Loop-switch sequence

A loop-switch sequence (also known as the for-case paradigm or Anti-Duff's Device) is a programming antipattern where a clear set of steps is implemented as a switch-within-a-loop. The loop-switch sequence is a specific derivative of spaghetti code.

It is not necessarily an antipattern to use a switch statement within a loop—it is only considered incorrect when used to model a known sequence of steps. The most common example of the correct use of a switch within a loop is an inversion of control such as an event handler. In event handler loops, the sequence of events is not known at compile-time, so the repeated switch is both necessary and correct (see event-driven programming, event loop and event-driven finite state machine).

This is not a performance antipattern, though it may lead to an inconsequential performance penalty due to the lack of an unrolled loop. Rather, it is a clarity antipattern, as in any non-trivial example it is much more difficult to decipher the intent and actual function of the code than the more straightforward refactored solution.

Example

An event-driven solution would implement a listener interface: <syntaxhighlight lang="java"> String key = null; String value = null; List<String> params = null; int column = 0;

public void addToken(token) {

   // parse a key, a value, then three parameters 
   switch (column) {
       case 0:
           params = new LinkedList<String>();
           key = token;
           break;
       case 1:
           value = token;
           break;
       default:
           params.add(token);
           break;
   }
   if (++column >= 5) {
       column = 0;
       completeRow(key, value, params);
   }

} </syntaxhighlight>

But without the listener, it becomes an example of the antipattern: <syntaxhighlight lang="java"> // parse a key, a value, then three parameters String key = null; String value = null; List<String> params = new LinkedList<String>();

for (int i = 0; i < 5; i++) {

   switch (i) {
       case 0:
           key = stream.parse();
           break;
       case 1:
           value = stream.parse();
           break;
       default:
           params.add(stream.parse());
           break;
   }

} </syntaxhighlight>

And here is the refactored solution: <syntaxhighlight lang="java"> // parse a key and value String key = stream.parse(); String value = stream.parse();

// parse 3 parameters List<String> params = new LinkedList<String>(); for (int i = 0; i < 3; i++) {

   params.add(stream.parse());

} </syntaxhighlight>