CODEWITHSUNDEEP
javamultithreadingarraylist
user1836132
user1836132

Reputation: 21

Java ArrayList removes non existing objects from parallel thread

Lately i was playing around with semaphores and multithreading, when i noticed, that something weird is going on with array list. More details:

I have some class with a single private ArrayList:

public class NewClass{

    private ArrayList list = new ArrayList();

    public void put(Object val){
        list.add(val);
    }

    public void del(Object val){
        list.remove(val);
    }
}

From some thread, i'm trying to remove elements from it (without putting anything before that):

public class SomeClass {

    public static void main(String[] args) throws InterruptedException {
        new SomeClass();
    }

    public SomeClass() throws InterruptedException {
        Thread tr2 = new Thread() {
            @Override
            public void run() {
                NewClass nc = new NewClass();
                for (int i = 0; i < 100; i++) {
                        nc.del(i);
                }
            }
        };
        tr2.start();
    }
}

And when the thread starts working - i have no errors, no exceptions, etc. While, if debugging, i can clearly see, that list.indexOf(val); return -1 value - it actually does not exist, but is "removed". Can anyone explain what is going on here?

Upvotes: 1

Views: 3085

Answers (4)

yushulx
yushulx

Reputation: 12160

when you call remove, the list will become shorter than before. do you think your loop code is right? The total length is not 100 any more.

Upvotes: 0

veritas
veritas

Reputation: 2444

Like as you said "playing around with stuff".

Am suspecting you have been caught between the two methods of ArrayList.

 public E remove(int index) // throws exception 

and 

 public boolean remove(Object o) // doesn't throws exception

When you are accessing through it thread you are actually passing it like an reference as an object. which actually calls the remove(Object o) method which doesn't throw exception.

While i think while playing around from single thread program you directly calling

list.remove(int) /// method which actually throws IndexOutOfBoundsException.

Thisis just my assumption as you haven't provided the code where it throws exception

Upvotes: 0

assylias
assylias

Reputation: 328855

Too long for a comment:

  • your NewClass instances are not shared across threads and each instance is accessed from only one thread.
  • calling List.remove(int index) on an empty list does nothing special (it returns -1).

Upvotes: 0

Kayaman
Kayaman

Reputation: 73568

Lists don't throw exceptions if you try to remove things that aren't in it with the remove(Object o) method. They return false.

Upvotes: 2

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