why java class WeakReference does not override hashcode and equals

Question

I was expecting the WeakReference class to override hashCode and equals methods like this

class WeakReference<T>{
  T ref;

  int hashCode(){
    return ref.hashCode();
  }

  boolean equals(Object o){
    return ref.equals(o);
  }
}

So that I could use We WeakReference directly as key in hashmaps like

Person p1 = new Person("p1");
WeakReference<Person> wr = new WeakReference<Person>(p1);

map.put(wr, "some value object");

But when I tested I found out that hashCode and equals are not overridden

Person p1 = new Person("p1");
WeakReference<Person> wr = new WeakReference<Person>(p1);
WeakReference<Person> wr2 = new WeakReference<Person>(p1);

System.out.println(wr.hashCode()); // prints x
System.out.println(wr2.hashCode()); // prints y

System.out.println(wr.equals(wr2)); // prints false

Any specific reasons reasons that hashCode and equals are not overridden in WeakReference class?

Answer 1

An important aspect of any key on a Map (or element of a Set) is that it must be immutable (or at least not change) once it has been added to the collection. Changing a key has undefined behaviour which is highly unlikely to work.

A WeakReference can change at any time due to a GC being performed i.e. in ways you have no control over, which makes the equals/hashCode inappropriate for general collections which use these.

I was trying to make MyWeakConcurrentHashMap

A simple way of doing this is to have an array of WeakHashMaps. e.g. 32 partitions . Use the hashCode() to determine which WeakHashMap to use. This way you can have a thread accessing each of the individual WeakHashMap at once (best case)

As you have more concurrency you can increase the number of partitions.