ArrayList removeAll() not removing objects

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?