Is there any Checkstyle/PMD/Findbugs rule to force “else if” to be on the same line?

Question

In our project for chained if/else/if we would like to have following formatting:

if (flag1) {
    // Do something 1
} else if (flag2) {
    // Do something 2
} else if (flag3) {
    // Do something 3
}

And forbid following one:

if (flag1) {
    // Do something 1
} else {
    if (flag2) {
        // Do something 2
    } else {
        if (flag3) {
            // Do something 3
        }
    }
}

Is there some predefined rule in either of listed above static code analysis tools to force this code style? If no - I know there is an ability to write custom rules in all of those tools, which one would you suggest to implement such a rule (not really familiar with writing custom rules in either of them)?

Answer 1

It can be done with CheckStyle, but you'll have to code a custom check.

Using a custom check allows you to completely ignore comments. The line number that a token is on can be determined by calling getLineNo() on the DetailAST. Here's what the AST looks like, with line number information (red circles):

The custom check's code will likely be quite short. You basically register for LITERAL_ELSE tokens and see if LITERAL_IF is their only child. Also remember to handle SLISTs. In those cases, LITERAL_IF and RCURLY should be the only children. Both cases are illustrated in the above picture.

---

Alternative using a RegExp check

For the record, I originally thought one could also configure a regex match using else[ \t{]*[\r\n]+[ \t{]*if\b for the format property (based on this post).

Here's the mentioned regex as a railroad diagram:

This turned out not to be feasible, because it produces false negatives when there are comments between between else and if. Worse, it also produces false positives when the nested if is followed by unrelated code (like else { if() {...} <block of code>}. Thanks @Anatoliy for pointing this out! Since comments and matching braces which are mixed with comments cannot be reliably grasped by regexes, these problems obsolete the RegExp approach.

Answer 2

This post says you can't do it in Checkstyle.

In PMD you definitely can. The AST (abstract syntax tree) is different.

For the pattern you don't want

if (true) {
  String a;
} else {
  if (true) {
    String b;
  }
}

The tree looks like:

<IfStatement>
  <Expression>...</Expression>
  <Statement>...</Statement>
  <Statement>
    <Block>
      <BlockStatement>
        <IfStatement>...

For the pattern you do want

if (true) {
  String a;
} else if (true) {
  String b;
}

The tree looks like:

<IfStatement>
  <Expression>...</Expression>
  <Statement>...</Statement>
  <Statement>
    <IfStatement>...

In PMD 4 (which I used to make these trees), you write a rule by writing a XPath expression matching the pattern you don't want to occur.