Question

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Why are variables declared with their interface name in Java?
How should lists be cast to their conrecte implementations?

Example:

Map<Integer, Integer> map = new HashMap<Integer, Integer>();

Thanks.

Answer 1

This is usually so that you can swap in another implementation later and still have the code operate the same. For example, if you then decided to use a TreeMap instead of a HashMap, you could just change the instantiation step, and the rest of your code would still work just fine.