Java – clone and subtract sets – is this useful?

private HashMap<DataObject,HashSet> AllDataObjects;

private HashMap<DataObject,HashSet> AllDataObjects;

...

/** Returns all DataObject elements that are NOT in the specified set. */
private DataObject[] invert( HashSet<DataObject> set )
{
    HashSet<DataObject> keys = (HashSet) AllDataObjects.keySet();
    keys = (HashSet) keys.clone();

    keys.removeAll( set );

    return (DataObject[]) keys.toArray();
}

Please note that I do not want to change alldataobjects. XML through this process I convert the keys of a set of alldataobjects (DataObjects I want to subtract from the set parameter) to HashSet to use clone, which should return a shallow copy, and then I can delete the set without affecting alldataobjects

Does this look good to you?

Solution

Create a new set and merge the set to be cloned as a parameter This avoids projection, so you won't lose generics

private DataObject[] invert( Set<DataObject> set ){
    Set<DataObject> keys = new HashSet<DataObject>(AllDataObjects.keySet());
    keys.removeAll( set );
    return keys.toArray(new DataObject[]{});
}

It is also worth noting that you should use set instead of HashSet as a parameter to avoid overburdening customers