Java generics type erasure
erasure, generics, java
Solution
Because the compiler doesn't know that `new Circle` is valid. Consider this code:
Base<? extends Shape> bs = new Base<Square>(); // Really a Base<Square>
bs.setX(new Circle());
(FYI, a very similar example is given in the Java tutorial on wildcards.)
You may now exclaim "But the compiler can see it's really a `Base<Square>`!". But not in general. Consider this:
Base<? extends Shape> bs = someInterface.getBaseOfSomeKindOfShape();
bs.setX(new Circle());
Problem
I'd like to ask about java type erasure rules. If we have classes: ``` public class Shape{} public class Circle extends Shape{} public class Base<T extends Shape>{ T x; public void setX(T t){} } public class MainClass(){ public static void main(String... _arg){ Base<? extends Shape> bs = new Base<Circle>(); bs.setX(new Circle()); // <- compilation problem } } ``` Can you please explain me why calling setX() method causes compilation problem?