Java 8 Lambda filter by Lists

Look this:

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

Predicate<Client> hasSameNameAsOneUser = 
    c -> users.stream().anyMatch(u -> u.getName().equals(c.getName()));

return clients.stream()
              .filter(hasSameNameAsOneUser)
              .collect(Collectors.toList());

But this is quite inefficient, because it's O(m * n). You'd better create a Set of acceptable names:

Set<String> acceptableNames = 
    users.stream()
         .map(User::getName)
         .collect(Collectors.toSet());

return clients.stream()
              .filter(c -> acceptableNames.contains(c.getName()))
              .collect(Collectors.toList());

Also note that it's not strictly equivalent to the code you have (if it compiled), which adds the same client twice to the list if several users have the same name as the client.

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.