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Hint: Create your own custom map (extending from HashMap)

If we extend a HashMap, we will have to use our custom HashMap in places where we would use the original HashMap. If we do that ,its easy. Declare a counter and override the put method modify it so that the counter is incremented each time it is called.

Actaully I heard a presentation on Javassist and it could be used to modify the bytecode of a class at runtime. So we could actually modify the byte code of put method of Hashmap at runtime to insert the counting logic. More about Javassist; http://www.csg.is.titech.ac.jp/~chiba/javassist/

overriding.. hashcode() method and putting static variable inside it.

every time put() and get() call java will call hashcode() method to find hashcode()

override hashcode() method of class whose object is used as key.

What if you have more than one class whose objects are used as keys?
Can we not use Decorator pattern to add responsibility to hashmap to count the get and put method calls at runtime?

hashcode() doesn't seem to be a good idea. Put and Get method of hashmap will call the key objects hashCode(); Hence it an overhead to override HashCode function in every key objects class. Imagine key being String class.

Cleaner way is to extend HashMap class and overide get, put call..and call super.put() and get method. Use get/put count per MyHashMap object.

When it comes to passing the MyHashMap to another method, reference would be created to same hashMap object. Hence continue to use the same get and put method.

Hence at the end . when you query MyHashmap objects getPutCount()/getGetCount() you should be getting the exact number of time the call was made to get/put method.

solution:

1) In case if map allows null keys then hashcode() of key doesn't get called. So hashcode() of key class can't be used for counting purpose. Hence Override will work as explained above. May be use decorator pattern.

2) How about implementing custom collection class called as MyHashMap which is very well supported by Java Collection Framework with the help of template such as AbstractMap etc.

Dude how about java Observer Class? Exploiting the observer design pattern.

Hint : Add such behaviour to Map using dynamic proxies.