Question

I am creating a generic class that has a method that is supposed to sort through an array collection that may either be a Book or it may be a CD. I implemented Comparable in the two type classes and am trying to sort the array in the GenericCollection class.

My method looks like this right now:

public void sort(){
    for (int i = 0; i < collection.length; i++) {
        for (int j = 0; j < collection.length; j++) {
            if (((T)collection[i]).compareTo((T)collection[j]) > 0){
                T t = (T)collection[i];
                collection[i] = collection[j];
                collection[j] = t;
            }                   
        }
    }
}

This gives me the error that Object is not comparable. If I try to cast to type T then I get the error that T is undefined for CompareTo. If I try something like:

<T extends Comparable>

I can compile but I get a runtime error even though I have Comparable defined in both types that are using this class.

How do I get the Generic type to be able to run the compareTo methods in their own classes.

Thanks for any help.

Answer 1

If you need to implement your own sort algorithm, try to declare the type parameter of your generic collection class as

 <T extends Comparable<T>>

This means, your class would be declared like this:

 class GenericCollection<T extends Comparable<T>> {  ... }

And of course, your book or CD classes should implement this exact type:

 class Book implements Comparable<Book> { ... }

 class CD implements Comparable<CD> { ... }

If you use the parameter <T extends Comparable>, you are using a raw type, and these are only for compatibility with old (pre-generics) code, should never be used in new code.