Make Encoding.toSchedule() return an optional to handle the case where the solution is not valid.

src/main/java/jobshop/Instance.java

@@ -49,7 +49,7 @@ public class Instance {
         return this.machine(t.job, t.task);
     }
 
-    /** among the tasks of the given job, returns the task index that uses the given machine. */
+    /** Among the tasks of the given job, returns the task number of the one that uses the given machine. */
     public int task_with_machine(int job, int wanted_machine) {
         for(int task = 0 ; task < numTasks ; task++) {
             if(machine(job, task) == wanted_machine)

src/main/java/jobshop/Main.java

@@ -8,6 +8,7 @@ import java.util.Arrays;
 import java.util.List;
 import java.util.stream.Collectors;
 
+import jobshop.encodings.Schedule;
 import jobshop.solvers.*;
 import net.sourceforge.argparse4j.ArgumentParsers;
 import net.sourceforge.argparse4j.inf.ArgumentParser;
@@ -122,13 +123,15 @@ public class Main {
                     long runtime = System.currentTimeMillis() - start;
 
                     // check that the solver returned a valid solution
-                    if(!result.schedule.isValid()) {
+                    if(result.schedule.isEmpty() || !result.schedule.get().isValid()) {
                         System.err.println("ERROR: solver returned an invalid schedule");
                         System.exit(1); // bug in implementation, bail out
                     }
+                    // we have a valid schedule
+                    Schedule schedule = result.schedule.get();
 
                     // compute some statistics on the solution and print them.
-                    int makespan = result.schedule.makespan();
+                    int makespan = schedule.makespan();
                     float dist = 100f * (makespan - bestKnown) / (float) bestKnown;
                     avg_runtimes[solverId] += (float) runtime / (float) instances.size();
                     avg_distances[solverId] += dist / (float) instances.size();

src/main/java/jobshop/MainTest.java

@@ -28,7 +28,7 @@ public class MainTest {
             System.out.println("\nENCODING: " + enc);
 
             // convert to a schedule and display
-            Schedule schedule = enc.toSchedule();
+            Schedule schedule = enc.toSchedule().get();
             System.out.println("VALID: " + schedule.isValid());
             System.out.println("MAKESPAN: " + schedule.makespan());
             System.out.println("SCHEDULE: " + schedule.toString());

src/main/java/jobshop/Result.java

@@ -2,10 +2,22 @@ package jobshop;
 
 import jobshop.encodings.Schedule;
 
+import java.util.Optional;
+
 /** Class representing the result of a solver. */
 public class Result {
 
-    public Result(Instance instance, Schedule schedule, ExitCause cause) {
+    /** Instance that was solved. */
+    public final Instance instance;
+
+    /** A schedule of the solution or Optional.empty() if no solution was found. */
+    public final Optional<Schedule> schedule;
+
+    /** Reason why the solver exited with this solution. */
+    public final ExitCause cause;
+
+    /** Creates a new Result object with the corresponding fields. */
+    public Result(Instance instance, Optional<Schedule> schedule, ExitCause cause) {
         this.instance = instance;
         this.schedule = schedule;
         this.cause = cause;
@@ -20,15 +32,4 @@ public class Result {
         /** The solver was not able to further improve the solution (e.g. blocked in a local minima. */
         Blocked
     }
-
-    /** Instance that was solved. */
-    public final Instance instance;
-
-    /** A schedule of the solution or null if no solution was found. */
-    public final Schedule schedule;
-
-    /** Reason why the solver exited with this solution. */
-    public final ExitCause cause;
-
-
 }

src/main/java/jobshop/encodings/Encoding.java

@@ -2,6 +2,8 @@ package jobshop.encodings;
 
 import jobshop.Instance;
 
+import java.util.Optional;
+
 /** Common class for all encodings.
  *
  * The only requirement for this class is to provide a conversion from the encoding into a Schedule.
@@ -11,10 +13,15 @@ public abstract class Encoding {
     /** Problem instance of which this is the solution. */
     public final Instance instance;
 
+    /** Constructor, that initializes the instance field. */
     public Encoding(Instance instance) {
         this.instance = instance;
     }
 
-    /** Convert into a schedule. */
-    public abstract Schedule toSchedule();
+    /** Attempts to convert this solution into a schedule.
+     *
+     * @return A empty optional if the solution is not valid. Otherwise the optional will contain a valid schedule of
+     *         the solution.
+     */
+    public abstract Optional<Schedule> toSchedule();
 }

src/main/java/jobshop/encodings/JobNumbers.java

@@ -4,6 +4,7 @@ import jobshop.Instance;
 
 import java.util.Arrays;
 import java.util.Comparator;
+import java.util.Optional;
 import java.util.stream.IntStream;
 
 /** Encoding of the solution of a jobshop problem by job numbers. */
@@ -56,7 +57,7 @@ public class JobNumbers extends Encoding {
     }
 
     @Override
-    public Schedule toSchedule() {
+    public Optional<Schedule> toSchedule() {
         // time at which each machine is going to be freed
         int[] nextFreeTimeResource = new int[instance.numMachines];
 
@@ -79,7 +80,7 @@ public class JobNumbers extends Encoding {
             nextTask[job] = task + 1;
         }
 
-        return schedule;
+        return Optional.of(schedule);
     }
 
     @Override

src/main/java/jobshop/encodings/ResourceOrder.java

@@ -2,6 +2,7 @@ package jobshop.encodings;
 
 import jobshop.Instance;
 
+import java.util.Arrays;
 import java.util.Comparator;
 import java.util.Optional;
 import java.util.stream.IntStream;
@@ -85,7 +86,7 @@ public class ResourceOrder extends Encoding {
     }
 
     @Override
-    public Schedule toSchedule() {
+    public Optional<Schedule> toSchedule() {
         // indicate for each task that have been scheduled, its start time
         Schedule schedule = new Schedule(instance);
 
@@ -130,16 +131,21 @@ public class ResourceOrder extends Encoding {
                 releaseTimeOfMachine[machine] = est + instance.duration(t.job, t.task);
             } else {
                 // no tasks are schedulable, there is no solution for this resource ordering
-                return null;
+                return Optional.empty();
             }
         }
         // we exited the loop : all tasks have been scheduled successfully
-        return schedule;
+        return Optional.of(schedule);
     }
 
     /** Creates an exact copy of this resource order. */
     public ResourceOrder copy() {
-        return new ResourceOrder(this.toSchedule());
+        var schedule = this.toSchedule();
+        if (schedule.isEmpty()) {
+            throw new RuntimeException("Cannot copy an invalid ResourceOrder");
+        } else {
+            return new ResourceOrder(schedule.get());
+        }
     }
 
     @Override

src/main/java/jobshop/encodings/Schedule.java

@@ -238,7 +238,7 @@ public class Schedule extends Encoding {
 
 
     @Override
-    public Schedule toSchedule() {
-        return this;
+    public Optional<Schedule> toSchedule() {
+        return Optional.of(this);
     }
 }

src/main/java/jobshop/solvers/RandomSolver.java

@@ -4,6 +4,7 @@ import jobshop.*;
 import jobshop.encodings.JobNumbers;
 import jobshop.encodings.Schedule;
 
+import java.util.Optional;
 import java.util.Random;
 
 /** A solver that generates random solutions until a deadline is met.
@@ -22,12 +23,14 @@ public class RandomSolver implements Solver {
                 sol.jobs[sol.nextToSet++] = j;
             }
         }
-        Schedule best = sol.toSchedule();
+        Optional<Schedule> best = sol.toSchedule();
         while(deadline - System.currentTimeMillis() > 1) {
             shuffleArray(sol.jobs, generator);
-            Schedule s = sol.toSchedule();
-            if(s.makespan() < best.makespan()) {
-                best = s;
+            Optional<Schedule> candidate = sol.toSchedule();
+            if(candidate.isPresent()) {
+                if (best.isEmpty() || candidate.get().makespan() < best.get().makespan()) {
+                    best = candidate;
+                }
             }
         }
 

src/test/java/jobshop/encodings/EncodingTests.java

@@ -24,7 +24,7 @@ public class EncodingTests {
         enc.jobs[enc.nextToSet++] = 0;
         enc.jobs[enc.nextToSet++] = 1;
 
-        Schedule sched = enc.toSchedule();
+        Schedule sched = enc.toSchedule().get();
         // TODO: make it print something meaningful
         // by implementing the toString() method
         System.out.println(sched);
@@ -42,7 +42,7 @@ public class EncodingTests {
         enc.jobs[enc.nextToSet++] = 0;
         enc.jobs[enc.nextToSet++] = 1;
 
-        sched = enc.toSchedule();
+        sched = enc.toSchedule().get();
         assert sched.isValid();
         assert sched.makespan() == 14;
     }
@@ -60,15 +60,15 @@ public class EncodingTests {
         enc.jobs[enc.nextToSet++] = 0;
         enc.jobs[enc.nextToSet++] = 1;
 
-        Schedule sched = enc.toSchedule();
+        Schedule sched = enc.toSchedule().get();
         assert sched.isValid();
         assert sched.makespan() == 12;
 
         Solver solver = new BasicSolver();
         Result result = solver.solve(instance, System.currentTimeMillis() + 10);
 
-        assert result.schedule.isValid();
-        assert result.schedule.makespan() == sched.makespan(); // should have the same makespan
+        assert result.schedule.get().isValid();
+        assert result.schedule.get().makespan() == sched.makespan(); // should have the same makespan
     }
 
 }