You want to ensure that only one copy of each unique value is stored in a collection.

Use a Set.

The Set interface is a collection that maintains only one instance of each value. If you add into it an object that is equal (as defined by the equals( ) method) to another object, only one of the objects is maintained. By definition, it does not matter to you which of the two objects it keeps -- the one in the collection or the one being added -- since your objects’ equals( ) method indicated they were both equal.

// SetDemo.java
HashSet h = new HashSet(  );
h.add("One");
h.add("Two");
h.add("One"); // DUPLICATE
h.add("Three");
Iterator it = h.iterator(  );
while (it.hasNext(  )) {
    System.out.println(it.next(  ));
}

Not surprisingly, only the three distinct values are printed.