Balakrishna Thati

Thanks Isberg and Ritchie... we done it flawlessly atlast

Is this ok ? or any thing else missed ??

public class ProveIt { public static void main(String[] args){ Abc obj1 = new Abc(); Abc obj2 = new Abc(); if(obj1.sobj == obj2.sobj) { System.out.println("We have only one instance"); } else { System.out.println("They are different"); } } } class Abc{ public static Sclass sobj = new Sclass(); //public Sclass sobj = new Sclass(); public Abc() { sobj.count++; System.out.println("count= "+sobj.count); } } class Sclass { public int count; }

How about this ?

if(obj.count == new Abc().count) { System.out.println("We have only one instance"); }

if the comparision is between the primitives, it again fall back to my option 1, that i stated earlier.

To conclude it finally, i changed the code little bit.

public class ProveIt { public static void main(String[] args){ Abc obj = new Abc(); if(obj.count.equals(new Abc().count)) { System.out.println("We have only one instance"); } } } class Abc{ public static Integer count=new Integer(10); }

Thanks Ritchie,

Here is my code
public class ProveIt { public static void main(String[] args){ Abc obj = new Abc(); if(obj.count == Abc.count) { System.out.println("We have only one instance"); } } } class Abc{ public static int count; }

Pragramatically how to show that there is only one instance of Static variable irrespective of number of objects...

I thought of doing it by
1. showing their values being equal.. But the counter argument is that the "having the same value doesn't sufficient to say they are from the same variable"
2. equating the static variable's hash code accessing it from different objects. But Hashcode can't guarantee uniqueness.

is there any other way to prove that irrespective of number of objects there is only one static variable exists for a Class.

-- KVT

Hi Vijay,

my question is about the requirement of downcast.

how a method declaration in base class will eliminate the need to downcast an object.

As you said, provide the method in base (remove the first comments) and try to compile. it is not getting compiling saying "incompatible types"

now provide the comments for the method in base class and remove the comments around casting. It is compiling and running normally.

my question is whether the thing mentioned in the book wrong or what??

Hi Santosh,

\u0038 is a unicode representation.

when java finds \u0038, it converts it to its equivalent value of 8.

so in when you run your code, f will print 8.0

observer this code

char c='\u0042';
int \u0042= \u0038;
System.out.println("x = "+B+", c = "+c); //B's unicode value is 0042 and 8's is 0038 (in Hex)

Here out put would be x=8, c = B

for unicode value look at
http://www.ssec.wisc.edu/~tomw/java/unicode.html

Hi,

This Q is from TIJ 4e, Interfaces, Ex-4

It is like this

Create an abstract class with no methods. Derive a class and add a method. Create a static method that takes a reference to the base class, downcasts it to the derived class and calls the method. In main(), demonstrate that it works. Now put the abstract declaration for the method in the base class, thus eliminating the need for the downcast.

How providing a abstract declaration for a method in base class will eliminate the need to downcast ??\

So i tried it with the following code
abstract class Base { // abstract public void print(); //toggle it } class Derived extends Base { public void print() { System.out.println("From Print of Derived"); } } class Runner { public static void main(String[] args) { Runner.TakeBase(new Derived()); } public static void TakeBase(Base b) { Derived obj = /*(Derived)*/ b; //toggle the comments obj.print(); } }

It still requires downcast. i checked the errara of this 4e.

But there is nothing regarding this.

Please clarify over this.

Thanks manoj and Cristian

Even though my doubt is cleared, i am more of feeling that this situation should be escalated at compile time itself rather than at runtime.


But don't know why they postponed it till run time. Any guesses ??

Hi James,

The show() method is public. So it will be available.

Then whats the issue with that. Why i am not been able to access it.

Even then The run time error is about the Class casting not with accessing a non-visible method.

Thanks Tsang

Hi Mumtaz,

As you said

Child class do have all the method of parent class, but Parent class does not know of any method of child class thats why it can not be downcasted to child class unless the object it referred to is of Chile class.

so based on this, the following code should compile and run properly
ABC obj1 = new ABC(); DEF obj2= (DEF)obj1; obj2.show();

But its throwing the same Exception.

Hi,

One of my friend told me that

Assertion checking can be controlled on a class, package, or package hierarchy basis.

can any one explain with an example how can we do this ??

or refer me to a good link

Hi,

i am performing the following downward casting.

I am thinking that the Child class will have all the methods of the Parent class so when a Parent object is downward casted to Child class, we should be access methods derived from the parent class.

Here is my code..
class ABC { public void show() { System.out.println("From ABC"); } } class DEF extends ABC { public void display() { System.out.println("From DEF"); } } class Test { public static void main(String[] args) { /* //Block 1 ABC obj1 = new ABC(); DEF obj2= (DEF)obj1; */ // DEF obj2 = new DEF(); //Block2 } }
The above code altering between Block 1 and Block2, compiling normally but Block1 code throwing an exception java.lang.ClassCastException.

Why is it??