import structure5.*;
import java.util.Iterator;


public class SetHashtable<T> extends AbstractSet<T> {
    Hashtable<T,Object> data;

    public SetHashtable() {
        data = new Hashtable<T,Object>();
    }

    public int size() {
        return data.size();
    }

    public boolean isEmpty() {
        return data.isEmpty();
    }

    public void clear() {
        data.clear();
    }

    public boolean contains(T value) {
        return data.containsKey(value);
    }
    
    public void add(T value) {
        if (!contains(value)) {
            data.put(value, null);
        }
    }

    public T remove(T value) {
        if (contains(value)) {
            data.remove(value);
            return value;
        } else {
            return null;
        }
    }

    public Iterator<T> iterator() {
        return data.keys();
    }
}