1", "cat", "ant",
                "we", "java", "cat");
        Set<String> items = new HashSet<>();
        Set<String> result = obj.stream().
             filter(n -> !items.add((String) n)).
                collect(Collectors.toSet());
        result.stream().forEach(System.out::println);
    }
}

Output:
ant
cat



Example 4: Using Collectors.toMap()
public class DriverClass {

    public static void main(String[] args) {

        List<String> obj = Arrays.
                asList("ant", "1", "cat", 
                    "ant", "we", "java", "cat");
        Map<String, Integer> map = obj.stream()
                .collect(Collectors.
                    toMap(Function.identity(), 
                        value -> 1, Integer::sum));

        System.out.println(map);
    }
}

Output:
{1=1, java=1, ant=2, cat=2, we=1}

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