Java: Why subList(0, 5).clear() doesn't work on my objects?

Question

If I run this operation on List<Integer> for example, it works as expected (removes first 5 elements), but when I run it on a list of my objects, nothing happens (list stays the same).

list.subList(0, 5).clear();

My class is a pojo that doesn't implement equals or hashCode, if that matters.

UPDATE: The implementation I am using is ArrayList, which is returned from Hibernate query. There is nothing to show, really. Sublist doesn't return an empty list.

Here is an example for those who don't beleive it works on a list of Integers:

    List<Integer> testList = new ArrayList<Integer>();
    for(int i=0;i<10;i++) {
        testList.add(i);
    }
    testList.subList(0, 5).clear();
    for(int i=0;i<testList.size();i++) {
        System.out.print(testList.get(i)+" ");
    }

The result is 5 6 7 8 9

UPDATE2: Actually everything is working as expected, don't know how I couldn't see that (got confused by numbers of results). Sorry for false alarm :) This question could be deleted.

Answer 1

edit - Looks like I was wrong, but leaving my original answer here anyway:

Are you sure that it works with a List<Integer>? It shouldn't.

The method subList() returns a separate List. If you remove elements from that list, it shouldn't affect the original list. The API docs for List.subList() say this:

Returns a view of the portion of this list between the specified fromIndex, inclusive, and toIndex, exclusive. (If fromIndex and toIndex are equal, the returned list is empty.) The returned list is backed by this list, so non-structural changes in the returned list are reflected in this list, and vice-versa.

Clearing a list is not a non-structural change; only changing the elements in the list are non-structural changes.

This has nothing to do with whether your POJO has equals or hashCode methods or not.

edit - I just tried it out with an ArrayList and it does work (not only with Integer, but also with my own object as a list element).

Answer 2

Two things I can think of are:

1. list.sublist(0, 5) returns an empty list, therefore .clear() does nothing.

2. Not sure of the inner workings of the List implementation you're using (ArrayList, LinkedList, etc), but having the equals and hashCode implemented may be important. I had a simiarl issue with Maps, where HashMap definitely needs the hashCode implementation.

Answer 3

It works on my machine^tm

import java.util.*;
import static java.lang.System.out;

class SubListExample {
    public static void main( String [] args ) {
        List<RandomObject> testList = new ArrayList<RandomObject>();
        for(int i=0;i<10;i++) {
            testList.add( new RandomObject() );
        }

        System.out.println( "Before: " + testList );
        testList.subList(0, 5).clear();
        System.out.println( "After: "+ testList );
    }
}
class RandomObject {
    static Random generator = new Random();
    int id = generator.nextInt(100);
    public String toString(){
        return "ro("+id+")";
    }
}

Produces:

$ java SubListExample
Before: [ro(68), ro(97), ro(48), ro(45), ro(43), ro(69), ro(45), ro(8), ro(88), ro(40)]
After: [ro(69), ro(45), ro(8), ro(88), ro(40)]

So, the problem is not in ArrayList nor in your objects.

I don't think Hibernate returns a plain old ArrayList ( may be it does )

Try printing

 System.out.println( "That hibernate list.class.name = "
        + listReturnedByHibernate.getClass().getName() );

And let us know if it is in fact an ArrayList

Answer 4

Have you tried creating a List of your objects manually and doing the same thing (without Hibernate)? It seems possible to me that this has to do with Hibernate's lazy loading of data... if you haven't read the data in the returned List, it may not have been loaded yet (since sublists themselves are just views). In that case, it's possible clear would do nothing.

Answer 5

Is it possible that the List returned from Hibernate is not modifiable? i.e. wrapped by Collections.unmodifiableList()