java，从A <t>转换为A <s extends =“”t =“”> </s> </t>

Question

我想知道下面的代码是如何工作的。我认为它投了A＆lt; T＆GT;输入到A＆lt; S扩展T>，这不应该起作用，例如，你不能用A＆lt;数＆GT;到A＆lt;整数取代。但是调用A.returnB（）只会导致警告：未经检查或不安全的操作。

class A<T>{
    public static <S extends Number> A<S>  returnB(){
    return (A<S>) B.INSTANCE;
    }
}

class B extends A<Number>{
    static final B INSTANCE = new B();
}

Answer 1

缺点是Java使用类型擦除实现泛型，并允许您回退到原始类型（pre-generics）以实现向后兼容性，因此允许 不安全的转换。它们带有警告，并且可能在运行时导致ClassCastExceptions，但在某些情况下它们也可以合法使用，所以它是允许的。