Checkstyle

src/edu/kit/informatik/Terminal.java

@@ -102,7 +102,7 @@ public final class Terminal {
             if ("breakpoint".equals(line)) {
                 return readLine();
             } else if (line != null && line.startsWith("#")) {
-                return readLine();
+                line = IN.readLine();
             }
             return line;
         } catch (final IOException e) {

src/edu/kit/informatik/model/ModelRailwaySimulation.java

@@ -322,10 +322,10 @@ public class ModelRailwaySimulation {
      * Move the trains in this simulation.
      *
      * @param speed amount of steps to move the trains
-     * @return train collisions, null if no trains are placed
+     * @return train collisions, empty if no trains are placed
      * @throws InvalidInputException if the simulation is not yet ready
      */
-    public List<SortedSet<Integer>> step(short speed) throws InvalidInputException {
+    public Optional<List<SortedSet<Integer>>> step(short speed) throws InvalidInputException {
         return trainManager.step(speed);
     }
 }

src/edu/kit/informatik/model/RailwayNetwork.java

@@ -197,7 +197,7 @@ public class RailwayNetwork {
             return null; // there is no rail to move on
         } else if (touchingRails.length == 1) { // simply move along the rail
             return touchingRails[0].move(position, direction, 1);
-        } // else: we check movement along both rails and pick the correct rail
+        } // else: check movement along both rails and pick the correct rail
         final Vector2D onRailOne = touchingRails[0].move(position, new Vector2D(direction), steps);
         final Vector2D onRailTwo = touchingRails[1].move(position, new Vector2D(direction), steps);
         // if we are stuck on the first rail
@@ -205,8 +205,8 @@ public class RailwayNetwork {
         //   or move in the correct direction on the second rail
         if (position.equals(onRailOne) || onRailOne == null
                 || onRailTwo != null && onRailTwo.subtract(position).directionEquals(direction)) {
-            return onRailTwo; // return that position
+            return onRailTwo; // return movement on rail two
-        } else if (position.equals(onRailTwo) || onRailTwo == null
+        } else if (position.equals(onRailTwo) || onRailTwo == null // see above, same logic for the other rail
                 || onRailOne.subtract(position).directionEquals(direction)) {
             return onRailOne;
         } else {
@@ -235,10 +235,10 @@ public class RailwayNetwork {
             // rail should not be orthogonal to the requested direction
             return touchingRails[0].allowsPlacement(position, direction)
                     || touchingRails[0].allowsMovement(position, direction);
-        } else if (!touchingRails[0].allowsPlacement(position, direction)
+        } else if (!touchingRails[0].allowsPlacement(position, direction) // train can not be placed on either rail
-                && !touchingRails[1].allowsPlacement(position, direction)
+                && !touchingRails[1].allowsPlacement(position, direction) //  in the specified direction
-                && !(touchingRails[0].allowsMovement(position, direction)
+                && !(touchingRails[0].allowsMovement(position, direction) // *and* it can not move in the specified
-                || touchingRails[1].allowsMovement(position, direction))) {
+                || touchingRails[1].allowsMovement(position, direction))) { // direction afterwards
             // rail network does not allow this direction according to
             // https://ilias.studium.kit.edu/goto.php?client_id=produktiv&target=frm_996924_139143_534378
             return false;

src/edu/kit/informatik/model/Train.java

@@ -8,6 +8,7 @@ import java.util.LinkedList;
 import java.util.List;
 import java.util.Optional;
 import java.util.Set;
+import java.util.function.Supplier;
 import java.util.stream.Collectors;
 
 /**
@@ -252,7 +253,7 @@ public final class Train implements Comparable<Train> {
      * @param backPosition position to move back of the train to
      */
     public void moveBackTo(RailwayNetwork railNetwork, Vector2D backPosition) {
-        // TODO: create common method for both directions
+        // a common method for both directions would be a huge mess: requires at least six (!) extra parameters
         final Optional<Rail> railUnderFrontOfTrain = railNetwork.findContainingRail(positions.getFirst());
         positions.getFirst().subtractInPlace(getDirection());
         if (positions.getFirst().equals(positions.get(1))) {
@@ -372,6 +373,12 @@ public final class Train implements Comparable<Train> {
         return stringBuilder.toString();
     }
 
+    /**
+     * Compare two trains based on their identifier.
+     *
+     * @param other train to compare to
+     * @return integer comparison of the identifiers
+     */
     @Override
     public int compareTo(Train other) {
         return Integer.compare(this.identifier, other.identifier);

src/edu/kit/informatik/model/TrainManager.java

@@ -333,24 +333,24 @@ public final class TrainManager {
      * Move the trains in this simulation.
      *
      * @param speed amount of steps to move the trains
-     * @return train collisions, null if no trains are placed
+     * @return train collisions, empty if no trains are placed
      * @throws InvalidInputException if simulation is not yet ready
      */
-    public List<SortedSet<Integer>> step(short speed) throws InvalidInputException {
+    public Optional<List<SortedSet<Integer>>> step(short speed) throws InvalidInputException {
         if (!railNetwork.isReadyForTrains()) {
             throw new InvalidInputException("rail tracks/switches not set up");
         }
         if (trains.values().stream().noneMatch(Train::isPlaced)) {
-            return null;
+            return Optional.empty();
         }
 
-        return IntStream.range(0, Math.abs(speed))
+        return Optional.of(IntStream.range(0, Math.abs(speed))
                 .mapToObj(step -> speed >= 0 ? getCollisionsOfOneStep() : getCollisionsOfOneReverseStep())
                 // replace train references with identifiers to prevent modification by caller
                 .flatMap(collisions -> collisions.stream()
                         .map(collision -> collision.stream().map(Train::getIdentifier)
                                 .collect(Collectors.toCollection(TreeSet::new))))
                 .sorted(Comparator.comparing(TreeSet::first))
-                .collect(Collectors.toList());
+                .collect(Collectors.toList()));
     }
 }

src/edu/kit/informatik/ui/command/Step.java

@@ -6,6 +6,7 @@ import edu.kit.informatik.model.Vector2D;
 import edu.kit.informatik.ui.InvalidInputException;
 
 import java.util.List;
+import java.util.Optional;
 import java.util.SortedSet;
 
 import static edu.kit.informatik.ui.CommandLine.OK;
@@ -26,12 +27,12 @@ public class Step extends Command {
 
     @Override
     public void apply(ModelRailwaySimulation simulation) throws InvalidInputException {
-        final List<SortedSet<Integer>> collisions = simulation.step(speed);
+        final Optional<List<SortedSet<Integer>>> collisions = simulation.step(speed);
-        if (collisions == null) {
+        if (!collisions.isPresent()) {
             Terminal.printLine(OK);
         } else {
             for (final int id : simulation.getTrains().keySet()) {
-                final SortedSet<Integer> collisionSet = collisions.stream()
+                final SortedSet<Integer> collisionSet = collisions.get().stream()
                         .filter(collision -> collision.first() == id)
                         .findFirst().orElse(null);
                 if (collisionSet != null) { // print collision