ArrayList removeAll() not removing objects

Question

I have the simple ArrayLists of the member class:

ArrayList<Member> mGroupMembers = new ArrayList<>();
ArrayList<Member> mFriends = new ArrayList<>();

Member class:

public class Member {
    private String userUID;
    private String userName;

    public String getUserUID() {
        return userUID;
    }

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName) {
        this.userName = userName;
    }

    public void setUserUID(String userUID) {
        this.userUID = userUID;
    }


}

The ArrayList for friends contains all the users friends. What I simply wish to do is remove from the friends list, group members if present with:

mFriends.removeAll(mGroupMembers);

Yet it does nothing to the mFriends list...

Looking at the log statements, the friend does in fact appear within the mGroupMember list.

Why doesn't this work?

Since I don't see overriding of equals method in Member class I suspect that your lists doesn't contain same objects but only similar ones (with similar states). — Pshemo
– Pshemo, Commented Mar 12, 2016 at 16:35
You haven't included enough information in your question, but at a guess, you have different (but equivalent) Member objects in the lists. As Pshemo said, since you haven't defined equals for Member, it will use Object's equals, which won't be true for equivalent Member objects. You need to override equals (and thus also hashCode). — T.J. Crowder
– T.J. Crowder, Commented Mar 12, 2016 at 16:37

Kevin DiTraglia · Accepted Answer · 2016-03-12 17:14:40Z

31

How are 2 members determined to be equal? I'm guessing if they have the same ID, you deem them equal, however java wants them to be the exact same reference in memory which may not be the case. To correct for this you can override the equals function to have it return if the ids are equal:

public class Member {
    //..

    @Override
    public boolean equals(Object anObject) {
        if (!(anObject instanceof Member)) {
            return false;
        }
        Member otherMember = (Member)anObject;
        return otherMember.getUserUID().equals(getUserUID());
    }
}

Also when you override .equals it is recommended to also override hashCode so that the objects also work correctly in hashing functions like Set or Map.

edited Mar 12, 2016 at 17:14

answered Mar 12, 2016 at 16:38

Kevin DiTraglia

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5 Comments

Sauron Over a year ago

Is the equals overridden within the Member object?

Yassin Hajaj Over a year ago

@TJCrowder also, do not forget to check if anObject is an instance of the class.

Kevin DiTraglia Over a year ago

@Sauron yes it would be overridden in your Member class. Also notice the edit, since userID is a string, it needs to be .equals. and not ==.

Sauron Over a year ago

@KevinDiTraglia, Yes this is what I was forgetting.

Yassin Hajaj Over a year ago

@KevinDiTragila No need to check if it's null. instanceof always gives false when a null is giving to it.

Yassin Hajaj · Accepted Answer · 2023-01-23 19:55:03Z

10

You have to understand the sequence of invocation underneath it

 -> ArrayList#removeAll(Collection)
 |
 | calls
 |  
 |--> ArrayList#contains(Object)
   |
   | calls
   |
   |--> ArrayList#indexOf(Object)
     |
     | calls
     |
     |--> Object#equals

So if equals is not correctly overridden (following the equals contract rules), you're not getting the correct behaviour.

edited Jan 23, 2023 at 19:55

answered Mar 12, 2016 at 16:41

Yassin Hajaj

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2 Comments

Yogendra Over a year ago

This was so simple to understand. I liked this answer more than the actual code presented here as its explains the reasoning for comparison not working.

Yassin Hajaj Over a year ago

@YogendraJ Thanks a lot :) Happy it helped

M.S. · Accepted Answer · 2019-01-01 04:50:29Z

As mentioned in the comments, elements from the ArrayList will only be removed if their equals() method returns true. The non-overridden method checks for equality based on reference (i.e. they must be the same object in memory).

What you probably want is to override equals to be based on the properties of Member, such as in the example below:

@Override
public void equals(Object o) {
    if(o == null) {
        return false;
    } else if (!(o instanceof Member)) {
        return false;
    } else {
        return ((Member) o).getUserUID().equals(this.userUID) && ((Member) o).getUserName().equals(this.userName);
    }
}

In addition, you should override hashCode() when overriding equals() so that when two objects are equal, they have the same hash code. The non-overriden implementation of hashCode is also based on equality by reference.

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