Checked Exception 2 kinds of behaviours

Question

I have noticed that there are 2 kinds of checked exceptions:

1)

public static void main (String []args)
{
    try{
        //no exception thrown here, it still compiles though.
    }
    catch(Exception e){System.out.println(e);}
}

2)

public static void main (String []args)
{
    try{
         // it does not compile if not written any code which potentially might throw it
    }
    catch(IOException io){System.out.println(io);}
}

Is there any rule to anticipate this behaviour? to know in advance which is not mandatory to be present and it is? Hope I have been quite clear with explaining the issue. Thanks, Indeed ItIs

score 4 · Accepted Answer · answered Sep 22 '13 at 08:00

4

The Java Language specification states

It is a compile-time error if a catch clause can catch checked exception class E1 and it is not the case that the try block corresponding to the catch clause can throw a checked exception class that is a subclass or superclass of E1, unless E1 is Exception or a superclass of Exception.

answered Sep 22 '13 at 08:00

Sotirios Delimanolis

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I believe the reason for this is because `Exception` includes the run-time exceptions. This is what I was trying to say in my answer. Is that not the case? – Nicole Sep 22 '13 at 08:06
1

@NickC Exception might be the superclass of all unchecked exceptions, but it is not itself an unchecked exception. It is a checked exception. – Sotirios Delimanolis Sep 22 '13 at 08:07
Correct, I am not saying that it is. I'm suggesting that it appears the reason for the clause in your answer is because it is the superclass of all run-time exceptions; so it cannot compute compile-time analysis on whether `Exception` can be caught or not. – Nicole Sep 22 '13 at 08:09
@nickC It can if you call a method that is declared as `throws Exception`, otherwise it follows the JLS. – Sotirios Delimanolis Sep 22 '13 at 08:10
Now you've lost me. It can know if it *is* possible, but the compiler cannot be sure that it's *not* possible. – Nicole Sep 22 '13 at 08:13
@NickC Maybe I misunderstood/read what you were saying. Undelete your answer. – Sotirios Delimanolis Sep 22 '13 at 08:14

Rohit Jain · Answer 2 · 2013-09-22T08:21:26.327

3

The code compiles fine with Exception, because it is a super class of all Checked and Unchecked exceptions. So, a catch clause with Exception can handle the unchecked exception too. And for Unchecked exception, compiler cannot check at compile time that the exception is thrown or not. So, it won't give any error for Exception.

You can even try this with RuntimeException:

try {

} catch (RuntimeException e) {

}

... this will compile too.

From JLS Section 11.2.3 - Exception Checking:

It is a compile-time error if a catch clause can catch checked exception class E1 and it is not the case that the try block corresponding to the catch clause can throw a checked exception class that is a subclass or superclass of E1, unless E1 is Exception or a superclass of Exception.

edited Sep 22 '13 at 08:21

answered Sep 22 '13 at 08:01

Rohit Jain

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Yes but `Exception` is not an unchecked exception. If it was thrown in the body, you would still have to catch it. – Sotirios Delimanolis Sep 22 '13 at 08:04
@SotiriosDelimanolis. Yeah true. But it can handle the unchecked exception too. I said, it's a super class of all checked and unchecked exception. This compiles for the reason of unchecked subclass exceptions. – Rohit Jain Sep 22 '13 at 08:04
@SotiriosDelimanolis. I'm searching for the precise reason in JLS, will post it if found. – Rohit Jain Sep 22 '13 at 08:05
See my answer below and http://docs.oracle.com/javase/specs/jls/se7/html/jls-11.html#jls-11.1.1 – Sotirios Delimanolis Sep 22 '13 at 08:05
@SotiriosDelimanolis. OOPs, missed your comment, while I was editing. – Rohit Jain Sep 22 '13 at 08:08
It'll be easier for OP to understand with redundancy. – Sotirios Delimanolis Sep 22 '13 at 08:09

score 2 · Answer 3 · edited May 23 '17 at 12:05

Exception is a checked exception itself, but is also the superclass for RuntimeException and other unchecked exceptions.

1) In the first case, because the Exception class hierarchy includes RunTimeException, the compiler cannot conclude that an Exception will not be thrown in your try { ... } block. So, it must allow the code.

It can confirm the positive case (if you call a method that declares throws Exception or a non-run-time subclass), but it cannot confirm the negative case.

2) In the second case, it can conclude at compile-time that IOException will not be thrown in the try { ... } block, because IOException (nor any other checked exception besides Exception itself) is not the superclass for any run-time exceptions.

Of the following behaviors, I believe that Exception is the only special case with a combination of the two types:

If thrown:
1A. Does not need to be declared or 1B. Must be declared

If declared:
2A. Does not need to be thrown or 2B. Must be thrown

If not thrown:
3A. May be caught or 3B. Must not be caught

RuntimeException: 1A, 2A, 3A
Any other checked exception: 1B, 2B, 3B
Exception: 1B, 2A, 3A

More info on checked vs. unchecked exceptions in this answer: https://stackoverflow.com/a/6116020/143295

@SotiriosDelimanolis Thank you, I am updating my answer to clarify that `Exception` itself is checked but is the parent class for unchecked exceptions, following our comments on your answer. Good answer btw. — Nicole, Sep 22 '13 at 08:18
@SotiriosDelimanolis Thank you, I learned something today. I updated my answer to describe the situation I was seeing when I ran test code. — Nicole, Sep 22 '13 at 08:38

Checked Exception 2 kinds of behaviours

3 Answers3