Confused !HashMap stores an object or reference ?

Question

HashMap<Integer,Temp> m = new HashMap<Integer, Temp>();
Temp t1 = new Temp(1); /* pass the id in the constructor */
m.put(1, t1);
System.out.println(m.get(1).id); /* This prints out "1" */
t1.id = 100;
System.out.println(m.get(1).id); /* This prints out "100" */
t1 = null;
System.out.println(m.get(1).id); /* This prints out "100" */
Since externally modifying the id value of an object of temp class, which has already been inserted in the map, updates the id value of the object stored in the map, I thought that HashMap actually stores a reference and not the entire object. However if I assign a null pointer to t1 externally , still the object in HashMap is not null which means that hashMap has its own copy ??
Or does java play a smart trick and check whether a non-null object reference is being modified, and if it is, then all of its copies in any of the collections would be updated ?
Or I am missing something really simple ?

Answer 1

Remember that t1 is also a reference to the same object as the map. When changing the object through t1's reference the accessing the object through the map will show this change as you specify.
But, when you set the t1 to reference null (point at nothing) it does not mean the actual object is deleted, lost or changed. It still exists, but now only referenced by the map.
Consider this:
Integer a = new Integer(1); // a points to the Integer object
Integer b = a; // b points to this same object. (not pointing to a, but the same object as a)
System.out.println( b );
b = null; // Now b point to null, but the Integer object is still referenced by a
System.out.println( b );
System.out.println( a );- Roy