HashMap declaration

what is difference between below threee HashMap declarations

1.HashMap<String,Integer> map=new HashMap<String,Integer>();
2.Map<String,Integer> map=new HashMap<String,Integer>();
3.HashMap<String,int> map=new HashMap<String,int>();

why i cannot pass primitive to generics?
please advise
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3.  Primitives are held on the Stack; Objects on the heap. You can't keep an Object on the stack. *

2. See this answer from Doug

1. Now, HashMap is tightly bound. See point 2 above.

* and complementary-wise, therefore, stack members - primitives such as int - are not held on the heap. The stack is accessible per method in any case, so it would not be possible to share members on a stack across different methods in the same programme, so that would make practically all code unshareable and hence effectively useless.

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System Programming

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