Difference between Pair and Hashmap?

Question

What is the necessity to introduce Pair class when Hashmap can do the same job?

I see Pair being introduced to Java version 8

Answer 1

Your choice of which class to use is not just a message to your computer. It's also a message to future developers - people who will maintain your code in the future, or even you yourself in a few months time.

By choosing whether to declare a particular variable as either HashMap or Pair, you're telling those future developers something. It's EITHER

This variable references some kind of map, which uses a hash algorithm for fast retrieval.

OR

This variable references a pair of values.

That will help the future developers to understand what your code is doing. Whereas you can certainly use a HashMap with a single entry instead of a Pair, it would be a very strange thing to do, and it would send entirely the wrong message to the future maintainers of your code.

Answer 2

A pair is basically a convenient way of associating a simple key to a value. Maps do the same thing to store key-value pairs but maps stores a collection of pairs and operate them as a whole.

Number of times we have a requirement where a key-value pair shall exist on its own, for instance:

• A key-value pair needs to be passed to a method as an argument, Or
• A method needs to return just two values in form of a pair

Map complicates the things when we just need a single pair of key-value.