Exception

[jomypgeorge]

hi friends

i am confused with strange behaviour of this program.


import java.lang.*;

public class ExceptionTest
{
public static void main(String ... args)
{
ExceptionTest obj = new ExceptionTest();
obj.method1();
}

void method1()
{
throw new Exception(); //line1
//throw new Throwable(); //line2
}

}



as we know Throwable is the superclass of Exception. in the above code it gives error on compiling as

ExceptionTest.java:21: incompatible types
found : Exception
required: java.lang.Throwable
throw new Exception();
^
1 error

but if we uncomment line 2, and comment line 1 then error will be

ExceptionTest.java:22: unreported exception java.lang.Throwable; must be caught
or declared to be thrown
throw new Throwable();
^
1 error

why does it provides a eoor saying incompatible types on throwing new exception()?
i expected following error in first case

ExceptionTest.java:21: unreported exception java.lang.Exception; must be caught
or declared to be thrown
throw new Exception();
^
1 error

whats happening here?

[goldest]

The thumb rule is that, when you are throwing the exception, you should declare the same in your method signature.

Does that ring the bell for you?

If it still doesn't, refer some good documentation here: How To Throw Exception

Hope that helps,

Goldest

[jomypgeorge]

yes thats right and i expect that itself...
but the error is not that

error produced is

ExceptionTest.java:21: incompatible types
found : Exception
required: java.lang.Throwable
throw new Exception();
^
1 error

[j2me64]

first of all your import java.lang.* is for nothing, since java.lang is imported by default. secondly if your code throw a exception you must handle it 1) by add throw declaration or by surrounding it with try/catch. the corrections i have made is with the try/catch block and your code will be compilable and runnable and produce

java.lang.Exception
at fuel.ExceptionTest.method1(ExceptionTest.java:11)
at fuel.ExceptionTest.main(ExceptionTest.java:6)

[goldest]

The rule is simple!

If you want to throw Exception, have your method1() to throw it by using throws Exception. And then throw new Exception() inside that.

If you want to throw Throwable, have your method1() to throw it by using throws Throwable. And then throw new Throwable() inside that.

Make sure that you either wrap the call to method1() in a try/catch block inside your main method OR have your main method throws the Throwable.

If you prefer to put your method1() inside try/catch, then make sure to catch (Exception e) when you are throwing Exception and make sure to catch (Throwable t) when you are throwing Throwable.

I don't see any issue in either approach.

Goldest

[jomypgeorge]

i know it is throw an exception which is never caught, or throws and sure we get an error
but why produce an error like this?

ExceptionTest.java:21: incompatible types
found : Exception
required: java.lang.Throwable
throw new Exception();

why not produce an error like " unreported exception java.lang.Exception; must be caught or declared to be thrown"????

[goldest]

When I ran your code, while throwing exception, it gave me,

ExceptionTest.java:13: unreported exception java.lang.Exception; must be caught
or declared to be thrown
throw new Exception(); //line1
^
1 error

While throwing Throwable, it gave me,

ExceptionTest.java:14: unreported exception java.lang.Throwable; must be caught
or declared to be thrown
throw new Throwable();   //line2
^
1 error

I really don't know from which code you are getting your version of exception on your console.

Goldest

[jomypgeorge]

thanks goldest

now i added import java.lang.Exception
and works fine

[Tolls]

I get unhandled exception as well (as expected), so I guess it's a case of how are you compiling this?

[Tolls]

You don't need to import anything from lang.
That's not the solution.

[goldest]

now i added import java.lang.Exception
and works fine

java.lang package comes default for any java class. We don't need to import it explicitly.

import java.lang.*;

The import above just doesn't make any difference.

Goldest

[jomypgeorge]

i think you are using some IDE like eclipse.
try it in a text file and compile using javac command.

i think you can then see the difference.....

[jomypgeorge]

at last i got the reason,
i have already created a class file with name Exception in my default package.
and compiler used it where i used Exception......
now i deleted Exception.class and now it works as expected.

but one doubt now....
how compiler choose one class if same class name exists in more than one package?


thanks a lot for you all.... sorry for the mistake....

[goldest]

That's why packages are meant to be, to organize your classes in proper manner. So that any naming collision will be avoided.

You are not expected to have the classes with same name as in java.lang package, because that will make the compiler freak out. What you were trying to do is to have Exception class in your package and the same name class is already present in java.lang package, so the errors were expected to come.

but one doubt now....
how compiler choose one class if same class name exists in more than one package?

In case, if you want to use same name classes, you need to use their full names in your code. For e.g.

import java.util.Date;

public class SameNameClasses{
	public static void main(String args[]) {
		Date d = new Date();

		java.sql.Date d1 = new java.sql.Date(4);
	}
}

In above snippet, we are using Date class from both java.util and java.sql packages. If you want to use same name classes in your code, one can be imported the normal way, and rest need to be used with their full names as shown above.

Hope that helps,

Goldest

[jomypgeorge]

thanks goldest