Checking if an object exists in a list Java Stream

I have 2 entity. One of them User, and the other one is Followers. I’m trying to make a follower system like Twitter. User can follow another user but, i want to check if user followed, don’t do it again.

This is my Follower Entity :

@Entity
public class Followers {

    @Id
    @GeneratedValue(strategy= GenerationType.AUTO)
    private long id;

    @ManyToOne(fetch = FetchType.EAGER)
    @JoinColumn(name="from_user_fk")
    @JsonIgnore
    private User from;

    @ManyToOne(fetch = FetchType.EAGER)
    @JoinColumn(name="to_user_fk")
    @JsonIgnoreProperties({ "password", "balance","id","mail" })
    private User to;

    public Followers() {};

    public Followers(User from, User to) {
        this.from = from;
        this.to = to;
    }

    public User getFrom() {
        return from;
    }

    public void setFrom(User from) {
        this.from = from;
    }

    public User getTo() {
        return to;
    }

    public void setTo(User to) {
        this.to = to;
    }
}

And this is the Service class :

public ResponseEntity<?> followUser(String username, User user) {
        User byUsername = getByUsername(username);
        List<Followers> followers1 = byUsername.getFollowers();
        List<Followers> collect = followers1.stream().filter(p -> p.getTo().getId().equals(user.getId())).collect(Collectors.toList());
        if(followers1.size()>0){
            return ResponseEntity.status(HttpStatus.FORBIDDEN).body("e");
        }
        Followers followers = new Followers();
        followers.setFrom(user);
        followers.setTo(byUsername);
        followersRepository.save(followers);
        return ResponseEntity.ok(new GenericResponse("Followed"));

    }

    public List<Followers> getUserFollowers(String username) {
        User byUsername = getByUsername(username);
        List<Followers> followers = byUsername.getFollowers();
        return followers;

    }

As you can see, I got the followers of the user I want to follow, and try to check if its following or not. But I couldn’t.

Any help would be appreciated

Answer

What you’ve built is incredibly inefficient:

• Go to the DB and fetch all followers for a given user.
• Then check through these if the person you’d like to add already exists.
• If no, add it.
• If yes, don’t do anything or show an error.

The fail whale is in your near future with this kind of inefficiency.

There’s a much, much simpler way. Just.. add it! Don’t do any check at all.

Your DB should be configured to disallow having the same user/follower pair, so if you attempt to pull that stunt when that user already has that follower, the DB will refuse and throw an exception that indicates that there’s a DB constraint violation. That’s your cue to render whatever error you please.

Note that ‘check if X is allowed, if yes, do X’ is fundamentally broken when talking about multicore architecture. What you’re forgetting: What if the user ‘double clicks’ the ‘add follower’ link? Then 2 requests start, simultaneously. They both check if X is already a follower (they are not), then they both add X as follower (and now X is a double-follower which you did not want).

Generally if a DB is involved, it is better at data consistency and transactions support, so use the best tool for the job: A DB constraint.