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 SLIST s. In those cases, LITERAL_IF and RCURLY should be the only children. Both cases are illustrated in the above picture.

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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.