Java Generics 简明教程

Java Generics - No Cast

不允许强制转换为参数化类型，除非该类型由无界通配符参数化。

Box<Integer> integerBox = new Box<Integer>();
Box<Number> numberBox = new Box<Number>();
//Compiler Error: Cannot cast from Box<Number> to Box<Integer>
integerBox = (Box<Integer>)numberBox;

要达到同样的效果，可以使用无界通配符。

private static void add(Box<?> box) {
   Box<Integer> integerBox = (Box<Integer>)box;
}