Hi, my name is Timo Ernst and I am a web expert.

.remove() and .contains() not working on your Java set?

Posted on: June 8th, 2011 by Timo

If you suddenly experience problems callling set.remove() and set.contains() on your Java Set instances, you might be using Hibernate, which replaces your Set instance with its own version (PersistentSet), which uses a HashSet internally to store your data.

For example, the problem I faced was the following:

Person.java

public class Person(){

 String _name;

 public Person(String name){
  _name = name;
 }

 @Override
 public String toString(){
  return _name;
 }

 @Override
 public int hashCode(){
  if (_name != null) return _name.hashCode();
  return -1;
 }
}

MyTestApplication.java

public class MyTestApplication{

 public static void main(String[] args){

  Person steve = new Person("Steve");
  Set persons = new Set();
  persons.add(steve);
  System.out.println(persons);

  boolean contains = persons.contains(steve);
  System.out.println(String.valueOf(contains));

  persons.remove(steve);
  System.out.println(persons);

  for (Person person : persons){
   System.out.println("Contains: " + person.toString());
  }

 }
}

The code above prints the following to the console:

[Steve]
false
[Steve]
Contains: Steve

This definitely was not expected.

The reason: If you call set.remove(obj) or set.contains(obj), the internal HashSet will automatically call obj.hashCode() and use the returned value to search for obj.
Normally, this works without any issues, but there is one case where it doesn’t and that’s when:

  • You use Hibernate
  • You load the Set data eagerly
  • You override the obj.hashCode() method

The reason for this is a bug in Hibernate.

For me, a solution was to not override the obj.hashCode() method.
Since this is not a solution for all cases, I recommend to follow the link above and read through the comments. There are quite some workarounds described there. I’m sure one of them will work for you.

Good luck, soldier.

(God, Hibernate! I officially hate you now. It took me a whole day to find the reason for this issue).

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One Response

  1. Anonymous says:

    Thank you very much !
    I started to think I was “as dumb as a chair” as we say in french. But alright, Hibernate tricked me once again. Thank you once again, very useful post.

    Thumb up 0 Thumb down 0

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