Remove duplicate from List java8

Question

I have a class below, and wanted to remove duplicate person which contain same name, how to do by using Java8 Lambda, expected List contains p1, p3 from the below.

Person:

public class Person {

public int id;
public String name;
public String city;

public int getId() {
    return id;
}

public void setId(int id) {
    this.id = id;
}

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

public String getCity() {
    return city;
}

public void setCity(String city) {
    this.city = city;
}

}

Testing:

import java.util.ArrayList;
import java.util.List;

public class Testing {

public static void main(String[] args) {

    List<Person> persons = new ArrayList<>();

    Person p1 = new Person();
    p1.setId(1);
    p1.setName("Venkat");
    p1.setCity("Bangalore");
    Person p2 = new Person();

    p2.setId(2);
    p2.setName("Venkat");
    p2.setCity("Bangalore");

    Person p3 = new Person();
    p3.setId(3);
    p3.setName("Kumar");
    p3.setCity("Chennai");

    persons.add(p1);
    persons.add(p2);
    persons.add(p3);

}
}

override equals & hashcode methods in Person class for duplication removal using Set — Noman Khan
– Noman Khan, Commented May 29, 2018 at 17:03
stackoverflow.com/questions/23699371/…enter link description here — 取一个好的名字
– 取一个好的名字, Commented May 9, 2019 at 7:30

Eugene · Accepted Answer · 2017-07-31 20:14:03Z

37

You could filter them out and generate a unique Set:

Set<Person> set = persons.stream()
            .collect(Collectors.toCollection(() -> 
                 new TreeSet<>(Comparator.comparing(Person::getName))));

Or even nicer:

Set<String> namesAlreadySeen = new HashSet<>();

persons.removeIf(p -> !namesAlreadySeen.add(p.getName()));

edited Jul 31, 2017 at 20:14

answered Jul 31, 2017 at 14:40

Eugene

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5 Comments

holi-java Over a year ago

ooh, you faster than me, +1! you should tell the OP to make the persons to List<Person> rather than List<Object>.

fps Over a year ago

You actually don't need a stream for this, do you? Just create the TreeSet with the comparator and then use addAll.

Holger Over a year ago

No need for block syntax here, you can use the compact expression syntax just like here.

Eugene Over a year ago

@FedericoPeraltaSchaffner you're right, I don't have to use streams... thx

Majid Ali Khan Over a year ago

Helped me with some other kind of problem :p

Tarun Kumar · Accepted Answer · 2024-05-04 06:41:36Z

36

The distinct() function can be used to removed duplicates using a natural comparison, for that first override equals() and hashCode() method in Person class.

List<Person> personsWithoutDuplicates = persons.stream()
 .distinct()
 .collect(Collectors.toList());

Note: for custom comparison Comparator interface can be used.

edited May 4, 2024 at 6:41

Tarun Kumar

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answered Nov 1, 2017 at 18:41

Taras Melnyk

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3 Comments

Robert Moskal Over a year ago

You sure that's going to work if the class does override equals and hash?

Priidu Neemre Over a year ago

@RobertMoskal: the question is, rather, why you'd want to sneak around implementing equals and hashCode in the first place.

Taras Melnyk Over a year ago

distinct - Returns a stream consisting of the distinct elements (according to Object.equals(Object)) of this stream.

Timbus Calin · Accepted Answer · 2020-01-27 09:38:38Z

0

List<Person> modified = pesrons.stream().collect(Collectors.toCollection(()->new TreeSet<>(Comparator.comparing(Person::getName)))).stream().collect(Collectors.toList());

This will return a list of non duplicates based on Name.

You can refer this also Remove duplicates from a list of objects based on property in Java 8

edited Jan 27, 2020 at 9:38

Timbus Calin

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answered Jul 31, 2017 at 18:07

Rohit

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