42

here is working java code

class Cup {
    public String sayColor() {
        return "i have a color .";
    }
}

class TCup extends Cup{
    public String sayColor(){
        System.out.println(super.getClass().getName());
        return super.sayColor()+"color is tee green.";
    }
}

class MyTCup extends TCup {
    public String sayColor(){
        System.out.println(super.getClass().getName());
        return super.sayColor()+"but brushed to red now!";
    }
}
class Test {
    public static void main(String[] args) {
        Cup c = new MyTCup();
        System.out.print(c.sayColor());
    }
}

and running the Test class prints

MyTCup
MyTCup
i have a color .color is tee green.but brushed to red now!

question 1: At the runtime, the type of object C is MyTCup, but it can always call the super method. Is there a method stack in the memory within MyTCup after initializing the object, and then can call through at runtime like the code ?

question 2: There is no way to call the super method in other objects. As I know ,c++ can cast to call parent method at any time. Why is it different in Java?

Dov Vachman
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sting
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4 Answers4

81

You can't call the super method in other objects - that would violate encapsulation. The whole point is that the object controls what its overridden methods do. For instance, you might override a collection's add method to throw an exception in certain circumstances, so it could ensure only "valid" items got added to the collection. That would be pointless if callers could just bypass it with a cast!

The only reason an object gets to call super.foo() for itself is to enable one call to be implemented by using the parent implementation. It's up to the code in the class to make sure it only ever does that sensibly. Again, to take the add-in-a-collection example, if the collection overrides add it would have to have some way of adding the validated item to the collection, which it would do with super.add().

Note that for the same reason of encapuslation, you can only call your parent implementation, not the grandparent implementation - so super.foo() is valid, but super.super.foo() isn't.

Jon Skeet
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1

1:

Your question is not quite clear. What do you mean by "call through at runtime like the code"? If you are asking how instance c knows what its super class is, then yes, the class hierarchy is stored in memory, and can be accessed by the VM.

2:

Java actually does allow you to cast an instance to its parent. It's just that calling a method on an instance always uses the instance's actual class, not its compile-time class. I.e. in Java all methods are what would be called "virtual" in C++. Why this was decided I do not know.

Edit: Actually Jon Skeet explains very nicely why you cannot call a method of a super class on an instance of a sub class, so now I know :-).

sleske
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  • i mean "sayColor" method called from MyTCup to TCup and Cup and the class displayed is "MyTCup".so the methods are belonged to MyTCup or belonged TCup,Cup.I dont familiar with jvm,maybe all in the method area. – sting Jun 23 '09 at 14:55
1

You can define a new helper method in the subclass so that you don't override the original--call that--it can call the super method or not, depending on what you want.

rogerdpack
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0

Actually you can, but you have to use a methodology, so it will only work on your code.
At the super class, add the parameter "Class" to the method, like in: 

public String sayColor(Class classFilter){...

now, at every override, you check to see if the current sub-class is the one of the specified filter like:

if(TCup.class!=classFilter)return super.sayColor(classFilter);
return "some text for TCup only";

I coded in a exclusive way. You can add more parameters, or compare with null, so you can join results with the super classes, use your creativity :)

Aquarius Power
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