Warning: [rawtypes] found raw type: Comparable?

Question

Java

Greetings! In Algorithms in Java by Robert Sedgwick, the second chapter uses an ADT like this . The compiler constantly issues warnings about its use of Comparable as a data type. Here is one of the warnings

Selection.java:3: warning: [rawtypes] found raw type: Comparable
public static void sort(Comparable[] a) {
missing type arguments for generic class Comparable where T is a type-variable:
T extends Object declared in interface Comparable

.
Why is this warning being issued? And how to avoid it?