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I have declared following HashMap

Map<Integer,String> map = new HashMap<>(); // Java 6+

I have two questions :
A) Is there any Issue(Performance or Other) , while I put element in HashMap with primitive data type for above declare HashMap for example 

map.put(1,"abc");

Or I have to use only 

map.put(new Integer(1),"abc");

B) Which is good candidate for HashMap if I have number(int) as a Key
String OR Integer

realPK
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mcacorner
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    You can't use generics with primitives. You'll end up with `Integer` either way due to autoboxing. In fact, autoboxing *might* be faster because you won't always allocate a `new Integer()`; you can negate this advantage by using `Integer.valueOf()` instead of `new Integer()`. – awksp Jun 13 '14 at 05:53

4 Answers4

2

You should use 

Integer.valueOf(1)

instead of using the new keyword. Java stores values -128 thru 127 in a IntegerCache, and will return the already allocated value from the heap.

As for primitive v/s Integer object, java internally autoboxes both ways, so it will not make a difference.

Siddhartha
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This is effectively the same. Java is using auto boxing to convert 

map.put(1,"abc");

to

map.put(Integer.valueOf(1),"abc");

during compile time.

If you need better performace, look at external libraries supporting primivites as key in maps.

rmuller
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I think you can use any one but the important thing is key should preferably not be mutable as this can cause aberrant behavior.

Java HashMap relies on two things:

  • the hashCode() method, which returns an integer that is generated from the key and used inside the map
  • the equals(..) method, which should be consistent to the hash calculated, this means that if two keys has the same hashcode than it is desiderable that they are the same element.

So if you are planning to use just simple types as keys (like you said integers or strings) just don't worry, there's no difference. In both cases two different items will have the same hashcode.

Rahul Tripathi
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When you are using the map.put(1,"abc");, the java compiler internally uses the Integer.valueOf(8) method

As for the performance question, I suggest you refer to String as key in hashmap

Community
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Ashish
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