Java多态中重寫static函數

Pitfall: fields and static methods

Once you learn about polymorphism, you can begin to think that everything happens polymorphically. However, only ordinary method calls can be polymorphic. For example, if you access a field directly, that access will be resolved at compile time, as the following example demonstrates:

//: polymorphism/FieldAccess.java
// Direct field access is determined at compile time.
class Super {
    public int field = 0;
    public int getField() { return field; }
}
class Sub extends Super {
    public int field = 1;
    public int getField() { return field; }
    public int getSuperField() { return super.field; }
}
public class FieldAccess {
    public static void main(String[] args) {
        Super sup = new Sub(); // Upcast
        System.out.println("sup.field = " + sup.field +
                  ", sup.getField() = " + sup.getField());
        Sub sub = new Sub();
        System.out.println("sub.field = " +
                  sub.field + ", sub.getField() = " +
                  sub.getField() +
                  ", sub.getSuperField() = " +
        sub.getSuperField());
}
} /* Output:
sup.field = 0, sup.getField() = 1
sub.field = 1, sub.getField() = 1, sub.getSuperField() = 0
*///:~
           

When a Sub object is upcast to a Super reference, any field accesses are resolved by the compiler, and are thus not polymorphic. In this example, different storage is allocated for Super.field and Sub.field. Thus, Sub actually contains two fields called field: its own and the one that it gets from Super. However, the Super version is not the default that is produced when you refer to field in Sub; in order to get the Super field you must explicitly say super.field.

Although this seems like it could be a confusing issue, in practice it virtually never comes up. For one thing, you’ll generally make all fields private and so you won’t access them directly, but only as side effects of calling methods. In addition, you probably won’t give the same name to a base-class field and a derived-class field, because its confusing.

If a method is static, it doesn’t behave polymorphically:

//: polymorphism/StaticPolymorphism.java
// Static methods are not polymorphic.
class StaticSuper {
    public static String staticGet() {
        return "Base staticGet()";
    }
    public String dynamicGet() {
        return "Base dynamicGet()";
    }
}

class StaticSub extends StaticSuper {
    public static String staticGet() {
        return "Derived staticGet()";
    }
    public String dynamicGet() {
        return "Derived dynamicGet()";
    }
}

public class StaticPolymorphism {
    public static void main(String[] args) {
        StaticSuper sup = new StaticSub(); // Upcast
        System.out.println(sup.staticGet());
        System.out.println(sup.dynamicGet());
    }
} /* Output:
Base staticGet()
Derived dynamicGet()
*///:~
           

static methods are associated with the class, and not the individual objects.