Java 8 How to filter not different entity from two collections?

Question

I want to use Java 8 stream filter collections but i found all example is one collections , Java 6 is :

public class User {
public Long id;
public String name;

public User(Long id, String name) {
    this.id = id;
    this.name = name;
}

   List<User> usersA = new ArrayList<>();

    usersA.add(new User(1l,"A"));
    usersA.add(new User(2l,"B"));
    usersA.add(new User(3l,"C"));

    List<User> usersB = new ArrayList<>();
    usersB.add(new User(33l,"A"));
    usersB.add(new User(34l,"B"));
    usersB.add(new User(35l,"D"));
    usersB.add(new User(36l,"C"));

    List<User> tempUser = new ArrayList<>();
    tempUser.addAll(usersB);

    for (User user : usersA) {
        for (User user1 : tempUser) {
            System.out.println(user1.getName().equalsIgnoreCase(user.getName()));
            if (user1.getName().equalsIgnoreCase(user.getName())){
                System.out.println("remove:"+user1.getName());
                tempUser.remove(user1);
                break;
            }
        }

    }

    System.out.println("last:"+tempUser);

And Java 8 I want to use stream api not use foreach ,can u take a example for me ? thanks

JB Nizet · Accepted Answer · 2015-11-11 08:06:08Z

So, if I understand correctly, you want to create a new list containing all the users of usersB, except those that have the same name as any of the users in usersA, right?

First of all, I would change the strategy: instead of adding all the users, then removing some, I would only add the users that belong to the list in the first place.

And in Java 8, that can be done with

List<User> result = 
    usersB.stream()
          .filter(u -> !userNameIn(u, usersA))
          .collect(Collectors.toList());

with userNameIn defined as

private boolean userNameIn(User u, List<User> users) {
    return users.stream().anyMatch(user -> user.getName().equalsIgnoreCase(u.getName()));
}

That won't be very efficient if usersA contains a large number of users. A more efficient solution would be to store all the lowercase names of usersA in a HashSet, and replace the method with

List<User> result = 
    usersB.stream()
          .filter(u -> !lowercaseNames.contains(u.getName().toLowerCase()))
          .collect(Collectors.toList());

Holger · Accepted Answer · 2015-11-11 08:18:34Z

You should avoid repeated lookups using linear search as you do with your nested loops. This can become a performance disaster when the collections grow.

First, collect the names of the first collection into a Set supporting the desired lookup:

Set<String> namesInA=usersA.stream().map(User::getName)
    .collect(Collectors.toCollection(() -> new TreeSet<>(String.CASE_INSENSITIVE_ORDER)));

Then you can perform the operation in either of two fashions:

In-place in a collection:

List<User> tempUser = new ArrayList<>(usersB);
tempUser.removeIf(u -> namesInA.contains(u.getName()));

Using the stream API to collect matching elements into a new collection:

List<User> tempUser = usersB.stream()
    .filter(u->!namesInA.contains(u.getName()))
    .collect(Collectors.toList());

DanieleDM · Accepted Answer · 2015-11-11 17:03:17Z

I would do something like that:

public static class User {
    public Long id;
    public String name;

    public User(Long id, String name) {
        this.id = id;
        this.name = name;
    }
}
public static void main(String[] args){


    List<User> usersA = new ArrayList<>();

    usersA.add(new User(1l,"A"));
    usersA.add(new User(2l,"B"));
    usersA.add(new User(3l,"C"));

    List<User> usersB = new ArrayList<>();
    usersB.add(new User(33l,"A"));
    usersB.add(new User(34l,"B"));
    usersB.add(new User(35l,"D"));
    usersB.add(new User(36l,"C"));

    List<User> list = usersB.stream()
                    .filter(userb -> !usersA.stream().
                                    filter(usera -> usera.name.equalsIgnoreCase(userb.name)).findFirst().isPresent())
                    .collect(Collectors.toList());

}

Emerson Cod · Accepted Answer · 2015-11-11 08:03:24Z

-1

this can be done without any streams in java 8 and even better before in previous java versions.

http://docs.oracle.com/javase/7/docs/api/java/util/Collection.html allows you to remove all items of one collection that are in the other collection. For the caller this is a one-liner then

userB.removeAll(userA)

this is correct when the objects of the collection implement equals

answered Nov 11, 2015 at 8:03

Emerson Cod

2,1003 gold badges22 silver badges44 bronze badges

Collectives™ on Stack Overflow

Java 8 How to filter not different entity from two collections?

4 Answers 4

Comments

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related