Universidad Carlos III de Madrid

Solution

The solution is included in the following listing:

public class Program {
  private String id;
  private int priority;

  public Program(String id, int priority) {
    this.id = id;
    this.priority = priority;
  } // Program

  public int getPriority() {
    return priority;
  } // getPriority

  public void print() {
    System.out.println("Id: " + id);
    System.out.println("Priority: " + priority);
  } // print

} // Program

          

public class RealTimeProgram extends Program {
  final static int MAX_PRIORITY = 4;

  public RealTimeProgram(String id) {
    super(id, MAX_PRIORITY);
  } // RealTimeProgram

  public void print() {
    System.out.println("Real Time Program");
    super.print();
  } // print

} // RealTimeProgram

          

public class BackgroundProgram extends Program {
  final static int MIN_PRIORITY = 0;

  public BackgroundProgram(String id) {
    super(id, MIN_PRIORITY);
  } // BackgroundProgram

  public void print() {
    System.out.println("Background Program");
    super.print();
  } // print

} // BackgroundProgram

          

public class Scheduler {
  private Program programs[];

  public Scheduler(int size) {
    programs = new Program[size];
  } // Scheduler

  /** Adds a program, sorted from low to high priority */
  public void add(Program program) throws Exception {
    boolean found = false;
    int i = 0;
    int size = programs.length;
    int currentPriority = program.getPriority();
    // Find a place to insert into
    while ((!found) && (i < size)) {
      if (programs[i] == null) {
        found = true;
      } else if (currentPriority < programs[i].getPriority()) {
        found = true;
      } else {
        i++;
      }
    } // while
    if (found) {
      // Insert the program into the null position
      if (programs[i] == null) {
        programs[i] = program;
      } else {
        // Move the elements...
        // ...but what if it is full?
        if (programs[size - 1] != null) {
          throw new Exception("List is full (Cond 1)");
        } else {
          // Move from the end
          for (int j = size - 1; j > i; j--) {
            programs[j] = programs[j - 1];
          }
          // Then, asign the value
          programs[i] = program;
        }
      }
    } else {
      throw new Exception("List is full (Cond 2)");
    }
  } // add

  public Program next() throws Exception {
    boolean found = false;
    int size = programs.length;
    int i = size - 1;
    while ((!found) && (i >= 0)) {
      found = (programs[i] != null);
      i--;
    }
    if (!found) {
      throw new Exception("Empty list, no program to execute");
    } else {
      return programs[i + 1];
    }
  } // next

} // Scheduler

          

public class Programs {
  public static void main(String args[]) {
    Scheduler miScheduler = new Scheduler(6);
    try {
      Program program = new Program("1", 1);
      miScheduler.add(program);
      program = new Program("2", 2);
      miScheduler.add(program);
      program = new Program("3", 3);
      miScheduler.add(program);
      program = new Program("4", 2);
      miScheduler.add(program);
      RealTimeProgram realtimeProgram = new RealTimeProgram("5");
      miScheduler.add(realtimeProgram);
      BackgroundProgram backgroundProgram = new BackgroundProgram("6");
      miScheduler.add(backgroundProgram);

      // Test the full list (Cond 2)
      // miScheduler.add(new Program("7", 7));

      Program next = miScheduler.next();
      next.print();
    } catch (Exception ex) {
      System.out.println(ex.getMessage());
    }
    System.out.println("End");
  } // main
} // Programs