Sort method and Generics

[Naresh Shanmugam]

Sort method in the Collections class looks like this:

public static <T extends Comparable<? super T>> void sort(List<T> list)

I understood the significance of <T extends Comparable> which means the list which implements the comparable interface could be passed as an argument to the sort method.

Consider the below example
class Animal{}
class Dog extends Animal implements Comparable{}

Now i could pass the instance of Dog as the parameter to sort method...

Can any body tell the significance of having <? super T> here Please??

[Rob Spoor]

Take as example java.sql.Timestamp. This class extends java.util.Date which implements Comparable<Date>. Therefore, Timestamp also implements Comparable<Date>.
The <? super T> simply takes these classes into account. If you would have limited it to <T extends Comparable<T>> then you could never use Timestamp, or for example any class that would extend Dog.

[Naresh Shanmugam]

So a class which directly implements Comparable interface or a class whose super class implements Comparable interface could be passed as a parameter to sort() method.
Rob, please correct me if i am wrong...

[Rob Spoor]

A class X that implements Comparable<X> -- in other words, any instance can be compared to any other instance. Or a sub class of such a class.

It's the <X> that's very important. I can write a class that can compare itself to Strings:

public class Test implements Comparable<String>
{
    String name;

    // other code

    public int compareTo(String s) { return name.compareTo(s); }
}

Because instances of Test cannot be compared to other instances of Test, only to String, it cannot be used in sort().

[Naresh Shanmugam]

Thank you so much for your valuable guidance Rob.. I got the point
I want to become stronger in Generics Could you please suggest me any URL where everything about Generics is explained clearly (or) could you suggest me any text book please...

[Rob Spoor]

Go to Sun's (or Oracle's these days) tutorial: http://download-llnw.oracle.com/javase/tutorial/