question about polymorphism

[becky]

hi

I've got a question regarding polymorphism in java:

public class Test1 {

    public Test1() {
        Test3 t3=new Test2();
        t3.output();
        t3.value1=9; //not possible
    }
    
    public static void main(String[] args) {
        new Test1();
    }
}

public class Test2 extends Test3 {

    public int value1=5;

    @Override
    public void output() {
        System.out.println("the value is "+value1);
    }
}

public class Test3 {

    public void output(){
        System.out.println("class: Test3");
    }

}

output of the program:
the value is 5

why do i have no access from "Test1" to the member variable "value1" which is initialized in class "Test2" although i'm calling the constructor "Test2()"?
and why is the output of the program "the value is 5" although I've got no access to "value1" from "Test1" like i said before?

[CJSLMAN]

OK... some comments...

... although i'm calling the constructor "Test2()"?

The Test2 class has no constructor. It only has an output() method.

why do i have no access from "Test1" to the member variable "value1"

Because when you execute the Test1 class, the main method gets executed (not the constructor). If you want the Test1() constructor to execute, you have to instantiate it from another class (or call it within the same class).

Test3 t3=new Test2();

Shouldn't this be:

Test2 t2=new Test2();

... or am I missing something here?

Did I make sense ? It can get very confusing sometimes.

Luck,
CJSL

[becky]

The Test2 class has no constructor. It only has an output() method.

sure it has! the compiler generates a default-constructor automatically if there is no constructor available.

Because when you execute the Test1 class, the main method gets executed (not the constructor). If you want the Test1() constructor to execute, you have to instantiate it from another class (or call it within the same class).

well, actually i did this in my main method :) take another look

Shouldn't this be:

Test2 t2=new Test2();

... or am I missing something here?

no, it should be the way i've written it. this is what is called polymorphism!

thx anyway

[becky]

i've already found answers to my questions:

why do i have no access from "Test1" to the member variable "value1" which is initialized in class "Test2" although i'm calling the constructor "Test2()"?

Because the type of t3 is reference to Test3, and Test3 does not have a value1 variable. The compiler doesn't know that at runtime that reference is going to point to a subclass of Test3 that does have that variable.

and why is the output of the program "the value is 5" although I've got no access to "value1" from "Test1" like i said before?

Because that's how Java's runtime polymorphism works. When you call an overridden method, it's the runtime class of the object that determines whose version of that method gets called. Since the object is a Test2, it's Test2's implementation of the method that gets called. The caller and type of reference don't have to know about the internal details of that class. Only that it's a subclass of Test3, and therefore exposes all the same public instance members as Test3.

[CJSLMAN]

um...uh... how do I start?...

sure it has! the compiler generates a default-constructor automatically if there is no constructor available.

I had heard of this, but never really paid that much attention to what that meant... So I learned somthing today. Thanks!

well, actually i did this in my main method take another look

I really have no excuse for this this blooper. Sorry about that... you're completly right...the main was calling Test1(). :o

no, it should be the way i've written it. this is what is called polymorphism!

I don't think I've ever seen this particular twist of polymorphism... or I don't remember seeing it like that.

So... sorry for the my misunderstandings and erronious statements. On the bright side, I learned a couple of things... not all was lost :)

Luck,
CJSL