java - 字符串到对象类型转换 - 区别

Question

有什么区别。

public class Test {

    public static void main(String args[]) {
        String toBeCast = "cast this string";
        A a = toBeCast; // error - Type mismatch: cannot convert from String to A
        Object object = toBeCast;
    }
}


public class A {

}

当我们说每个对象都扩展Object class时，为什么不允许A a = toBeCast;，但是这个Object object = toBeCast;工作正常。

Answer 1

记住几何课上的那句老话 - “每个正方形都是矩形，但并非每个矩形都是正方形”。将其概括为:“每个正方形/平行四边形/菱形都是多边形，但并非每个多边形都是正方形/平行四边形/菱形”。

这是你正在做的:

String toBeCast = "cast this string" //this rhombus is a rhombus: cool!
A a = toBeCast; //this parallelogram is that rhombus : WTF? that doesn't make sense!  
Object object = toBeCast; //this polygon is that rhombus: cool!