Why it is necessary to override hashcode and equals method of key for Hashmap? [duplicate]

问题

This question already has answers here:

What issues should be considered when overriding equals and hashCode in Java? (11 answers)

Closed 6 years ago.

I created a HashMap having Student as key and String as value.

Now everywhere i have read It is necessary to override equals and hashcode method if using as a key for hashmap.

But I did not override it. And insert multiple key value pairs in hashmap. I am also able to fetch it back.

So why it is necessary?

回答1:

They are required when you want to use your objects as useful hashing keys into collections that use hashing.

Have your tried to add the Student objects which are meaningfully equal? First understand the significance of equals method in java

Read the significance of equals and hashcode

来源：https://stackoverflow.com/questions/18606447/why-it-is-necessary-to-override-hashcode-and-equals-method-of-key-for-hashmap