Generic Question

[Sagar Shroff]

[b]

abstract class Test<K extends Number>
{
 public abstract <K> Test<K> useMap(Test<K> s);
}

[/b]
Can some one make me explain why this is not compiling.The compiler is telling me that Parameter is not within Bounds......

Also not understanding then how this is working ??
[b]

abstract class Test<K extends Number>
{
 public abstract <? extends Number> Test<K> useMap(Test<? super K> s);
}

[/b]

Please Help me ??

Examples would be really appreciated !!

Can someone please help me with an example of my 2 Questions ??

[Ankit Garg]

abstract class Test<K extends Number>
{
 public abstract <K> Test<K> useMap(Test<K> s);
}

In this case you are declaring a new K with your method declaration. Basically the code is equivalent to this code

abstract class Test<K extends Number>
{
 public abstract <M> Test<M> useMap(Test<M> s);
}

Now here M might be resolved to something not compatible with Number, like lets say String. But Test class can only have a type which is a Number. So you'll have to bound M to a number.

abstract class Test<K extends Number>
{
 public abstract <? extends Number> Test<K> useMap(Test<? super K> s);
}

Here first of all the type declaration in the method is useless. So we'll strip it out first

abstract class Test<K extends Number>
{
 public abstract Test<K> useMap(Test<? super K> s);
}

Now the return type would clearly work. As for the parameter, since you are using a wildcard syntax, so compile would be able to maintain type safety here. Lets actually put some code into the class to see how

abstract class Test<K extends Number>
{
   K me;
   public Test<K> useMap(Test<? super K> s) {
       s.me = new Integer();  //not allowed, type of s is not known
       return null;
   }
}

Basically you can't make changes to s variable based on its type as the type is not known. So even though ? super K seems to be breaking the bounds of K, it will still result in type safe code which is the goal of generics...

[Sagar Shroff]

Thanks Ankit !!