GENERICS - Introduction to Java Programming: Comprehensive Version - page 749

When is the wildcard <? super T> needed? Consider the example in Listing 19.9. The
example creates a stack of strings in stack1 (line 3) and a stack of objects in stack2 (line 4),
and invokes add(stack1, stack2) (line 8) to add the strings in stack1 into stack2 .
GenericStack<? super T> is used to declare stack2 in line 13. If <? super T> is
replaced by <T> , a compile error will occur on add(stack1, stack2) in line 8, because
stack1 's type is GenericStack<String> and stack2 's type is GenericStack<Object> .
<? super T> represents type T or a supertype of T . Object is a supertype of String .

why <? Super T>

L ISTING 19.9

SuperWildCardDemo.java
1 public class SuperWildCardDemo {
2 public static void main(String[] args) {
3 GenericStack<String> stack1 = new GenericStack<>();
4 GenericStack<Object> stack2 = new GenericStack<>();
5 stack2.push( "Java" );
6 stack2.push( 2 );
7 stack1.push( "Sun" );
8 add(stack1, stack2);
9 AnyWildCardDemo.print(stack2);
10 }
11
12 public static <T> void add(GenericStack<T> stack1,
13 GenericStack<? super T> stack2) {
14 while (!stack1.isEmpty())
15 stack2.push(stack1.pop());
16 }
17 }

GenericStack<String>
type

<? Super T> type

This program will also work if the method header in lines 12-13 is modified as follows:

public static <T> void add(GenericStack<? extends T> stack1,
GenericStack<T> stack2)

The inheritance relationship involving generic types and wildcard types is summarized in
Figure 19.6. In this figure, A and B represent classes or interfaces, and E is a generic type
parameter.

Object Object

? super E E's superclass A<?>

?

E A<? extends B> A<? super B>

E's subclass ? extends E A<B's subclass> A<B> A<B's subclass>

F IGURE 19.6

The relationship between generic types and wildcard types.

19.14

✓ ✓

Is GenericStack the same as GenericStack<Object> ?

Check

19.15

What are an unbounded wildcard, a bounded wildcard, and a lower-bound wildcard?

Point

19.16

What happens if lines 12-13 in Listing 19.9 are changed to

public static <T> void add(GenericStack<T> stack1,
GenericStack<T> stack2)