About Question enthuware.ocajp.i.v7.2.1328 :

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Re: About Question enthuware.ocajp.i.v7.2.1328 :

Post by admin »

Try it out :)
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baptize
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Re: About Question enthuware.ocajp.i.v7.2.1328 :

Post by baptize »

admin wrote:Try it out :)
already did :shock:

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Re: About Question enthuware.ocajp.i.v7.2.1328 :

Post by admin »

So what did you find out?
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Re: About Question enthuware.ocajp.i.v7.2.1328 :

Post by baptize »

this:
A a = (B)(I) b;
can be broken down to..
I i = b; // i don't even need the (I) here because B is I (upward cast).
A a = (B) i; // here (B) is needed to pass the compiler time error. At runtime i points to an Object of type B.

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Re: About Question enthuware.ocajp.i.v7.2.1328 :

Post by admin »

Very good :)
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Codeguru
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Re: About Question enthuware.ocajp.i.v7.2.1328 :

Post by Codeguru »

So, I i = (C) a fails because you can't cast an object to a class that I is not, you can only reference to a class that is an I, like that has been said I i = b, and then using polymorphism do a A a = (C) b, since C is a B is an A is an I? Or does the cast on the b here need to be the actual class that it is in runtime, which is B?

If it looks like I have no idea how polymorphism works, it's because I don't. :) God help me when I attain a Java developer position...

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Re: About Question enthuware.ocajp.i.v7.2.1328 :

Post by bptoth »

Just a small question about

Code: Select all

I i = (C) a;
The explanation is as follows: It will compile because a C is-a A, which is-a I, and a reference of class A can point to an object of class C. But it will fail at runtime because a does not point to an object of class C.

Didn't the bold part possibly wanted to state: a reference of interface I can point to an object of class C?
Just because this is what actually seems to happen here
It is definitely true in its current form as well, but does not seem to exactly cover the scenario, and also seems to be redundant, given that "C is-a A" has been already stated in the first half of the sentence

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Re: About Question enthuware.ocajp.i.v7.2.1328 :

Post by admin »

No, 'a' is declared as a reference of class A. Since the given code is "(C) a" the explanation says that a reference of class A can point to an object of class C to explain why 'a' can be cast to C.

The fact "A implements I" is the basis for a valid assignment of a to i.

HTH,
Paul.
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Re: About Question enthuware.ocajp.i.v7.2.1328 :

Post by admin »

BTW, the exact rules for casting are given by JLS here: http://docs.oracle.com/javase/specs/jls ... #jls-5.5.1
One should try these out by going through the specification and writing sample programs to improve understanding.

HTH,
Paul.
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javaman
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Re: About Question enthuware.ocajp.i.v7.2.1328 :

Post by javaman »

Hello,

I thought I got this icasting concept but am afraid I don't follow...Keep answering these questions wrong!
We have this (slight variation of the question's class hierarchy but esentially the same):

Code: Select all

package LearnJava;

public class LearnJava {
	public static void main(String args[]) {
		A a = new A();
		B b = new B();
		a = (B)a;
	}
}
interface I{
}
class A implements I{
	void a(){
		System.out.println("Method a() from A");
	}	
}

class B extends A {
	void b(){
		System.out.println("Method b() from B");
	}
}
If C is-a B is-a A is I why can't I cast var a to b by:

a = (B)a;?

The compiler says
"Exception in thread "main" java.lang.ClassCastException: LearnJava.A cannot be cast to LearnJava.B"
Thanks,

Marc
Last edited by admin on Mon Nov 25, 2013 1:44 pm, edited 2 times in total.
Reason: Please use code tags

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Re: About Question enthuware.ocajp.i.v7.2.1328 :

Post by admin »

I don't see any C in your code.
The code you've quoted above will fail at run time because a is pointing to an object of class A, and you are trying to cast it as B. So why do you think it should work? B is A but A is not B.

-Paul.
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javaman
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Re: About Question enthuware.ocajp.i.v7.2.1328 :

Post by javaman »

Here is code like in question. Added functions in class for clarity:

package LearnJava;

interface I{
}
class A implements I{
void a1(){
System.out.println("A.m1()");
}
}

class B extends A {
void b1(){
System.out.println("B.m1()");
}
}

class C extends B{
void c1(){
System.out.println("C.m1()");
}
}
//And the following declarations:
class LearnJava{
public static void main(String[] args) {
A a = new A();
B b = new B();
a = (B)(I)b;
a.b1();
}
}

Line a = (B)(I)b; compiles ok and gives no RE as the correct answer says. I don't understand what's happening here. Is it a cast from a to b? If so, why does a.b1(); give an compiler error "Exception in thread "main" java.lang.RuntimeException: Uncompilable source code - Erroneous sym type: LearnJava.A.b1"?

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Re: About Question enthuware.ocajp.i.v7.2.1328 :

Post by admin »

It will be a bit difficult to explain this. But I can try:

At runtime, b refers to an object of class B. Since B implement I, (I)b is valid as compile as well as runtime. Let's calls (I)b as i.
Now, since I is an interface, (B)i is valid at compile time and is valid at runtime because i indeed points to an object of class B. Let's call (B)i as x.
a = x is valid at compile time because x is of type B and B is a A. It is also valid at run time because x indeed points to an object of class B.


a.b1() won't work because the class of reference a is A, which does not have the method b1().


I would suggest you refer to a book to understand this better.

HTH,
Paul.
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javaman
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Re: About Question enthuware.ocajp.i.v7.2.1328 :

Post by javaman »

Ok, I understand this is not a cast from a to B...
Thanks for explanation.

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Re: About Question enthuware.ocajp.i.v7.2.1328 :

Post by javaman »

Hello,

I try to get my finger behind casting but not 100% sure I get it...

In below code, a.m1(); prints "C.m1()". In plain English it does the following:

B b = new C(); ==>
"Create a reference of type B b and assign to it (aka 'point it to') an instance of subclass C
B a = (B)(I)b; ==>
I tmp = (I)b; // Treat reference to instance C as reference to interface I;
B a = (B)tmp; // treat reference to interface I as reference to
an instance of class B (this is possible because B implements I via A);

WHY does a.m1() not give me the output "B.m1()"? if a points to instance of B it should?
======

Code: Select all

class LearnJava{
	public static void main(String[] args) {
		B b = new C();
		B a = (B)(I)b;
		a.m1();
	}
}
interface I{}

class A implements I{
    public void m1(){
         System.out.println("A.m1()");
    }
}
class B extends A{
    public void m1(){
        System.out.println("B.m1()");
    }
}
class C extends B{
    public void m1(){
        System.out.println("C.m1()");
    }
}
class D extends C{
    public void m1(){
        System.out.println("D.m1()");
    }
}

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Re: About Question enthuware.ocajp.i.v7.2.1328 :

Post by admin »

>>WHY does a.m1() not give me the output "B.m1()"? if a points to instance of B it should?
Because a does not point to an instance of B. It points to an instance of C. That what is polymorphism. http://docs.oracle.com/javase/tutorial/ ... phism.html

HTH,
Paul.
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tn1408
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Re: About Question enthuware.ocajp.i.v7.2.1328 :

Post by tn1408 »

Hello, Paul I ran the code:
class TestClass{
public static void main(String[] args){
A a = new A();
B b = new B();

a = (B)(I)b;
b = (B)(I) a;
//I i = (C) a;
//I i = (C) a;
}
}
interface I{}
class A implements I{}
class B extends A {}
class C extends B{}

and answer #2: b = (B)(I) a; actually didn't fail.

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Re: About Question enthuware.ocajp.i.v7.2.1328 :

Post by admin »

That is because you already have "a = (B)(I)b;". You need to comment it out first.
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Re: About Question enthuware.ocajp.i.v7.2.1328 :

Post by seanthemill »

Hi,
Given the following code:

Code: Select all

class A{}
class B extends A {}

public class Test {
    public static void main(String[] args) {
        A a = new A();
        A a1=new A();
        B b = new B();
//        a=b;// ok
//        b=(B)a;// ClassCastException
//        a=(A)a1;  // ok
//        a=a1;  // ok
        
      a=(B)a1; // compiles ok, ClassCastException???
    }
}
My question is with the line in bold/red. My understanding is that for the compiler to be ok, it just needs to be satisfied that the classes are in the same hierarchy and as a result it may work (up the tree implicit casting, down the tree requires explicit casting). Whenever I have come across ClassCastException it is because the reference was pointing to an object up the tree e.g. a ref of type B pointing to an object of type A.
The line in question appears to be a ref of type A pointing to an object of type A. The cast to (B) obviously is what is causing the ClassCastException. Can someone explain please what it does to effect this? Is the A ref a, a B ref all of a sudden??

Thanks,
Sean.

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Re: About Question enthuware.ocajp.i.v7.2.1328 :

Post by admin »

You've written about your understanding about compiler. But then you are talking about ClassCastException, which happens only at run time.
I am not really clear about what you are asking? ClassCastException is thrown whenever the object that you are trying to cast to a reference of another class cannot be cast to that class.

I don't think in your code b=(B)a should throw ClassCastException because a is actually pointing to an object of class B because of a=b; written earlier.
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Re: About Question enthuware.ocajp.i.v7.2.1328 :

Post by seanthemill »

Thanks for the reply. I think I have it figured: my confusion was arising because of the fact that given the assignment

Code: Select all

 A a = (B)new A(); // where B extends from A
the type on the left hand side was an A; however this has nothing to do with the cause of the ClassCastException at runtime. Quite simply, a B reference can never point at an object of type A (and that is what the right hand side of the assignment was trying to create).

Given that is the case, the following also holds true:

Code: Select all

A a = (B)new B();   // no issues at compile time or runtime
because a reference of type A can point at any object of type B (because B is-an A via inheritance).

Is that accurate?

Regards,
Seán.

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Re: About Question enthuware.ocajp.i.v7.2.1328 :

Post by admin »

Yes, that is correct.
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Re: About Question enthuware.ocajp.i.v7.2.1328 :

Post by iamslrr »

Consider the following classes :

interface I{
void iMsg();
}

class A implements I{
void iMsg() { }
void aMethod() { }
}

class B extends A {
void bMethod() { }
}

class C extends B{
void cMethod() { }
}


And the following declarations:
A a = new A();
B b = new B();
a = (B)(I)b;
a.bMethod();


Why does a.bMethod cause a compiler error:
Test4.java:35: error: cannot find symbol
a.bMethod();
^
symbol: method bMethod()
location: variable a of type A


After the cast a instanceof B is true.

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Re: About Question enthuware.ocajp.i.v7.2.1328 :

Post by iamslrr »

Ahhh...
Nevermind!

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Re: About Question enthuware.ocajp.i.v7.2.1328 :

Post by AristPhil »

I've summarized my understanding of this multiple casting question here: http://www.coderanch.com/forums/posts/l ... 31#3066425
I'd appreciate if somebody interested and competent would find the time to verify, if my understanding is correct. Many thanks!

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