Interfaces and Inheritance and exceptions..question

[Esther Kak]

Could someone explain exceptions and inheritance for this code...All I need to know is how to use methods that throw exceptions in the child class and parent class. I am confused

public class Parent(){
public m1() throws FileNotFoundException, SSLException
{ 
 System.out.println("Parent m1");
}
public Interface I1(){
public m1() throws IOException
}
public class MyQuestion extends Parent implements I1
{
 public m1(){
 System.out.println("Print Yes");
  }
}

[Mark Uppeteer]

When thinking about this, you should always think about a case where an exception could 'slip through' the try catch net. It should never be possible.
e.g. If a parent has a method 'a' that throws a FileNotFoundException , then
it can not have a child that overides that method 'a' and that throws an
exception that is broader than its FileNotFoundException (broader than
FileNotFoundException is e.g. IOException because FileNotFoundException
extends IOException). Why not ? Because of polymorphism. One could easily
write Parent p = new Child(); and if somebody would call that method 'a' on
p, then a try{}catch(FileNotFoundException){} would have to be done. And if
the child would be able to throw an IOException (like HttpException), then
the parent's try catch would not suffice. Therefore it can not be allowed.

If the child overides the method 'a' and choses to throw a subclass of the
parents exception or nothing, then there is never a problem since the
FileNotFoundException would always catch that.

public class Parent(){
public m1() throws FileNotFoundException, SSLException
{ 
 System.out.println("Parent m1");
}
 
public Interface I1(){
public m1() throws IOException // whoever instatiates this will need to catch this exception
   // because it might be thrown
}
 
public class MyQuestion extends Parent implements I1
{
 public m1(){ /* THIS implementation of the interface I1 choses 
     not to throw an IOException and concerning inheritance, the child can 
     always throw more specific or no exception when overiding a method. 
 
     If I would write 
     [I]I1 myI = new MyQuestion(); try{myI.m1();}catch(IOException)[/I]
     then this can never throw an exception I'm not expecting. */
 
 System.out.println("Print Yes");
  }
}   
 

[ November 08, 2007: Message edited by: Mark Uppeteer ]
long line edited
[ November 09, 2007: Message edited by: Burkhard Hassel ]

[Esther Kak]

Thanks. That gives me a clear understanding.

[Esther Kak]

I am new to using java exceptions...but could you please explain why this code is not working. All the following three files are in the same folder. All I am trying to do is... to do an Example on Inheritance, Interfaces and Exceptions. What do I have to do to make this code function properly. Can I define a method to throw exceptions without actually causing them to happen in the method? This code does not compile though. Need help.

Parent.java - has a method that throws two exceptions

public class Parent
{
 
public void m1() throws FileNotFoundException, EOFException
{
System.out.println("Parent throwing FNF, EOF Exceptions");
}
}

I1.java-has a method defined which throws IOException

public interface I1
{
public void m1() throws IOException;
}

MyQuestion.java - has a method with same name but throws nothing.
In this class, I created an instance for this class and want to call the method from the interface.I understand that there is no real use, but I still want to call the interface's method.My intention here is to call the method from the interface and then implement it.
AM I RIGHT OR AM I CONFUSED?

public class MyQuestion extends Parent implements I1
{
public void m1()
{
System.out.println("My question is answered");
}
public static void main(String args[])
{
MyQuestion mq= new MyQuestion();
try{
mq.m1();
}
catch(IOException ioe){}
}
}