Nested forEach in Java 8 throws an exception [duplicate]

Question

EDIT its worked

        group.getUsers().forEach(user -> {
        user.removeGroup(group);
        privilegeRepository.findAll().stream().
                filter(privilege -> privilege.getName().startsWith(slugname.toUpperCase()))
                .forEach(privilege -> {
            user.removePrivilege(privilege);
            privilegeRepository.delete(privilege);
        });
    });

I have nested for-loops and I want to replace that for-loops with a forEach:

for (User user : group.getUsers()) {
     user.removeGroup(group);

     for(Privilege privilege : user.getPrivileges()){

         if (privilege.getName().startsWith(slugname.toUpperCase())){
             user.removePrivilege(privilege);
             privilegeRepository.delete(privilege);
         }
     }
}

to

group.getUsers().forEach(user -> {

     user.removeGroup(group);

     user.getPrivileges().stream().filter(privilege -> privilege.getName().startsWith(slugname.toUpperCase()))
          .forEach(privilege -> {
              user.removePrivilege(privilege);
              privilegeRepository.delete(privilege);
          });
});

And it throws a exception:

java.util.ConcurrentModificationException

the complete stack trace would be useful, so we can see what line throw the exception — NickJ
– NickJ, Commented Feb 26, 2019 at 8:28
user.getPrivileges() vs privilegeRepository.findAll(): doesn't look like the two are doing the same thing — ernest_k
– ernest_k, Commented Feb 26, 2019 at 8:29
Instead of using streams here it might make more sense to use an old fashioned iterator which you can call remove() on to get the privilege removed from the user's privileges. — Thomas
– Thomas, Commented Feb 26, 2019 at 8:31
You cannot call user.removePrivilege while iterating over user.getPrivileges. You need to make a snapshot copy to iterate over. — Thilo
– Thilo, Commented Feb 26, 2019 at 8:32
It is interesting why you do not get java.util.ConcurrentModificationException in the first example (with the nested for-loops). I think user.removePrivilege(privilege); still can make modification on the collection that you are iterating in the same time. — Level_Up
– Level_Up, Commented Feb 26, 2019 at 8:52

László Stahorszki · Accepted Answer · 2019-02-26 08:33:55Z

0

It is what it says. In the loop, you are modifying the array you are looping through, which is not allowed in Java foreach loops.

I'd suggest you create a temporary array in the outer foreach loop (where you loop the users) for the privileges and delete the items from that array, instead of the user's privilege array. So instead of user.removePrivilege(privilege); you should use something like temp.remove(privilege).

After the inner loop, you can clear the user's privilege array and add the items in the temporary array.

answered Feb 26, 2019 at 8:33

László Stahorszki

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3 Comments

zblash Over a year ago

So in this case no need replace for-loops with a forEach ?

László Stahorszki Over a year ago

no, you can keep the forEach

Holger Over a year ago

@zblash the for-loops have exactly the same problem. The iteration policy does not change when you use a Stream, so you’re using a collection not allowing modifications while iterating and it’s a pure coincidence if you did not hit the exception previously.

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