Java - Can not create objects in a shortend line if

Question

Lately I started programming in Java again after a short break and whilst doing a project for a friend I noticed something odd: It appears you can not create objects in shortened ifs, example:

if( condition )
     Statement statement = con.createStatement();

(I cut the code short for simplicity's sake.)

I receive an error saying that Statement (a class from the java.sql package) can not be resolved to a variable, however, if I were to wrap the code with braces it would work fine.

I assume this is some problem with the compiler turning this in to a multiline statement but I'm uncertain, I'd like to know the reason for this behaviour, thanks in advance!

That has something to do with `statement`'s `scope`!!! Please post the relevant code to clarify more,cutting short the code won't help us! — Am_I_Helpful, Sep 06 '14 at 08:16
Yes I am sure adding braces fixed the problem, yes I have imported the required classes. Full example: `if( ! (con == null) ) Statement statement = con.createStatement();` Gives an error whilst: `if( ! (con == null) ){ Statement statement = con.createStatement(); }` Works fine — Mickey695, Sep 06 '14 at 08:17
Well, I tried similar code : `if (i > 3) String str = "aaa";`. I got a different error - `error : not a statement`. — Eran, Sep 06 '14 at 08:21
And this is why you should always use `{}` in your *if statements*. — christopher, Sep 06 '14 at 08:21

T.J. Crowder · Accepted Answer · 2014-09-06T08:50:12.683

You can't declare a variable there (the current error, from Java 8, is error: variable declaration not allowed here). If you think about it, it makes sense: You haven't created a new scope (but using a block), but you're creating a situation where sometimes in the current scope, there will be a statement variable, and other times there won't. E.g.:

if (condition)
    Statement statement = con.createStatement();

// Does `statement` exist here? What would Schrodinger say?

If you use a block, it clarifies the matter: The variable exists, but only within the block.

if (condition) {
    Statement statement = con.createStatement();
    // `statement` exists here
}
// `statement` does not exist here

If you want statement to exist in the current scope, you have to separate your declaration from your initialization:

Statement statement;

if (condition)
    statement = con.createStatement();

But then you run into the issue that statement may not have been initialized. To avoid that, you have a couple of options:

Statement statement;

if (condition)
    statement = con.createStatement();
else
    statement = null;

or

Statement statement = condition ? con.createStatement() : null;

Or of course, just use the block and only use statement within it. FWIW — and this is totally up to you — I (and many style guides) recommend always using blocks, because not doing so can introduce maintenance issues when you need (inevitably!) to add a second statement to the body of the if...

This is quite intriguing, thank you for the clarification @T.J.Crowder ! — Mickey695, Sep 06 '14 at 08:30
I like how I never knew this. I guess that's one of the perks of following standards. :) — lxcky, Sep 06 '14 at 08:32

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

2

As per Java specification,

  A local variable, one of the following:
       A local variable declared in a block (§14.4)
       A local variable declared in a for statement (§14.14)

If you don't wrap the statement within braces, it immediately falls out of scope.

More info here : https://stackoverflow.com/a/9206679/978501

edited May 23 '17 at 12:05

Community

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answered Sep 06 '14 at 08:34

Sridhar

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Java - Can not create objects in a shortend line if

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