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Generics

Minu Sam
Greenhorn

Joined: Apr 26, 2012
Posts: 10
I am having hard time understanding generics.
Could someone explain why line 2 compilation fails.??


}
Assuming Dog extends Animal
Henry Wong
author
Sheriff

Joined: Sep 28, 2004
Posts: 18702
    
  40

Minu Sam wrote:I am having hard time understanding generics.
Could someone explain why line 2 compilation fails.??


}
Assuming Dog extends Animal



Simply, lower bounds are not supported with generic methods. That's how it is defined.

Henry

Books: Java Threads, 3rd Edition, Jini in a Nutshell, and Java Gems (contributor)
Minu Sam
Greenhorn

Joined: Apr 26, 2012
Posts: 10
Henry,

Appreciate your response, however can you please explain what you mean by lower bounds?I still dont understand
Henry Wong
author
Sheriff

Joined: Sep 28, 2004
Posts: 18702
    
  40

Minu Sam wrote:
Appreciate your response, however can you please explain what you mean by lower bounds?I still dont understand


Java generics allow you to bound the type. You can specify an upper bound via <T extends UpperBound>, or you can specify a lower bound via <T super LowerBound>. With generic methods, only upper bounds are supported.

Henry
Helen Ma
Ranch Hand

Joined: Nov 01, 2011
Posts: 451


This wont' compile because p can be Animal, Animal's super type or Object.
What happens if a programmer does this inside addAgainWithSuper method :
List<Animal> l = new ArrayList<Animal>();
l.add(p);

?
What happens if p is an object? The list l cannot add p to it.

Or what happens if a program writes this:
Animal a = p; //where p is an object.
It will have type mismatch compilation error.

Since the programmer can write code in the method that may violation compilation rules or not type safe, the compiler won't let the addAgainWithSuper to be compiled as p may be Animal, Animal's super type or even object. During compilation time, the compiler does not know what type of p the programmer will pass as the argument.

Correct me if I am wrong.
 
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subject: Generics