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Upcasting & Downcasting !

1 reply on 1 page. Most recent reply: Jul 16, 2003 7:50 AM by Boris Pruessmann

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Jaideep

Posts: 1
Nickname: djai
Registered: Jul, 2003

Upcasting & Downcasting !

Posted: Jul 14, 2003 11:39 PM

All,
Can someone let me know how things go inside the JVM when we try to do the following:

class parent {
int x;
public void m() {.../}
}
class child extends parent {
int x;
int y;
public void m() {../} //overiding
}

Now when we assign :
Parent po = new Parent();
Child co = new Child();
po = co;
po.x; // The value of x in parent class is called. Why ???
po.m; // The m in child is called. Why ???

co = po; // Can't Do why ???

Assumming that the JVM is implemeted as a heap with handle pool & object pool. Is it not that when we do po = co Or co = po , we are merely changing the object reference to point to other ?? Thanks in advance.

Jaideep.

Boris Pruessmann

Posts: 2
Nickname: docbobo
Registered: Jul, 2003

Re: Upcasting & Downcasting !

Posted: Jul 16, 2003 7:50 AM

> po.x; // The value of x in parent class is called. Why???

Polymorphism doesn't work for variables because late-binding is not possible. The decision which variable to access is based on the compile-time informat and that says: "this is of type Parent"

> po.m; // The m in child is called. Why ???

Here, polymorphism is working because you're calling a method. Decision which method to call is left to the VM which decides at runtime, based on the run-time type of the object stored in the variable.

> co = po; // Can't Do why ???

The compiler can only check information that is available at compile-time and that indicates that po is of type parent (which is not a child). The compiler will therefor refuse this line of code. Instead you have to do 'co = (Child) po', telling the compiler: "I know that po will be of type Child". However, there is the risk that po isn't a child at runtime and this'll result in a ClassCastException.

-Boris

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