Difference in "? extends Object" and "Object" in Generics

[amit punekar]

Hello,
I want to create a HashMap that would hold String keys and values of type [Integer,Date,String.Boolean,Double].
I did

HashMap<String,? extends Object> map = new HashMap<String,? extends Object>();   

This gave me error while trying to put the different types of Object into the Map as value.
When I reverted back to the code below, everything compiled fine.

HashMap<String,Object> map = new HashMap<String,Object>();   

I could not relate Generic fundamentals I have read to this difference.
AFAIK instead of talking in terms of Base Object class , that might give ClassCastException at later point of time, its better to use Generic types.

Thanks in Advance

Regards,
Amit

[Ryan Beckett]

Java Generics - Wildcards

[Rob Spoor]

With Map<String,? extends Object>, you don't know the actual type - it may be Map<String,Object>, but also Map<String,String> or Map<String,Integer>. Therefore, you cannot put anything into the map. With Map<String,Object>, you know the actual type is Object so you can put any object into the map.

[Dhruva Mistry]

amit punekar wrote:Hello,
I want to create a HashMap that would hold String keys and values of type [Integer,Date,String.Boolean,Double].
I did

HashMap<String,? extends Object> map = new HashMap<String,? extends Object>();   

This gave me error while trying to put the different types of Object into the Map as value.
When I reverted back to the code below, everything compiled fine.

HashMap<String,Object> map = new HashMap<String,Object>();   

I could not relate Generic fundamentals I have read to this difference.
AFAIK instead of talking in terms of Base Object class , that might give ClassCastException at later point of time, its better to use Generic types.

Thanks in Advance

Regards,
Amit

by default every API classes are children of Parent- Object class so for any of Wrapper classes you need not no extend that way.
else, better to use

 HashMap<String, Object> 

so that you can convert the needed values also.

[amitabh mehra]

amit punekar wrote:

HashMap<String,? extends Object> map = new HashMap<String,? extends Object>();   

Also, AFAIK, you cannot have a wildcard notation in object creation i.e.

new HashMap<String, ? extends Object> 

will throw compile time error. but not this:

new HashMap<String, Object> 

And if you are using wildcard in your references like in your case, you will not be able to *add* to the collection.
This is what I had read in SCJP Sun Certified Programmer for Java5 Study Guide.

[amit punekar]

Hi
@Rob,
Thanks for the explanation.
It does explain me what I understood wrong about wild-cards. But then I really did not understand in which case I can use "? extends Object" .
@amitabh,
The code compiles fine. But when you try to put Integer,String of different Java Wrapper classes inside it, compiler does not allow as Rob explained.
@Ryan.
I had gone through the Generics tutorial and hence thought of using the "? extends Object". It was my mistake that what I understood from tutorial could not apply to the scenario I was handling.

thanks to all once again for your kind help.

Regards,
Amit