[java] Java 8 Lambda filter by Lists

I have two list and i want filter thoose elements which are both list contains. And i want to do this with lambda expression.

Users getName and Clients getUserName both are return with String.

Here is my sample code:

List<Client> clients = new ArrayList<>();
List<User> users = new ArrayList<>();
List<Client> results = new ArrayList<>();

for (Client user : users) {
    for(Client client: clients){
        if(user.getName().equals(client.getUserName())){
            result.add(client);
        }
    }
}

Look this:

List<Client> result = clients
    .stream()
    .filter(c -> 
        (users.stream().map(User::getName).collect(Collectors.toList())).contains(c.getName()))
        .collect(Collectors.toList());

I would like share an example to understand the usage of stream().filter

Code Snippet: Sample program to identify even number.

import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;

public void fetchEvenNumber(){
        List<Integer> numberList = new ArrayList<>();
        numberList.add(10);
        numberList.add(11);
        numberList.add(12);
        numberList.add(13);
        numberList.add(14);
        numberList.add(15);

        List<Integer> evenNumberListObj = numberList.stream().filter(i -> i%2 == 0).collect(Collectors.toList());
        System.out.println(evenNumberListObj);
}

Output will be : [10, 12, 14]

List evenNumberListObj = numberList.stream().filter(i -> i%2 == 0).collect(Collectors.toList());

numberList: it is an ArrayList object contains list of numbers.

java.util.Collection.stream() : stream() will get the stream of collection, which will return the Stream of Integer.

filter: Returns a stream that match the given predicate. i.e based on given condition (i -> i%2 != 0) returns the matching stream.

collect: whatever the stream of Integer filter based in the filter condition, those integer will be put in a list.

Something like:

clients.stream.filter(c->{
   users.stream.filter(u->u.getName().equals(c.getName()).count()>0
}).collect(Collectors.toList());

This is however not an awfully efficient way to do it. Unless the collections are very small, you will be better of building a set of user names and using that in the condition.