"Exception is never thrown in body of corresponding try statement." When does this error occur?

[Bindu Tadivaka]

I got the Compiler error "IOException is never thrown in body of corresponding try statement" for the below code. But I have got no error when I caught "NumberFormatException" instead of "IOException". When does this error occur? And when does it not??


import java.io.*;

public class abc
{

public static void main(String args[]) throws Exception
{
try
{
System.out.println("Try ");
}
catch (IOException e) {}
finally
{
System.out.println("Finally ");
}
}
}

[Koushik Ghosh]

IOException can never occur from System.out.println(). But NUmberFormatException can occur in the following case :

public static void main(String[] args) throws Exception {
		try {
			System.out.println(Integer.valueOf("Try"));
		} catch (NumberFormatException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}

	}

I hope this clears your doubt.

[James X Peterson]

Hi Bindu ,

If the underlying stream is a socket stream and the network dies while you're reading, if you're attempting to read from a stream that's been closed, etc.There are many instances where the IOException will be raised. If you want to raise the IOException you can do it explicitly by using keyword throw.Look at the following code which will throw the IOException explicitly.

public static void main(String... args){  
           try
           {
               throw new IOException();               
           }
           catch(IOException e)
           {
               System.out.println("This is from IOExceptiom");
           }        

  } 

[Bindu Tadivaka]

Thank you Koushik and James.

What I understood is that, Compiler does not generate an Error if we try to catch an Exception that might possibly be thrown in the try block. But if there is absolutely no possibility of that Exception occurring in the try block, then the Compiler would complain.

I guess an empty try block guarantees no exceptions, whatsoever. In this case, compiler should complain if we try to catch any kind of Exception. But, I still see no error in case of NumberFormatException and error in case of IOException. Why is this discrepancy?

try
{

}
catch (NumberFormatException e) {}

finally
{
System.out.println("Finally ");
}

[Henry Wong]

Bindu Tadivaka wrote:Thank you Koushik and James.

What I understood is that, Compiler does not generate an Error if we try to catch an Exception that might possibly be thrown in the try block. But if there is absolutely no possibility of that Exception occurring in the try block, then the Compiler would complain.

I guess an empty try block guarantees no exceptions, whatsoever. In this case, compiler should complain if we try to catch any kind of Exception. But, I still see no error in case of NumberFormatException and error in case of IOException. Why is this discrepancy?

try
        {
               
        }
catch (NumberFormatException e) {}
	
finally
        {
            System.out.println("Finally ");
        }

The compiler only error checks checked exceptions -- hence, the name. IOException is a checked exception. NumberFormatException is an unchecked exception.

Henry

[Bindu Tadivaka]

Thanks, Henry :-)

What I finally understood is, if we catch a checked exception, that has no possibility of being thrown in the try block, compiler would generate an error. In case of unchecked exception, even if there is no possibility of it being thrown in the try block, compiler does not complain.

I hope I am correct this time :-)