Java exception try-catch Exception vs IOException

Question

The following code compiles ok even though the try block doesn't actually throw any Exception.

public static void main(String[] args)  {
    try {} 
    catch (Exception e) {} // compiles ok
}

But if the catch is replaced with a sub-class of Exception, the code won't compile.

public static void main(String[] args)  {
    try {} 
    catch (IOException e) {} // won't compile. 
}

The compiler error is: Unreachable catch block for IOException. This exception is never thrown from the try statement body.

How come this behavior when both Exception & IOException are checked exceptions? I'm using Java 7.

`RuntimeException`s are also `Exception`s, so not all Exceptions are checked. — Gábor Bakos, Nov 10 '14 at 21:19

score 8 · Answer 1 · answered Nov 10 '14 at 21:19

The compiler can know exactly what part of the code can throw an IOException because it is a checked exception, so every method that can throw this kind of exception must specify it in the method signature.

On the other hand, runtime or unchecked exceptions are not meant to be expected, and since RuntimeException (the parent class of unchecked exceptions) also extends the class Exception, then the compiler is ok with it.

score 3 · Answer 2 · answered Nov 10 '14 at 21:19

Exception has subclasses that are unchecked exceptions (RuntimeException extends Exception). Throwable will behave similarly. RuntimeException or subclasses of RuntimeException will behave similarly. Subclasses of Exception other than RuntimeException will not.

score 3 · Answer 3 · edited Nov 10 '14 at 21:34

Manouti's answer seems correct, but according to the java documentation:

It is a compile-time error if a catch clause catches checked exception type E1 but there exists no checked exception type E2 such that all of the following hold:

E2 <: E1 The try block corresponding to the catch clause can throw E2 No preceding catch block of the immediately enclosing try statement catches E2 or a supertype of E2. unless E1 is the class Exception.

There is an explicit case for the throwing of an Exception instance (the Exception class is exceptional, one might say). This is the Java 5 documentation, but unless someone sees otherwise, I highly doubt this has changed since

Looking at the inheritance tree for Exception and IOException

https://docs.oracle.com/javase/7/docs/api/java/lang/Exception.html

and

https://docs.oracle.com/javase/7/docs/api/java/io/IOException.html?is-external=true

in Java 7, I don't see that the discussion on Checked/Unchecked exceptions is directly relevant - while it's true that unchecked exceptions don't follow the same rules, unchecked exceptions must inherit from RuntimeException, which Exception of course does not (it is the parent of that class)

https://docs.oracle.com/javase/specs/jls/se5.0/html/classes.html#308526

(again, Java 5 docs, but it hasn't changed) https://docs.oracle.com/javase/specs/jls/se5.0/html/exceptions.html

Java exception try-catch Exception vs IOException

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