String as a key in HashMap

Question

I had seen, only the String is used as a key in HashMap.Although the put() method takes Object as a parameter.What is the significant of it.If any other object can also used as a Key or not? Please provide the answers.

Answer 1

Any object that provides a meaningful implementation of hashCode() is a perfect key candidate in a map: see Understanding the workings of equals and hashCode in a HashMap.

Also, as @Jon mentioned, all keys in your map should be of the same type.

EDIT: Of course, you need to implement both equals() and hashcode() . I thought the title of the link to the other question made that clear. But a dumb implementation of hashcode() will just bring you a degenerate HashMap which performance is poor.

EDIT2: As @Adrian mentioned in his answer, generics will help you constrain the type of keys and values for the map.

References:

• Effective Java Programming Language Guide, sample chapter about equals() and hashCode()
• Java theory and practice: Hashing it out
• Equals and HashCode