Chain of Responsibility Design Pattern

The chain of responsibility pattern is used to pass a request through a chain of processors until it is resolved. It usually has a interface or abstract class that defines the common fields, request levels,  and methods used by all the processors. Each implementing processor is specialized to process only requests below a certain level.

For example, a company is hiring new employees. The policy is that if the position is entry level, the team leader interviews the person and makes the decision; if it is middle level, the project leader does the interview and makes the decision; else if the position is senior level, the department chair does the interview and makes the decision.

//Create the HireProcessor common interface
public abstract class HireProcessor {
      public static final int ENTRY = 1;
      public static final int MIDDLE = 2;
      public static final int SENIOR = 3;

      protected int level;

      protected HireProcessor nextProcessor

      public void setNextProcessor(HireProcessor np) {
            nextProcessor = np;
      }

      public void process(int lev) {
            if (lev <= level) {
                  printDecision();
            } else {
                  nextProcessor.process(lev);
            }
      }

      protected abstract void printDecision();
}

//Create each processor
//Entry level processor
public class EntryHireProcessor extends HireProcessor {
      public EntryHireProcessor(int level) {
            this.level = level;
      }

      protected void printDecision() {
            System.out.println("Hire as an entry level employee.");
      }
}

//Middle level processor
public class MiddleHireProcessor extends HireProcessor {
      public MiddleHireProcessor(int level) {
            this.level = level;
      }

      protected void printDecision() {
            System.out.println("Hire as a middle level employee.");
      }
}

//Senior level processor
public class SeniorHireProcessor extends HireProcessor {
      public SeniorHireProcessor(int level) {
            this.level = level;
      }

      protected void printDecision() {
            System.out.println("Hire as a senior level employee.");
      }
}

//The Runner
public class ChainDemo {
      public static void main(String[] args) {
            //create the chain
            HireProcessor ep = new EntryHireProcessor(HireProcessor.ENTRY);
            HireProcessor mp = new MiddleHireProcessor(HireProcessor.MIDDLE);
            HireProcessor sp = new SeniorHireProcessor(HireProcessor.SENIOR);

            ep.setNextProcessor(mp);
            mp.setNextProcessor(sp);

            //call the first element in the chain to process a request
            ep.process(HireProcessor.SENIOR);
            ep.process(HireProcessor.MIDDLE);
            ep.process(HireProcessor.ENTRY);
            ep.process(HireProcessor.SENIOR);
      }
}

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