Using the if-then-else framework, Part 2

The final steps to coding branching logic without nested ifs

By Paul Corazza, JavaWorld.com, 04/28/00

Listing 1. Sample nested-if code

void decideUrl() {
     DataBank.Region region = db.getRegion();
     double limit = db.getLimit();
     String id = db.getUserId();
     if(region.equals(DataBank.EAST_REGION)) {
         if(limit > db.LIMIT_THRESHOLD) {
             setUrl(EAST_PRIVILEGED);
         }
         else {
             setUrl(EAST_NOT_PRIVILEGED);
         }
     }
     else if(region.equals(DataBank.WEST_REGION)) {
         if(isMemberWestAlliance(id)) {
             if(limit > db.LIMIT_THRESHOLD) {
                 setUrl(WEST_MEMBER_PRIVILEGED);
             }
             else {
                 setUrl(WEST_MEMBER_NOT_PRIVILEGED);
             }
         }
         else {
             if(limit > db.LIMIT_THRESHOLD) {
                 setUrl(WEST_NONMEMBER_PRIVILEGED);
             }
             else {
                 setUrl(WEST_NONMEMBER_NOT_PRIVILEGED);
             }
         }
     }
     else {
         setUrl(OTHER_REGION);
     }
 }

TEXTBOX:

TEXTBOX_HEAD: Using the if-then-else framework: Read the whole series!

• Part 1. Code maintainable branching logic with the if-then-else framework
• Part 2. The final steps to coding branching logic without nesting ifs
• Part 3. Enhance the framework to support large-scale projects

:END_TEXTBOX

Recall that for the sake of having runnable code, I have wrapped this version of the decideURL() method in a class URLProcessor_bad in the downloadable sample code. The implementation of the if-then-else framework in this example will result in a rewrite of decideURL(). (This new version of the method appears in the class URLProcessor_good; see Resources.)

In Part 1, I outlined the main framework classes with which you will work and the four steps required to implement the framework, listed again here:

1. Determine the conditions. Determine the individual conditions involved in the logic at hand, and implement each as an instance of Condition (or possibly of a Condition subclass).
2. Determine the actions. Determine the possible actions that need to be executed and implement each as an instance of Action (or possibly of an Action subclass).
3. Implement the Updateable interface. Decide which object(s) should receive the actions implemented in Step 2 and make sure that it (they) implements the Updateable interface (which means you need to implement the doUpdate() method).
4. Subclass the Invoker class. Create a concrete invoker subclass of Invoker and implement the loadConditions() and loadRules() methods.

By the end of Part 1, I completed the analysis of the conditions involved in rewriting the decideURL() method. Here is a quick review:

The code in Listing 1 involves three main conditions that must be evaluated for the main action -- setting the value of a URL -- to be fired off: a location condition, a condition on the value of a limit, and a condition concerning membership to "West Alliance." Since the possible values of location cannot be reduced to a single Boolean-valued condition, I broke down the location condition into three finer-grained conditions: whether or not the specified region is EAST_REGION, whether or not it is WEST_REGION, and whether or not it is neither. The analysis of conditions resulted in the creation of five objects, which completed the work for Step 1: