References and casting

[Ismael Upright]

Here's a bunch of definitions and reference declarations from jqplus6:

interface I1 { }
interface I2 { }
class C1 implements I1 { }
class C2 implements I2 { }
class C3 extends C1 implements I2 { }
C1 o1;
C2 o2;
C3 o3;

1. Let's say that we do the following:

o3 = o1;

It won't compile because superclass reference cannot be assigned to subclass reference without explicit cast. So I did the cast:

o3 = (C3)o1;

And Eclipse wrote:

Syntax error on token "o3", VariableDeclaratorId expected after this token.

Why?


2. There is another assignment:

o3 = o2;

It also seems to be not correct. Why? Both o2 and o3 IS A interface I2 so...?

[Arie Prastowo]

I don't know if you wrote the code correctly, but the last 3 code is written outside any class/interface definition

interface I1 { }
interface I2 { }
class C1 implements I1 { }
class C2 implements I2 { }
class C3 extends C1 implements I2 { }
C1 o1;
C2 o2;
C3 o3;

[Harshit Rastogi]

Originally posted by Ismael Upright:
Here's a bunch of definitions and reference declarations from jqplus6:

1. Let's say that we do the following:

o3 = o1;

It won't compile because superclass reference cannot be assigned to subclass reference without explicit cast. So I did the cast:

o3 = (C3)o1;

And Eclipse wrote:

Syntax error on token "o3", VariableDeclaratorId expected after this token.

Why?

becuase you cannot downcast an object , you can only upcast
it means you can do
Parent obj = new ChildBoj()
or
Parent obj = (Parent) new ChildObj();
but not
Child obj = new Parentobj()
or Child obj = (ChildObj) new ParentObj()

2. There is another assignment:

o3 = o2;

It also seems to be not correct. Why? Both o2 and o3 IS A interface I2 so...?

in this case 03 is an object of type C3 and o2 of C2 and not of type interface I2

[Ismael Upright]

I figured out my mistake, it was lack of the main method

After fix the code looks like this:

public class v6_2_493 {
interface I1 { }
interface I2 { }
class C1 implements I1 { }
class C2 implements I2 { }
class C3 extends C1 implements I2 { }
static C1 o1;
static C2 o2;
static C3 o3;

public static void main(String[] args)
{
o3 = (C3)o1; //IT IS OKAY NOW
o3 = o2; //BUT THIS STILL IS A PROBLEM
I1 i1 = o3; I2 i2 = (I2) i1;
I1 b = o3;
}
}

It is possible to make a downcast.

But still I don't understand why we cannot assign o3 to o2. For me it's assigning an object which IS-A interface I2 to the reference which also IS-A interface I2 so it should work.

Isn't it?
[ April 21, 2008: Message edited by: Ismael Upright ]

[Arie Prastowo]

In my understanding, you can't do that because o3 was declare as C3 so it expected also C3 and all it's subclasses

[Ismael Upright]

But C3 was also declared as I2, which means that every C3 IS-A I2..

[Arie Prastowo]

when you said C3 IS-A I2, only valid in class definition where C3 IS-A C1 and I2.
But when you declare
static C3 o3;
the compiler expect something that come from C3 or it subclasses.
It would make a different if you code
static I2 o3;
since it can accept anything that implement I2 including o2

public class Test1 {
interface I1 { }
interface I2 { }
class C1 implements I1 { }
class C2 implements I2 { }
class C3 extends C1 implements I2 { }
static C1 o1;
static C2 o2;
static I2 o3; -->> change here
public static void main(String[] args){
o3 = (C3)o1; 
o3 = o2; 
}
}

[Ismael Upright]

You're right, I forgot about that.


Thanks!