java - 在父类之间来回转换后，子对象是否会丢失其独特的属性

Question

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;
    }
}

如果我要创建一个 Blackberry 实例，将其转换为 Phone 对象，然后再将其转换回 Blackberry，原始 Blackberry 对象丢失其成员变量？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

转换根本不会改变底层对象 - 它只是向编译器发出的消息，它可以将 A 视为 B。

如果 A extends B，也没有必要将 A 转换为 B，即您不需要转换子类型到它的父类(super class)型；如果它是从父类(super class)型到子类型，你只需要强制转换