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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) > 0) {
max = y; // y is the largest so far
}
if(z.compareTo(max) > 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 %dnn",
3, 4, 5, maximum( 3, 4, 5 ));
System.out.printf("Max of %.1f,%.1f and %.1f is %.1fnn",
6.6, 8.8, 7.7, maximum( 6.6, 8.8, 7.7 ));
System.out.printf("Max of %s, %s and %s is %sn","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
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