Using generics in Comparable

Question

I am trying to implement generics in Java using Comparable<T> interface.

public static <T> T[] sort(T[] a) {
    //need to compare 2 elements of a
}

Let's say, I want to override the compareTo method for the above type T in the Comparable interface. Ie I need to compare two elements of my type T , how will I do it? I don't know what my T type will be.

Answer 1

You need to set a type constraint on your method.

public static <T extends Comparable<? super T>> T[] sort (T[] a)
{
         //need to compare 2 elements of a
}

This forces the type T to have the compareTo(T other) method. This means you can do the following in your method:

if (a[i].compareTo(a[j]) > 0) }

}

Answer 2

Old question but...

As jjnguy responded, you need to use:

public static <T extends Comparable<? super T>> T[] sort(T[] a) {
  ...
}

Consider the following:

public class A implements Comparable<A> {}
public class B extends A {}

The class B implicitly implements Comparable<A> , not Comparable<B> , hence your sort method could not be used on an array of B 's if used Comparable<T> instead of Comparable<? super T> Comparable<? super T> . To be more explicit:

public static <T extends Comparable<T>> T[] brokenSort(T[] a) {
   ...
}

would work just fine in the following case:

A[] data = new A[3];
...
data = brokenSort(A);

because in this case the type parameter T would be bound to A . The following would produce a compiler error:

B[] data = new B[3];
...
data = brokenSort(B);

because T cannot be bound to B since B does not implement Comparable<B> .

Answer 3

尝试使用<T extends Comparable<T>>然后compareTo