Wildcard in Java Generics

he ? (question mark) symbol represents the wildcard element. It means any type. If we write <? extends Number>, it means any child class of Number, e.g., Integer, Float, and double. Now we can call the method of Number class through any child class object.

We can use a wildcard as a type of a parameter, field, return type, or local variable. However, it is not allowed to use a wildcard as a type argument for a generic method invocation, a generic class instance creation, or a supertype.

Let's understand it by the example given below:

1. import java.util.*;  
2. abstract class Shape{  
3. abstract void draw();  
4. }  
5. class Rectangle extends Shape{  
6. void draw(){System.out.println("drawing rectangle");}  
7. }  
8. class Circle extends Shape{  
9. void draw(){System.out.println("drawing circle");}  
10. }  
11. class GenericTest{  
12. //creating a method that accepts only child class of Shape  
13. public static void drawShapes(List<? extends Shape> lists){  
14. for(Shape s:lists){  
15. s.draw();//calling method of Shape class by child class instance  
16. }  
17. }  
18. public static void main(String args[]){  
19. List<Rectangle> list1=new ArrayList<Rectangle>();  
20. list1.add(new Rectangle());  
21.   
22. List<Circle> list2=new ArrayList<Circle>();  
23. list2.add(new Circle());  
24. list2.add(new Circle());  
25.   
26. drawShapes(list1);  
27. drawShapes(list2);  
28. }}