IOException vs RuntimeException Java

Question

class Y {
    public static void main(String[] args) throws RuntimeException{//Line 1
        try {
            doSomething();
        }
        catch (RuntimeException e) {
            System.out.println(e);
        }
    }
    static void doSomething() throws RuntimeException{ //Line 2
        if (Math.random() > 0.5) throw new RuntimeException(); //Line 3
        throw new IOException();//Line 4
    }
}

When I throw two types of exception (IOException in Line4 and RunTimeException in Line3), I found that my program does not compile until I indicate "IOException" in my throws clauses in Line 1 & Line 2.

Whereas if I reverse "throws" to indicate IOException being thrown, the program does compile successfully as shown below.

class Y {
    public static void main(String[] args) throws IOException {//Line1
        try {
            doSomething();
        }
        catch (RuntimeException e) {
            System.out.println(e);
        }
    }
    static void doSomething() throws IOException {//Line 2
        if (Math.random() > 0.5) throw new RuntimeException();//Line 3
        throw new IOException();//Line 4
    }
}

Why should I always use "throws" for IOException even though RuntimeException is also thrown (Line 3) ?

score 21 · Accepted Answer · edited May 23 '17 at 12:32

21

Because IOException is a Checked Exception, which should be either handled or declared to be thrown. On contrary, RuntimeException is an Unchecked Exception. You don't need to handle or declare it to be thrown in method throws clause (Here I mean, it would be syntactically correct if you don't handle the unchecked exception. Compiler won't be angry). However there are some situation, where you need to handle and act accordingly for some Unchecked Exception.

Related Post:

References:

edited May 23 '17 at 12:32

Community

1
1

answered Aug 24 '13 at 08:22

Rohit Jain

209,639
45
409
525

You don't need to handle RuntimeException, does not sound correct? – Juned Ahsan Aug 24 '13 at 08:25
There are some `RuntimeException`s that should be handled, even though they don't have to be caught/declared. `NumberFormatException` is a common example. – chrylis -cautiouslyoptimistic- Aug 24 '13 at 08:25
2

@chrylis, **should** be handled and **need** to be handled are actually a big difference. – svz Aug 24 '13 at 08:27
1

@JunedAhsan. I said that in terms of Compiler error not being thrown. – Rohit Jain Aug 24 '13 at 08:32
@RohitJain Yes i wondered, there should be some rationale ;-) – Juned Ahsan Aug 24 '13 at 08:44

score 1 · Answer 2 · answered Aug 24 '13 at 08:23

1

The reason is that there are two different types of exceptions in this case. One is a checked exception which ought to be handled by the programmer and the other one is an unchecked exception which doesn't require special handling.

answered Aug 24 '13 at 08:23

svz

4,516
11
40
66

IOException vs RuntimeException Java

2 Answers2

Linked