Java 8 Stream Distinct

stream.distinct() method to filter or collect all distinct elements from a collection,
distinct() uses hashCode() and equals() methods to get distinct elements. 
Hence our class must implement hashCode() and equals() methods.

Stream.distinct() method declaration:

Stream distinct()

In bellow example we have list of string data type tat contains duplicate elements:
Sample Example.java

Used distinct() with List of Objects:
In bellow example,class will be overried the hashCode() and equals() methods.

import java.util.ArrayList;
import java.util.List;
public class DistinctWithUserObjects {
    public static void main(String[] args) {
        List list = new ArrayList<>();
        {
        list.add(new PersonInfo("Ramesh", 20));
        list.add(new PersonInfo("Ramesh", 20));
        list.add(new PersonInfo("Komal Kumar", 15));       
        list.add(new PersonInfo("Kiran Kumar", 30));
        list.add(new PersonInfo("Kiran Kumar", 30));
        }
        long l = list.stream().distinct().count();
        System.out.println("No. of distinct books:"+l);
        list.stream().distinct().forEach(b ->
        System.out.println(b.getPerson()+ "," + b.getAge()));
    }
}


public class PersonInfo {
private String person;
private int age;
public PersonInfo(String person, int age) {
this.person = person;
this.age = age;
}
public String getPerson() {
return person;
}
public int getAge() {
return age;
}
@Override
    public boolean equals(final Object obj) {
      if (obj == null) {
         return false;
      }
      final PersonInfo personInfo = (PersonInfo) obj;
      if (this == personInfo) {
         return true;
      } else {
         return (this.person.equals(personInfo.age) && this.age == personInfo.age);
      }
    }
    @Override
    public int hashCode() {
      int hashno = 7;
      hashno = 13 * hashno + (person == null ? 0 : person.hashCode());
      return hashno;
    }
}



Distinct by attribute and Property
distinct() does not provide distinct elements by property or key. It works on the basis of hashCode() and equals().
If we want distinct element by a property or key, we can achieve it by a work around code. 

static Predicate distinctByKey(Function keyExtractor) {
        Map
        return t -> apple.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
} 

The above code can be used with Stream filter() method as follows.


list.stream().filter(distinctByKey(b -> b.getName()));