[JAVA] Problema banale

[Ilmalcom]

Qualcuno sa dirmi perchè questo semplice frammento di codice:

import java.util.*;

class IntSet {
        private Vector elems = new Vector ();

        public IntSet () {
                elems = new Vector ();
        }

        public void insert (int x) {
                if (!elems.contains (x))
                        elems.add (x);
        }

        public void remove (int x) {
                elems.remove (x);
        }

        public boolean isIn (int x) {
                return (elems.contains (x));
        }

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

Genera i seguenti errori?

IntSet.java:11: cannot resolve symbol
symbol : method contains (int)
location: class java.util.Vector
if (!elems.contains (x))
^
IntSet.java:12: cannot resolve symbol
symbol : method add (int)
location: class java.util.Vector
elems.add (x);
^
IntSet.java:20: cannot resolve symbol
symbol : method contains (int)
location: class java.util.Vector
return (elems.contains (x));
^
3 errors