Instance Initializer Exception Handling

Question

I didn't understand the difference between these codes. One of them is compiled, the other one isn't.

{
    if (true) {
        try {
            throw new IOException();
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }
}

This is a instance initializer, i throw a checked exception and then handle it, and this code compile.

But this one isn't compile.

{
    while (true) {
        try {
            throw new IOException();
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }
}

Exception says: Initializer must be able to complete normally

Here is the image of both Instance Initializer

Answer 1

Stripping down your examples - because the try/catch/rethrow doesn't matter here:

{
    if (true) {
        throw new RuntimeException();
    }
}

vs

{
    while (true) {
        throw new RuntimeException();
    }
}

What matters here is whether Java considers that the statements can complete normally, because JLS 8.6 says:

It is a compile-time error if an instance initializer cannot complete normally (§14.22).

Consult the rules on unreachable statements:

• An if-then statement can complete normally iff it is reachable.
• ...
• A while statement can complete normally iff at least one of the following is true:
  • The while statement is reachable and the condition expression is not a constant expression (§15.29) with value true.
  • There is a reachable break statement that exits the while statement.

(You can also read this same section to find out why I say the try/catch/rethrow didn't matter).

Notice that if doesn't have the same conditions on normal completion as while.