Internal implementation of java.util.HashMap and HashSet

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I have been trying to understand the internal implementation of java.util.HashMap and java.util.HashSet.

Following are the doubts popping in my mind for a while:

  1. Whats is the importance of the @Override public int hashcode() in a HashMap/HashSet? Where is this hash code used internally?
  2. I have generally seen the key of the HashMap be a String like myMap<String,Object>. Can I map the values against someObject (instead of String) like myMap<someObject, Object>? What all contracts do I need to obey for this happen successfully?

Thanks in advance !

EDIT:

  1. Are we saying that the hash code of the key (check!) is the actual thing against which the value is mapped in the hash table? And when we do myMap.get(someKey); java is internally calling someKey.hashCode() to get the number in the Hash table to be looked for the resulting value?

Answer: Yes.

EDIT 2:

  1. In a java.util.HashSet, from where is the key generated for the Hash table? Is it from the object that we are adding eg. mySet.add(myObject); then myObject.hashCode() is going to decide where this is placed in the hash table? (as we don't give keys in a HashSet).

Answer: The object added becomes the key. The value is dummy!

language-implementation hashcode hashmap hashset java
9 Answers

HashCode method for collection classes like HashSet, HashTable, HashMap etc – Hash code returns integer number for the object that is being supported for the purpose of hashing. It is implemented by converting internal address of the object into an integer. Hash code method should be overridden in every class that overrides equals method. Three general contact for HashCode method

  • For two equal objects acc. to equal method, then calling HashCode for both object it should produce same integer value.

  • If it is being called several times for a single object, then it should return constant integer value.

  • For two unequal objects acc. to equal method, then calling HashCode method for both object, it is not mandatory that it should produce distinct value.

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