Generic Methods and Bounded Type Parameters

Bounded type parameters are key to the implementation of generic algorithms. Consider the following method that counts the number of elements in an array T[] that are greater than a specified element elem.

public static <T> int countGreaterThan(T[] anArray, T elem) {
 int count = 0;
 for (T e : anArray)
 if (e > elem) // compiler error
 ++count;
 return count;
}

The implementation of the method is straightforward, but it does not compile because the greater than operator (>) applies only to primitive types such as short, int, double, long, float, byte, and char. You cannot use the > operator to compare objects. To fix the problem, use a type parameter bounded by the Comparable<T> interface:

public interface Comparable<T> {
 public int compareTo(T o);
}

The resulting code will be:

public static <T extends Comparable<T>> int countGreaterThan(T[] anArray, T elem) {
 int count = 0;
 for (T e : anArray)
 if (e.compareTo(elem) > 0)
 ++count;
 return count;
}