Why so much difference between two similar class?

[james gong]

read following code,why the code1 compile while the code2 not.Give me something about the item.
code1:
public class AQuestion
{
public static void main(String args[])
{
System.out.println("Before Try");
try
{
}
catch(Throwable t)
{
System.out.println("Inside Catch");
}
System.out.println("At the End");
}
}
code2:
public class AQuestion
{
public static void main(String args[])
{
System.out.println("Before Try");
try
{
}
catch(java.io.IOException t)
{
System.out.println("Inside Catch");
}
System.out.println("At the End");
}
}
the code1 ans :3(No compiler error. The lines "Before Try" and "At the end" are printed on the screen. )
the code2 ans :1(Compiler error complaining about the catch block where no IOException object can ever be thrown. )

[Jane Griscti]

Hi James,
The first example passes the compile ok as all exceptions, runtime and checked, extend Throwable. Since a runtime exception can be thrown any time, anywhere, there is no problem.
In the second example, IOException is a checked exception. The compiler can tell from the code that there is no possibility that one will be thrown and coughs up the error.
Hope that helps.

------------------
Jane Griscti
Sun Certified Programmer for the Java� 2 Platform

[Guoqiao Sun]

As far as I know, in code 1, you are trying to catch Throwable, which includes both Exception and Error. And it is possible that during the

try
{
}

there are some Error or RuntimeException(which are indeed, unchecked exception) happens, that is the reason compiles let you go with the compiling.
But with code 2, you are trying to catch <code>IOException</code>, which is a checked exception, the compiler definitely knows that it will not happen during

try
{
}

and as a result, failed the compiling.
Correct me if i am wrong.
Guoqiao

[james gong]

thanks a lot !I think you are right.