GENERICS - Introduction to Java Programming: Comprehensive Version - page 748

11 /** Find the maximum in a stack of numbers */

12 public static double max(GenericStack<Number> stack) {

GenericStack<Number>
type

13 double max = stack.pop().doubleValue(); // Initialize max

14
15 while (!stack.isEmpty()) {
16 double value = stack.pop().doubleValue();
17 if (value > max)
18 max = value;
19 }
20
21

return max;

22 }
23 }

The program in Listing 19.7 has a compile error in line 8 because intStack is not an instance
of GenericStack<Number> . Thus, you cannot invoke max(intStack) .
The fact is that Integer is a subtype of Number , but GenericStack<Integer> is not
a subtype of GenericStack<Number> . To circumvent this problem, use wildcard generic
types. A wildcard generic type has three forms: ? and ? extends T , as well as ? super T ,
where T is a generic type.
The first form, ? , called an unbounded wildcard , is the same as ? extends Object . The
second form, ? extends T , called a bounded wildcard , represents T or a subtype of T . The
third form, ? super T , called a lower-bound wildcard , denotes T or a supertype of T .
You can fix the error by replacing line 12 in Listing 19.7 as follows:

unbounded wildcard
bounded wildcard
lower-bound wildcard

public static double max(GenericStack<? extends Number> stack) {

<? extends Number> is a wildcard type that represents Number or a subtype of Number ,
so it is legal to invoke max(new GenericStack<Integer>()) or max(new
GenericStack<Double>()) .
Listing 19.8 shows an example of using the ? wildcard in the print method that prints
objects in a stack and empties the stack. <?> is a wildcard that represents any object type. It
is equivalent to <? extends Object> . What happens if you replace GenericStack<?>
with GenericStack<Object> ? It would be wrong to invoke print(intStack) ,
because intStack is not an instance of GenericStack<Object> . Please note that
GenericStack<Integer> is not a subtype of GenericStack<Object> , even though
Integer is a subtype of Object .

L ISTING 19.8

AnyWildCardDemo.java
1 public class AnyWildCardDemo {
2 public static void main(String[] args ) {
3 GenericStack<Integer> intStack = new GenericStack<>();
4 intStack.push( 1 ); // 1 is autoboxed into new Integer(1)
5 intStack.push( 2 );
6 intStack.push( -2 );
7
8 print(intStack);
9 }
10
11 /** Prints objects and empties the stack */
12 public static void print(GenericStack<?> stack) {
13 while (!stack.isEmpty()) {
14 System.out.print(stack.pop() + " " );
15 }
16 }
17 }

GenericStack<Integer>
type

wildcard type