[英]Java generics type erasure
I'd like to ask about java type erasure rules. 我想问一下Java类型擦除规则。
If we have classes: 如果我们有课程:
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? 您能解释一下为什么调用setX()方法会导致编译问题吗?
Because the compiler doesn't know that new Circle
is valid. 因为编译器不知道
new Circle
是有效的。 Consider this code: 考虑以下代码:
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 .) (仅供参考, Java教程中关于通配符的例子非常相似。)
You may now exclaim "But the compiler can see it's really a Base<Square>
!". 您现在可能会惊叹“但是编译器可以看到它实际上是
Base<Square>
!”。 But not in general. 但不是一般。 Consider this:
考虑一下:
Base<? extends Shape> bs = someInterface.getBaseOfSomeKindOfShape();
bs.setX(new Circle());
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.