wirte my own Exception throw

[Kee Kee moon]

Can anyone please provide some examples how to write Exception throw. I always use standard library.
As now, I like to learn how to write my own Exception throw.

Thank you very much

[Jesper de Jong]

I don't know what you mean by "writing my own Exception throw". Are you asking how to write your own Exception class?

That's easy: just create a class that extends Exception, or RuntimeException if you want it to be an unchecked exception. Since class Exception has 4 constructors, you might want to add the same 4 constructors to your own Exception class:

public class MySpecialException extends Exception {
    public MySpecialException() {
    }

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

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

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



// Now you can throw your own exception just like any other:
throw new MySpecialException("Something went wrong!");

[Kee Kee moon]

For example, if users enter a number that is less than 0. I like to use try and catch to handle exception.
It seems that I could not find a standard library for the exception handling.

Jesper de Jong wrote:I don't know what you mean by "writing my own Exception throw". Are you asking how to write your own Exception class?

That's easy: just create a class that extends Exception, or RuntimeException if you want it to be an unchecked exception. Since class Exception has 4 constructors, you might want to add the same 4 constructors to your own Exception class:

public class MySpecialException extends Exception {
    public MySpecialException() {
    }

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

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

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



// Now you can throw your own exception just like any other:
throw new MySpecialException("Something went wrong!");

[Ashutosh M Kulkarni]

Kee Kee moon wrote:For example, if users enter a number that is less than 0. I like to use try and catch to handle exception.
It seems that I could not find a standard library for the exception handling.

Oh, it seems you have a very simple problem. If the requirement is like quoted above, then you could do with a simple if statement.

public executeMyMethod(int userEntry) throws Exception {
   if (userEntry < 0)  {
      throw new Exception ("Invalid input !!");
   }
}

Although, it should be noted that this is a very generic example and it is NOT advised to use Exception class itself for throwing exceptions, a more specific Exception subclass would be more appropriate (the kind that Jasper suggested).
You'd best write your own exception class as demonstrated in the earlier post, that only caters to specific situations (in you case, less than zero).

Also, please notice that you must use the "throws" keyword to declare the exception while declaring the method, otherwise compiler will complain.

[Paul Beckett]

IllegalArgumentException would seem to be a good choice here. You could subclass it if you want to make the exception even more specific.