Bounded Type Parameters

There may be times when you’ll want to restrict the kinds of types that are allowed to be passed to a type parameter. For example, a method that operates on numbers might only want to accept instances of Number or its subclasses. This is what bounded type parameters are for.

To declare a bounded type parameter, list the type parameter’s name, followed by the extends keyword, followed by its upper bound.

Example

Following example illustrates how extends is used in a general sense to mean either “extends” (as in classes) or “implements” (as in interfaces). This example is Generic method to return the largest of three Comparable objects −

public class MaximumTest {
   // determines the largest of three Comparable objects
   
   public static <T extends Comparable<T>> T maximum(T x, T y, T z) {
  T max = x;   // assume x is initially the largest
  
  if(y.compareTo(max) &gt; 0) {
     max = y;   // y is the largest so far
  }
  
  if(z.compareTo(max) &gt; 0) {
     max = z;   // z is the largest now                 
  }
  return max;   // returns the largest object   
   }
   
   public static void main(String args[]) {
  System.out.printf("Max of %d, %d and %d is %d\n\n", 
     3, 4, 5, maximum( 3, 4, 5 ));
  System.out.printf("Max of %.1f,%.1f and %.1f is %.1f\n\n",
     6.6, 8.8, 7.7, maximum( 6.6, 8.8, 7.7 ));
  System.out.printf("Max of %s, %s and %s is %s\n","pear",
     "apple", "orange", maximum("pear", "apple", "orange"));
   }
}

This will produce the following result −

Output

Max of 3, 4 and 5 is 5

Max of 6.6,8.8 and 7.7 is 8.8

Max of pear, apple and orange is pear