Overiding with Exceptions

[Nikhil Shah Jain]

   import java.io.IOException;
import java.sql.SQLException;

class Code{

	public int a= 6;

	public Code(){
		System.out.println("Slum");
	}

	public  void amethod()   {
		System.out.println("Hi");
	}
}

public class ScrapCode extends Code{

	public int a= 12;

	public ScrapCode(){
		System.out.println("Dog");
	}

	public static void main(String[] args)  {
		Code ref = new ScrapCode(); 
		ref.amethod();
	}

	public void amethod() throws Exception  {
		System.out.println("Hello");
	}

}

 

Above code gives a compilation error stating
Exception is not compatible with throws clause in Code.amethod()
- overrides Code.amethod in Eclipse

But below code gives no compilation error even though there's no NullPointerExcetion clause in superclass.
It gives the following output:
Slum
Dog
Hello

  import java.io.IOException;
import java.sql.SQLException;

class Code{

	public int a= 6;

	public Code(){
		System.out.println("Slum");
	}

	public  void amethod()   {
		System.out.println("Hi");
	}
}

public class ScrapCode extends Code{

	public int a= 12;

	public ScrapCode(){
		System.out.println("Dog");
	}

	public static void main(String[] args)  {
		Code ref = new ScrapCode(); 
		ref.amethod();
	}

	public void amethod() throws NullPointerException  {
		System.out.println("Hello");
	}

}

  

Can you please explain why is NullPointerException spared ?

[sakthi moorthy]

NullPointerException is an Unchecked Exception (not the compile time Exception) whereas Exception is a checked Exception's SuperClass.....

[Debasis behera]

Rule for Overriding is if in superclass method thows a checked exception then the overrideen method must throws that exception or subtype of that exception or it may not throws any exception;
But in case of if subclass overriden method cant thows checked exception if its superclass method doesnot throws Exception
but there is no condition for unchecked exception
NullPointerException is unchecked RuntimeException
Exception is Checked

[Nikhil Shah Jain]

Ok.Can a subclass overriding method throw a narrower Exception?
i.e
if super class throws Exception and subclass IOException

[Mark Uppeteer]

yes it can. you got it

[Ankit Garg]

N Lv please check your private messages for some administrative matter...

[Nikhil Shah Jain]

In the above code snippet if
super class throws Exception and subclass IOException
It still gives a compile error, hence forcing the main method of subclass to throw
Exception

[Ankit Garg]

If we take this code into consideration

   import java.io.IOException;
import java.sql.SQLException;

class Code{
	public  void amethod()  throws Exception {
		System.out.println("Hi");
	}
}

public class ScrapCode extends Code{

	public static void main(String[] args)  {
		Code ref = new ScrapCode(); 
		ref.amethod();
	}

	public void amethod() throws IOException  {
		System.out.println("Hello");
	}

}

 

Since the reference is of type Code, so the main method will have to handle Exception and not IOException. This is because the method call will be resolved to amethod of class Code at compile time. Although at runtime amethod in class ScrapCode will be called, but the compiler doesn't know about that. The compiler only knows that the type of ref is Code and amethod in Code throws Exception...

[Nikhil Shah Jain]

Ankit,

[Nikhil Shah Jain]

On changing the reference to
ScrapCode ref = new ScrapCode();

It still give compile error

[Debasis behera]

if you throws a checked exception you are bound to either catch it or throws it to JVM

[Ankit Garg]

Nikhil you can edit your message using button if you write something wrong or press submit by mistake.

On changing the reference to
ScrapCode ref = new ScrapCode();

It still give compile error

But if you see the error, when the code is Code ref = new ScrapCode();, then the error will be Exception must be caught or thrown, and when the code is ScrapCode ref = new ScrapCode();, then it will say IOException must be caught or thrown. Do you understand the difference now??