Java HashSet vs HashMap

The map holds unique keys. When you invoke put with a key that exists in the map, the object under that key is replaced with the new object. Hence the size 1.

The difference between the two should be obvious:

• in a Map you store key-value pairs
• in a Set you store only the keys

In fact, a HashSet has a HashMap field, and whenever add(obj) is invoked, the put method is invoked on the underlying map map.put(obj, DUMMY) – where the dummy object is a private static final Object DUMMY = new Object(). So the map is populated with your object as key, and a value that is of no interest.