Don’t Extract Everything Into a Method

Every now and then, I tweet something like this, just to piss off some clean coders:

Apart from the obvious trolling factor (why can’t I ever resist?), I do think there’s something thought provoking in such a tweet.

First off, given how rare break and continue statements are in Java code, many people probably don’t know that you can break out of an if-else block, or any labelled block, for that matter.

Secondly, we’ve been following cargo cults about clean code so much that we completely forget about some more obscure language features of our favourite languages that would be so handy in edge cases. Like these ones!

Breaking out of loops

Most people know how this works:

for (int i = 0;; i++)

    // Stop at 42
    if (i == 42)
        break;

    // Ignore even numbers
    else if (i % 2 == 0)
        continue;

    // Main action
    else
        System.out.println("i = " + i);

Of course, this code is bogus just to illustrate the syntax. A much simpler, refactored, equivalent version is this:

// Stop criteria in loop, duh
for (int i = 0; i < 42; i++)

    // Inverse if and else for even numbers
    if (i % 2 != 0)
        System.out.println("i = " + i);

This refactoring was obvious and trivial because the loop is so simple. But sometimes it isn’t, or sometimes the break or continue statement makes a loop just simpler to read (“simplicity” like “beauty” is in the eye of the beholder, of course). Parsers are the most obvious example, but also, for example, consider jOOQ’s AbstractRecord.compareTo() method, which contains this loop:

for (int i = 0; i < size(); i++) {
    final Object thisValue = get(i);
    final Object thatValue = that.get(i);

    if (thisValue == null && thatValue == null)
        continue;
    else if (thisValue == null)
        return 1;
    else if (thatValue == null)
        return -1;
    else {
        // ... Actually interesting comparisons
    }
}

// If we got through the above loop, the two records are equal
return 0;

jOOQ Records are compared with each other like SQL Records (if they are comparable, i.e. of equal degree): From the the first attribute to the last.

The interesting bit here is the continue statement, that skips to the next attribute, if both attributes are null.

Of course, if you are a feverish hater of such explicit, goto-style control flow, you would probably refactor this to something “much better”, namely:

for (int i = 0; i < size(); i++) {
    final Object thisValue = get(i);
    final Object thatValue = that.get(i);

    if (!(thisValue == null && thatValue == null)) {
        if (thisValue == null)
            return 1;
        else if (thatValue == null)
            return -1;
        else {
            // ... Actually interesting comparisons
        }
    }
}

// If we got through the above loop, the two records are equal
return 0;

Drawbacks:

  • There’s one more level of indentation
  • The if-else branch series is no longer “regular”, i.e. some conditions lead to quite different control flow than others
  • This worked fine for a single continue statement, but we might complicate this loop and add a second continue statement, or better: A nested loop that also contains break or continue statements…

With loops, the choice of using break or continue is sometimes “obvious”. Refactoring the imperative programming style to something more “elegant” might take quite a while of thinking, introduce regressions, and (ghasp) make the code more complex and less readable. Yes, I said it. Sometimes, imperative style is simpler (not better, just simpler).

Of course, this isn’t necessarily true in this particular example, so don’t haunt me 😉

Breaking out of an if

While breaking out of loops (or continuing them from in the middle) is somewhat common in complicated loops, breaking out of an if is certainly less common. In fact, you can break out of any labeled statement in Java (label-less breaks are possible with loops only):

// No-op
label1:
break label1;

// More interesting, Russian Roulette:
label2: {
    System.out.println("Pulling trigger");

    if (Math.random() < 0.16) {
        System.out.println("Click");
        break label2;
    }

    System.out.println("Whew");
}

So, why not break out of an if-else?

public class FunWithBreak {
    public static void main(String[] args) {
        label:
        if (args.length > 0) {
            System.out.println("We got args!");

            for (String arg : args)
                if ("ignore-args".equals(arg))
                    break label;

            for (String arg : args)
                System.out.println("Arg : " + arg);
        }
        else {
            System.out.println("No args");
        }

        System.out.println("End of program");
    }
}

Now run this program from the commandline

> java FunWithBreak
No args
End of program

> java FunWithBreak a b c
We got args!
Arg : a
Arg : b
Arg : c
End of program

> java FunWithBreak a b c ignore-args
We got args!
End of program

It’s kind of neat, no? The break in the middle of the if statement. It makes sense because there’s an early abort condition for the if-block, and only for the if-block.

Alternatives

Someone responded to my original tweet, that I should refactor the code and factor out a method for the if block, which would allow for replacing the break by return. Here’s how that would work:

public class FunWithBreak {
    public static void main(String[] args) {
        if (args.length > 0)
            nameThisLater(args);
        else
            System.out.println("No args");

        System.out.println("End of program");
    }

    // TODO find a meaningful name for this method later
    private static void nameThisLater(String[] args) {
        System.out.println("We got args!");

        for (String arg : args)
            if ("ignore-args".equals(arg))
                return;

        for (String arg : args)
            System.out.println("Arg : " + arg);
    }
}

Now, that certainly looks more common, which doesn’t mean it’s better. We’ve now factored out a useless method that is called only exactly once, and that is inlined (hopefully) by the JIT to become the original code again.

The advantage is that our original main() method got a bit simpler to read (once we’ve found a meaningful name for the method), but again, simplicity is in the eye of the beholder. But the price we pay here is that we have to pass all local state as method arguments. Simple in this case (single local variable args), but that can become quite complex, especially when we have local mutable collections that need to be passed around.

Besides, others might argue that one shouldn’t return from the middle of a method. They are proponents of “single-return-statement methods”.

Sure, why not. Here’s how that would look:

public class FunWithBreak {
    public static void main(String[] args) {
        if (args.length > 0) {
            ifBlock(args);
        }
        else {
            System.out.println("No args");
        }

        System.out.println("End of program");
    }

    private static void ifBlock(String[] args) {
        System.out.println("We got args!");

        boolean ignoreArgs = false;
        for (String arg : args) {
            if ("ignore-args".equals(arg)) {
                ignoreArgs = true;
                break; // We need break again!
            }
        }

        if (!ignoreArgs)
            for (String arg : args)
                System.out.println("Arg : " + arg);
    }
}

Is it really better?

I don’t know. I’ll just stick with the original break-out-of-if.

Bonus

Surprise your coworkers with an even more unusual placement of your label. Between the if statement and the ensuing block. After all, the curly braces are just an ordinary block, not part of the if statement

public class FunWithBreak {
    public static void main(String[] args) {
        if (args.length > 0) label: {
            System.out.println("We got args!");

            for (String arg : args)
                if ("ignore-args".equals(arg))
                    break label;

            for (String arg : args)
                System.out.println("Arg : " + arg);
        }
        else {
            System.out.println("No args");
        }

        System.out.println("End of program");
    }
}

Second bonus

Now that we’ve gotten all hooked on imperative control flow… What does this print?

for (int i = 0; i < 3; i++) {

    hmmm:
    try {
        System.out.println("Try      : " + i);
        if (i % 2 == 0)
            break hmmm;
        else
            throw new RuntimeException("Hmmm");
    }
    finally {
        System.out.println("Finally  : " + i);
        if (i < 2)
            continue;
    }

    System.out.println("Loop end : " + i);
}

Solution (select text below)

Try      : 0
Finally  : 0
Try      : 1
Finally  : 1
Try      : 2
Finally  : 2
Loop end : 2

Feature Request for the JLS: Auto-Rethrow

Java 7 has eased some pain in the area of exception handling when the new try-with-resources and multi-catch syntaxes were introduced. The latter is very interesting from a syntax perspective because it is the only place in Java where formal union types are allowed, similar to what Ceylon offers. Remember, a union type A | B is a type has all the properties that both types share. For example:

try {
    // ...
}
catch (SQLDataException | SQLWarning e) {

    // Properties that both types share:
    System.out.println(e.getSQLState());
}

The above can be quite useful in cases when we simply want to re-throw a variety of similar but disjoint exceptions in the same way. But often, we want do this:

// Some code
try {
}

// Rethrow
catch (OurException e) {
    throw e;
}

// Wrap other types of exceptions in OurException
catch (Exception e) {
    throw new OurException(e);
}

That’s quite a few lines of code for something that we do all the time and that could be called “rethrow-or-wrap” pattern. How about if we could rewrite the above as such:

// Some code
try {
}

// Re-throw exceptions of type OurException
// ... and wrap all other types in OurException
throws OurException;

Without a catch clause, this would be exactly the same as our previous example. Rethrow or wrap. Java is a language that heavily recycles keywords, so a dedicated keyword is out of question. throws would be a good keyword here as it is somewhat similar to the throws clause of a method signature, i.e. the throws clause of the try block.

If we only want to rethrow some exceptions and write tailor-made catch blocks for other types of exceptions, we could still do the same:

// Some code
try {
}

// Re-throw exceptions of type OurException
throws OurException

// But deal with all other types here
catch (Exception e) {
    throw new OurException("Custom message", e);
}

Java should have a “rethrow Exception” statement. tweet this

What are your thoughts on this? Would such a syntax pull its own weight?