Contents
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?
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? extends T
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? super T ,
where T is a generic type.
Examples
unbounded wildcard example
Example 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 .
import DefineGenericClassAndInterface.GenericStack;
public class AnyWildCardDemo {
public static void main(String[] args) {
GenericStack<Integer> intStack = new GenericStack();
intStack.push(1); // 1 is autoboxed into new Integer(1)
intStack.push(2);
intStack.push(-2);
print(intStack);
}
/**
* Prints objects and empties the stack
*/
public static void print(GenericStack<?> stack) {
while (!stack.isEmpty()) {
System.out.print(stack.pop() + " ");
}
}
}
If we change print method to below will cause an error.
public static void print(GenericStack<Object> stack) {
while (!stack.isEmpty()) {
System.out.print(stack.pop() + " ");
}
}
Exception in thread "main" java.lang.ClassCastException: java.base/java.lang.Integer cannot be cast to java.base/java.lang.String
at BadExample.main(BadExample.java:14)
Process finished with exit code 1
lower-bounded wildcard example
package superwildcarddemo;
import DefineGenericClassAndInterface.GenericStack;
public class SuperWildCardDemo {
public static void main(String[] args) {
GenericStack<String> stack1 = new GenericStack();
GenericStack<Object> stack2 = new GenericStack();
stack2.push("Java");
stack2.push(2);
stack1.push("Sun");
add(stack1, stack2);
}
public static <T> void add(GenericStack<T> stack1, GenericStack<? super T> stack2) {
while (!stack1.isEmpty())
stack2.push(stack1.pop());
}
//If we change GenericStack<? super T> to GenericStack<T> will cause an error
// public static <T> void add(GenericStack<T> stack1, GenericStack<T> stack2) {
// while (!stack1.isEmpty())
// stack2.push(stack1.pop());
// }
}
bound wildcard example
package boundWildcard;
import DefineGenericClassAndInterface.GenericStack;
public class BoundWildCardDemo {
public static void main(String[] args) {
GenericStack<String> stack1 = new GenericStack();
GenericStack<Object> stack2 = new GenericStack();
stack2.push("Java");
stack2.push(2);
stack1.push("Sun");
add(stack1, stack2);
diaplyStackElement(stack2);
}
public static <T> void add(GenericStack<? extends T> stack1, GenericStack<T> stack2) {
while (!stack1.isEmpty())
stack2.push(stack1.pop());
}
public static <T> void diaplyStackElement(GenericStack<T> stack) {
while (!stack.isEmpty()) {
System.out.println(stack.pop());
}
}
}