Java Secret: Labels as an Anti-Pattern

Overview

It is well known that Java has no goto. (Technically goto is a keyword, but it cannot be used any where)

However, Java has continue and break which can be mis-used just like goto can.

NOTE: This article is not an example what to do. It is an article which will help you better understand Java's edge cases and help you recognise mis-use.

Recognising a label

A Java Puzzler uses a label to create some strange looking code.

public static void main(String... args) {
    http://www.google.com/search?q=Hello+World
    System.out.println("Hello World");
}

This compiles and runs. This is because what appears to be a URL is actually a label and a comment. ;)

goto backwards

If you want to go back you can use the following structure.

while(true) {
   // do something.
   if(go-back) 
      continue;
   // do something else.
   if(go-back) 
      continue;
   break;
}

The only purpose of the loop is to allow code to jump back to the start. Using labels it could look like this.

LOOP1: while(true) {
   // do something.
   LOOP2: while(true) {
       // do something.
       if(go-back) 
         continue LOOP1;
       // do something.
   }
   // do something else.
   if(go-back) 
      continue LOOP1;
   break;
}

This creates the sort of spaghetti code which removing goto was supposed to avoid. :(

goto forwards

You can use break to goto forwards

ONE: do {
   // do something
   if(goto-forwards)
      break ONE;
   // do something.
} while(false);

The while(false) is a loop which only loops once. i.e. not really a loop.

What is perhaps surprising and that unlike continue, break doesn't need a loop.

HERE:
{
    System.out.println("First");
    if (true)
        break HERE;
    System.out.println("Last");
}

prints

First