problem with user defined exception

[Naveen Megharaj]

I have created my own exception class and i m trying to throw the exception explicitly. But when i m compiling, its giving me the following error : Incompatible types error.

This is my code:

  
class MyException extends Exception {
	String errorMsg=null;
	MyException() {
		System.out.println("This is MyException");
	}
	
	public void setErrorMessage(String errorMsg) {
		this.errorMsg=errorMsg;
	}
	
	public String getErrorMessage() {
		return errorMsg;
	}
}

public class ExceptionTest {
		static int num=10;
		public static void main(String [] n) {			
			try {
				checkOne();
			}
			catch(MyException me) {
				System.out.println(me.getErrorMessage());
			}
		}

		public static void checkOne() throws MyException {
			if(num<100) {
				throw new MyException().setErrorMessage("Number is less than 100");				
			}	
		}
}		  

Can anybody tell me what's going wrong..??

Thanks in advance

[Campbell Ritchie]

Which line is it on? And does throw new Exception().setErrorMessage("xxx") actually add up?

[Jessica Sant]

I'm going to answer your question in two ways -- 1: to explain why your code doesn't compile, 2: to explain the proper way to write this method.

1: So as for the first part -- why your code doesn't compile:

  throw new MyException().setErrorMessage("Number is less than 100");    

When you use the "throw" clause, the method you call needs to return an Exception object

"throw new MyException()" in fact returns an Exception object, but you then chain that method with a call to setErrorMessage() which doesn't return anything, and so you get an incompatible Type error.

There are 2 ways you can fix this.
-- have setErrorMessage() return the MyException object.
-- construct and modify the Exception in one line, and throw the exception in the next line.

2- That being said, this isn't the "proper" way to use and define your own exception, a better way would be to define a constructor that takes the error message as an argument:

     MyException(String errorMsg) {
         this.errorMsg = errorMsg;
     }

and then call the exception like this:

 throw new MyException("Number is less than 100");

[Jesper de Jong]

To improve it even further:

You do not need a member variable to store the message in your own exception class, because the superclass Exception already has such a member variable. Write your own exception class by implementing four constructors, like this:

public class MyException extends Exception {

    public MyException() {
    }

    public MyException(String message) {
        super(message);
    }

    public MyException(String message, Throwable cause) {
        super(message, cause);
    }

    public MyException(Throwable cause) {
        super(cause);
    }
}

Those four constructors correspond to the constructors of the base class Exception.

And then use it like this:

throw new MyException("Something went wrong!");

Use the standard getMessage() method from class Exception to see the message:

catch (MyException ex) {
    System.out.println("Exception: " + ex.getMessage());
}

[Jessica Sant]

much better -- nice Jasper.

[Naveen Megharaj]

thanks a lot guys.....!!!
the information given by you was very helpful....