confusion

[Rippon Jalali]

when i run the program below it gives compiler error saying that myref.test() does not throw exception.but as i know it is not necessary that the code should throw the exception


import java.io.*;
public class TechnoSample {
public static void main(String[] args) {
TechnoSampleSub myref = new TechnoSampleSub();
try{
myref.test();
}catch(IOException ioe){}
}
void test() throws IOException{
System.out.println("In TechnoSample");
throw new IOException();
}
}
class TechnoSampleSub extends TechnoSample {
void test() {
System.out.println("In TechnoSampleSub");
}
}

[Paul Sturrock]

Indeed. The test method you call does not declare it throws IOException, so trying to catch this exception will result in an "unreachable catch block" compiler error. Is this what you wanted to know?

[Rippon Jalali]

But whenver we write a try catch block.It is not necessary that exception will occur .It may or may not occur.
The error comes:
t.java:7: exception java.io.IOException is never thrown in body of corresponding
try statement
}catch(IOException ioe){}

[Ernest Friedman-Hill]

No, it's not true that it may or may not occur. It will not occur, since your reference variable is of type TechnoSampleSub, and TechnoSampleSub.test() does not declare the exception. If the reference variable were of type TechnoSample, then it would be a different story.

[Rippon Jalali]

But as shown in the simple program below,never a exception can be thrown in try block.BUt prog compiles fine and runs


public class t {
public static void main(String[] args) {
int i = 2;
try {
i=9;

} catch (Exception ae){
System.err.println(" exception caught");
}
System.out.println(i);
}
}

[Milan Jagatiya]

hi

Originally posted by Rippon Jalali:
But as shown in the simple program below,never a exception can be thrown in try block.BUt prog compiles fine and runs


public class t {
public static void main(String[] args) {
int i = 2;
try {
i=9;

} catch (Exception ae){
System.err.println(" exception caught");
}
System.out.println(i);
}
}

yes , above code work well. because it catch Exception class not the special kind of exception classes. you can not catch IOException if your block not having IO operation statements. Like wise you can not catch SQLException if your block not having SQL operations statements.

milan.

[Rippon Jalali]

but even if you throw exception other than general 'Exception' like 'ClassCastException'.Still it doesn't throw exception.

[Rippon Jalali]

"sorry the previuos one had a mistake".I am repeating again.But even if you throw exception other than general 'Exception' like 'ClassCastException'.Still it doesn't catch that exception.

[Milan Jagatiya]

because they are runtime unchecked exceptions. and IOException is compile time. so compiler never allow you to throw if there is no IO statements.

milan.

[Rippon Jalali]

Thanks for all of your help.

[Anton Uwe]

Well, let's try to explain it: You are free to "catch" all unchecked exceptions and errors, even Throwable. But if you want to "catch" checked exceptions, there are some restrictions.
1st, it's no problem to catch an exception using "catch (Exception e)".

2nd, when you want to catch a special checked exception (subclass of Exception), the compiler will check if the try-block "is able" to throw this special exception (or a subclass of it).

import java.io.*;
class A
{
   public static void main(String[] args) {
      try{
   throw new IOException();
      } catch (IOException ioex){
      }
   }
}

This is perfectly legal. The code in the try-block throws an IOException, so you can catch it.

import java.io.*;
class A
{
   public static void main(String[] args) {
      try{
   foobar();
      } catch (IOException ioex){
      }
   }
   public static void foobar() throws IOException {
   }
}

Perfectly legal, too. The method foobar() declares that it might throw an IOException (though it never does), so, for the compiler, the code in the try-block is able to throw an IOException.

import java.io.*;
class A
{
   public static void main(String[] args) {
      try{
   foobar();
      } catch (IOException ioex){
      }
   }
   public static void foobar(){
   }
}

Not legal, because foobar() is unable to throw an IOException.

Hope this helps a bit...

[anup sachan]

This was a extermely good explanation.Thanks a lot.all my doubts are cleared now.