HashMap can store only object key and value

Question

HashMap does not take primitive types for the key and value, but we can still store values of primitive types easily as follows:

HashMap h = new HashMap();

h.put(1,1);

How is it possible?

Learn about generics and interfaces. Your code should be `Map h = new HashMap();` — Paul Boddington, Dec 22 '14 at 11:55
yes java converts `int` to `Integer` by using auto-boxing and in-boxing so what you are expecting? — atish shimpi, Dec 22 '14 at 11:56
Your question is very unclear (and could do with better formatting too). Please read http://tinyurl.com/stack-hints and edit your question to clarify it. — Jon Skeet, Dec 22 '14 at 11:58

Eran · Answer 1 · 2014-12-22T12:17:55.297

2

When you store primitives in a HashMap (or any Collection), they are boxed into their reference types. An int is boxed to an Integer.

edited Dec 22 '14 at 12:17

answered Dec 22 '14 at 11:53

Eran

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downvoted for answering without a question. – Suresh Atta Dec 22 '14 at 11:54
Can we encourage this type of questions by answering them in seconds? – Suresh Atta Dec 22 '14 at 11:58

score 2 · Answer 2 · edited May 23 '17 at 10:26

That is because 1 is autoboxed to an Integer (actually the same as this: Integer.valueOf(1)). Find more info about autoboxing here.

int is boxed to Integer
long is boxed to Long
double is boxed to Double
float is boxed to Float
boolean is boxed to Boolean

In your example you are using the raw type of the Map. The Map-object that you declared should most likely be declared like this:

Map<Integer, Integer> h = new HashMap<>();
h.put(1, 1);

Read more about raw types in this SO-question.

score 1 · Answer 3 · answered Dec 22 '14 at 11:54

Primitives are autoboxed by the compiler and inserted as a wrapper object. So in your case it will be automatically boxed as Integer object. See this for details. Every primitive type has corresponding wrapper object with some of them like:

primitive - Object  
   byte   -  Byte
   char   -  Character
   short  -  Short
   int    -  Integer
   long   -  Long
   double -  Double
   float  -  Float
   boolean - Boolean

score 0 · Answer 4 · answered Dec 22 '14 at 11:59

As any Java programmer knows, you can't put an int (or other primitive value) into a collection.

Collections can only hold object references, so you have to box primitive values into the appropriate wrapper class (which is Integer in the case of int).

When you take the object out of the collection, you get the Integer that you put in; if you need an int, you must unbox the Integer using the intValue method. All of this boxing and unboxing is a pain, and clutters up your code.

The autoboxing and unboxing feature automates the process, eliminating the pain and the clutter.

refer java docs for more details

HashMap can store only object key and value

4 Answers4