Q5 from Dan's top exam (2nd)

[Barkat Mardhani]

class ColorException extends Exception {}
class WhiteException extends ColorException {}
class White {
  void m1() throws ColorException {throw new WhiteException();}
  [B]void m2() throws WhiteException {}[/B]
  public static void main (String[] args) {
    White white = new White();
    int a,b,d,f;
    a = b = d = f = 0;
    try {white.m1();a++;} catch (ColorException e) {b++;}
    try {white.m2();d++;} catch (WhiteException e) {f++;}
    System.out.print(a+","+b+","+d+","+f);
  }
}

m2() does not have any code, nor it is inherited. And nothing is inherited from it. What is the purpose of throws clause. Should it not give compile error?
Thanks
Barkat

[Anthony Villanueva]

A method can declare a checked exception in its throws clause (assuming this does not collide with an overriden method's throws clause) even if the implementation doesn't really throw a checked exception.
For example:

public class Test74
{
public static void main (String[] args) throws java.io.IOException
{
System.out.println("Done");
}
}

Is that what you meant?

[Barkat Mardhani]

Yes that is what I mean. But should it not be redundant when such exception will never be thrown?

[Jose Botella]

If you omit the throws clause in m2() there will be the following error

 exception WhiteException is never thrown in body of corresponding try statement

That is you need to declare, but not really throw, those checked exceptions that are caught.
If the catch wasn't present, notice that the compiler won't complain about declaring an exception that is not thrown. In this way a method is able to declare the type or supertypes of the exceptions that possibly overriding methods are allowed to throw.

[Barkat Mardhani]

I got it thank you.