Question

I have a code to return an arrayList with the duplicates of an ArrayList
but seems it’s not working, I am comparing all items in the array…

public ArrayList<ObjectList> duplicates(ArrayList<ObjectList> someObjectsList) {

    ArrayList<ObjectList> ret = new ArrayList<ObjectList>();
    for ( ObjectList aSomeObjectsList: someObjectsList) {

        String field1 = aSomeObjectsList.get1();
        String field2 = aSomeObjectsList.get2();
        String field3 = aSomeObjectsList.get3();
        String field4 = aSomeObjectsList.get4();
        for (ObjectList someObject : ret) {
            if (
                field1.trim().equals(someObject.get1())&& 
                field2.trim().equals(someObject.get2())&&
                field3.trim().equals(someObject.get3())&&
                field4.trim().equals(someObject.get4())     
                ){
                ret.add(aSomeObjectsList);

            }
        }

    }
    return ret;
} 

But i guess I am doing something wrong because it doesn’t return anything, and I know it has duplictates under this 4 field criteria

Thanks in advance

Answer 1

Here’s a way you can do this. I have defined a basic class ObjectList that shows a way to implement equals and hashCode. Note that this assumes that all the internal variables are non-null. If these variables can contain null then you will need to check for that when computing the equals/hashCode. Also, the objects in this class must also themselves properly implement equals/hashCode.

public class ObjectList {

    private int h;

    private Object obj1;
    private Object obj2;
    private Object obj3;
    private Object obj4;

    @Override
    public boolean equals(final Object o) {
        if (!(o instanceof ObjectList))
            return false;

        final ObjectList that = (ObjectList) o;
        return that.obj1.equals(obj1) && that.obj2.equals(obj2)
            && that.obj3.equals(obj3) && that.obj4.equals(obj4);
    }

    @Override
    public int hashCode() {
        // caches the hashcode since it could be costly to recompute every time
        // but this assumes that your object is essentially immutable 
        // (which it should be if you are using equals/hashCode. If this is not
        // true and you want to just temporarily use this when doing the duplicate
        // test, move the h variable definition from the object level to this method
        // and remove this if statement.
        if (h != 0)
            return h;

        h = obj1.hashCode();
        h = h * 31 + obj2.hashCode();
        h = h * 31 + obj3.hashCode();
        h = h * 31 + obj4.hashCode();
        return h;
    }

}

public Collection<ObjectList> duplicates(
        final Collection<ObjectList> someObjectsList) {

    final Set<ObjectList> unique = new HashSet<ObjectList>(someObjectsList);
    final ArrayList<ObjectList> ret = new ArrayList<ObjectList>(someObjectsList);
    for (final ObjectList o : unique) {
        ret.remove(o);
    }

    // The ret list now contains the duplicate instances; instances 
    // with more than two occurrences will occur multiple times still in
    // this list.
    return ret;

    // If you want a list of unique duplicate instances then, comment out the above
    // return and uncomment this one.
    // return new HashSet<ObjectList>(ret);
}

Using Collection<ObjectList> is better, if you can do that, for both the parameter and returned value so you can vary the implementations (ArrayList, Set, etc).