在父类之间来回转换后，子对象是否失去其唯一属性

Question

请考虑以下类：

public class Phone {
    private boolean has3g;

    public boolean has3g() {
        return has3g;
    }

    public void setHas3g(boolean newVal) {
        has3g = newVal;
    }
}

public class Blackberry extends Phone {
    private boolean hasKeyboard;

    public boolean hasKeyboard() {
        return hasKeyboard;
    }

    public void setHasKeyboard(boolean newVal) {
        hasKeyboard = newVal;
    }
}

如果我要创建blackberry的实例，将其转换为phone对象，然后将其转换回blackberry，那么原始的blackberry对象是否会丢失其成员变量？例如：

Blackbery blackbery = new Blackberry();
blackbery.setHasKeyboard(true);

Phone phone = (Phone)blackbery;

Blackberry blackberry2 = (Blackberry)phone;

// would blackberry2 still contain its original hasKeyboard value?
boolean hasKeyBoard = blackberry2.hasKeyboard();

Answer 1

强制转换根本不会更改底层对象--它只是向编译器发出一条消息，说明它可以将a视为B。

如果a扩展了B，也不需要将a强制转换为B，即不需要将子类型强制转换为它的超类型；只有从超类型转换为子类型时才需要转换