Downcasting gives compiler error.

[sarojni agrwal]

In my program Downcasting gives compiler error even if I cast it, please see in my below program:

package multipleintrfce;


class sup1
{
	void same1()
	{
		
		System.out.println("same are 1");
	}
	
	
	void same2()
	{
		System.out.println("same1");
	}
	void alag1()
	{
		System.out.println("alg");
	}
}


public class dymnandupcastng extends sup1{


	void same1()
	{
		System.out.println("same are 2");
	}
	
	
	void same2()
	{
		System.out.println("same2");
	}
	
	void saperate1()
	{
		System.out.println("sateerate");
	}
	
	
	public static void main(String[] args) {
		
		//sup1 o = new sup1();
		
		dymnandupcastng gu1 = (dymnandupcastng) new sup1();
		
		gu1.same1();
		
		
		
		gu1.same2();
		
		gu1.alag1();
		
		gu1.saperate1();
		
		

	}
}

error:

Exception in thread "main" java.lang.ClassCastException: multipleintrfce.sup1 cannot be cast to multipleintrfce.dymnandupcastng
at multipleintrfce.dymnandupcastng.main(dymnandupcastng.java:48)

[Henry Wong]

sarojni agrwal wrote:In my program Downcasting gives compiler error even if I cast it, please see in my below program:

error:

Exception in thread "main" java.lang.ClassCastException: multipleintrfce.sup1 cannot be cast to multipleintrfce.dymnandupcastng
at multipleintrfce.dymnandupcastng.main(dymnandupcastng.java:48)

While all dymandupcasting objects are sup1 objects, and hence, cast between them... not all sup1 objects are dymnandupcasting objects, and hence, not always can be cast. In your case, your example instantiated a sup1 class instance directly, which can't be cast to a dymnandupcasting type.

Henry

[Tapas Chand]

I tried to compile.
No compilation error in my system.

[sarojni agrwal]

what should I do then for downcasting?

[Henry Wong]

sarojni agrwal wrote:what should I do then for downcasting?

You can only downcast objects that IS-A that subtype. This means that you can downcast a reference to something that was previously upcasted.

Henry

[Tim Holloway]

First of all, rename your classes to conform with Java recommended usages.

Class names should begin with an uppercase letter. Names beginning with lowercase letters are used for instances and properties. Some tools can become confused if you do not conform to this standard.

I have a suspicion that possibly you're doing this exercise under Windows, and something is converting (or not converting) classfile names in accordance with that rule, then failing to recognize the corresponding file. Windows doesn't care about upper/lower case when locating files by name. Java does, even under Windows.

[Tapas Chand]

I copy/pasted OP's code and compiled like this. No compilation error.

javac multipleintrfce\dymnandupcastng.java

2 class files got generated.
dymnandupcastng.class and sup1.class

[Dave Tolls]

Tapas Chand wrote:I copy/pasted OP's code and compiled like this. No compilation error.

javac multipleintrfce\dymnandupcastng.java

2 class files got generated.
dymnandupcastng.class and sup1.class

Try and run it.

The OP posted an exception, not a compilation error.