Removing an object from TreeSet

Question

So I have the following relevant code:

public static TreeSet<Object> set;



public static void remove(Object object){
    for (Object o : set){
        if (o.equals(object)){
            System.out.println("removing " + o);
            set.remove(o);
        }
    }
}

I add a few objects to that TreeSet, then I call remove with a certain object1 as an argument. The object1 is in the set, as

removingobject1

is printed out.

Yet, when I then print out the whole set after that using a for-loop like this:

for (Object o: set){
        System.out.println(o);
    }

It still prints out the whole set including object1. It was clearly in the collection before, which Java was able to recognize, yet calling set.remove(o) results in absolutely nothing.

edit: I tried to keep the question as general as possible, but here are the objects that I'm using:

public class Player implements Comparable<Player>{

public String firstname;
public String lastname;
public int value;
public Position position;



public Player(String firstname, String lastname, int value, Position position){
this.firstname = firstname;
this.lastname = lastname;
this.value = value;
this.position = position;

}

public String toString(){
    return(firstname + " " + lastname")
}


public int compareTo(Player player){
    if (this.value > player.value){
        return -1;
    } else {
        return 1;
    }
}

Does your object implement `hashCode()`? Please post a [mcve]. — shmosel, Mar 06 '17 at 17:47
Please show some code that shows manipulations of your set and objects. How do you create your objects, how do they implement hashCode(), how do you add them to the set? — Natalja Olefire, Mar 06 '17 at 18:03
You are using Object Class are you also setting any property? If would be easier for us to help you if you paste your code here. — Atul, Mar 06 '17 at 18:10
A `TreeSet` needs either objects that implement the `Comparable` interface or a `Comparator` to compare the elements. Which do you use? — Thomas Kläger, Mar 06 '17 at 18:52
Return 0 when this.value == player.value in your compareTo(...) method and it will work. — Natalja Olefire, Mar 06 '17 at 23:15
@shmosel it doesn't - I assumed this wouldn't be a problem since the `o.equals(object)` part worked fine. — Lukas Tilmann, Mar 06 '17 at 23:16
@NataljaOlefire is right. Your `compareTo()` is broken when both values are 0. See the contract for [`Comparable.compareTo()`](https://docs.oracle.com/javase/8/docs/api/java/lang/Comparable.html#compareTo-T-): *The implementor must ensure `sgn(x.compareTo(y)) == -sgn(y.compareTo(x))` for all `x` and `y`.* — shmosel, Mar 06 '17 at 23:20
@shmosel She was indeed right, your suggestions fixed my problem. Thank you for your help! — Lukas Tilmann, Mar 06 '17 at 23:21
@shmosel not really. compareTo should return 0 when objects are equal. (Math.signum(0) == -Math.signum(0)) is true. — Natalja Olefire, Mar 07 '17 at 00:47

score 1 · Answer 1 · answered Mar 06 '17 at 17:53

1

Your remove method could look like:

public static void remove(Object object){
   set.remove(o);
}

Method remove removes element if it is present in the set, and returns true if it was removed. You should not modify your set during the iteration (docs):

The iterators returned by this class's iterator method are fail-fast: if the set is modified at any time after the iterator is created, in any way except through the iterator's own remove method, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

answered Mar 06 '17 at 17:53

Natalja Olefire

442
5
9

This is true, but doesn't answer the question. And I suspect the iteration was just to prove the element is in the set. – shmosel Mar 06 '17 at 17:56
1

In this case then it should throw ConcurrentModificationException as one element is removed and the modCount value is changed so the iterator should fail. – Rahul Vedpathak Mar 06 '17 at 17:56
I have actually tried it this way and somehow it didn't work. I implemented the for loop to see if Java recognizes that the object is still in the set (which it is). If remove(o) actually worked naturally I'd delete the unnecessary for loop. ;) – Lukas Tilmann Mar 06 '17 at 23:01

Removing an object from TreeSet

1 Answers1