Using Set

    Set set = new HashSet(); // Instantiate a concrete set
    ...
    set.add(obj); // insert an element
    ...
    if (set.contains(obj)) { // check membership
        ...
    }
    // iterate through the set
    Iterator iter = set.iterator();
    while (iter.hasNext()) {
        object e = iter.next();
        // downcast e
        // ...
    }