Add tests.

src/main/java/ch/fritteli/labyrinth/model/Walls.java

@@ -13,8 +13,8 @@ public class Walls {
     private final SortedSet<Direction> directions = new TreeSet<>();
     private final Set<Direction> hardened = new HashSet<>();
 
-    public boolean set(@NonNull final Direction direction) {
-        return this.directions.add(direction);
+    public void set(@NonNull final Direction direction) {
+        this.directions.add(direction);
     }
 
     public void setAll() {

src/test/java/ch/fritteli/labyrinth/model/DirectionTest.java

@@ -0,0 +1,15 @@
+package ch.fritteli.labyrinth.model;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class DirectionTest {
+    @Test
+    void invert() {
+        assertEquals(Direction.BOTTOM, Direction.TOP.invert());
+        assertEquals(Direction.LEFT, Direction.RIGHT.invert());
+        assertEquals(Direction.TOP, Direction.BOTTOM.invert());
+        assertEquals(Direction.RIGHT, Direction.LEFT.invert());
+    }
+}

src/test/java/ch/fritteli/labyrinth/model/PositionTest.java

@@ -0,0 +1,27 @@
+package ch.fritteli.labyrinth.model;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+class PositionTest {
+    @Test
+    void move() {
+        // arrange
+        final Position sut = new Position(0, 0);
+
+        // act
+        final Position resultTOP = sut.move(Direction.TOP);
+        final Position resultRIGHT = sut.move(Direction.RIGHT);
+        final Position resultBOTTOM = sut.move(Direction.BOTTOM);
+        final Position resultLEFT = sut.move(Direction.LEFT);
+
+        // assert
+        // Original is unchanged
+        assertEquals(new Position(0, 0), sut);
+        assertEquals(new Position(0, -1), resultTOP);
+        assertEquals(new Position(1, 0), resultRIGHT);
+        assertEquals(new Position(0, 1), resultBOTTOM);
+        assertEquals(new Position(-1, 0), resultLEFT);
+    }
+}

src/test/java/ch/fritteli/labyrinth/model/WallsTest.java

@@ -0,0 +1,137 @@
+package ch.fritteli.labyrinth.model;
+
+import io.vavr.collection.Stream;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+class WallsTest {
+    @Test
+    void testConstruct() {
+        // arrange / act
+        final Walls sut = new Walls();
+
+        // assert
+        assertFalse(sut.isSet(Direction.TOP));
+        assertFalse(sut.isSet(Direction.RIGHT));
+        assertFalse(sut.isSet(Direction.BOTTOM));
+        assertFalse(sut.isSet(Direction.LEFT));
+    }
+
+    @Test
+    void testSet() {
+        // arrange
+        final Walls sut = new Walls();
+
+        //act
+        sut.set(Direction.RIGHT);
+
+        // assert
+        assertFalse(sut.isSet(Direction.TOP));
+        assertTrue(sut.isSet(Direction.RIGHT));
+        assertFalse(sut.isSet(Direction.BOTTOM));
+        assertFalse(sut.isSet(Direction.LEFT));
+
+        //act: Setting twice has no effect
+        sut.set(Direction.RIGHT);
+
+        // assert
+        assertFalse(sut.isSet(Direction.TOP));
+        assertTrue(sut.isSet(Direction.RIGHT));
+        assertFalse(sut.isSet(Direction.BOTTOM));
+        assertFalse(sut.isSet(Direction.LEFT));
+    }
+
+    @Test
+    void testSetAll() {
+        // arrange
+        final Walls sut = new Walls();
+
+        //act
+        sut.setAll();
+
+        // assert
+        assertTrue(sut.isSet(Direction.TOP));
+        assertTrue(sut.isSet(Direction.RIGHT));
+        assertTrue(sut.isSet(Direction.BOTTOM));
+        assertTrue(sut.isSet(Direction.LEFT));
+    }
+
+    @Test
+    void testHarden() {
+        // arrange
+        final Walls sut = new Walls();
+        sut.set(Direction.TOP);
+        sut.set(Direction.RIGHT);
+
+        // act
+        sut.harden(Direction.TOP);
+
+        // assert
+        assertTrue(sut.isSet(Direction.TOP));
+        assertTrue(sut.isSet(Direction.RIGHT));
+        assertFalse(sut.isSet(Direction.BOTTOM));
+        assertFalse(sut.isSet(Direction.LEFT));
+
+        // act: try to clear hardened wall
+        final boolean result = sut.clear(Direction.TOP);
+
+        // assert: TOP wall is still set
+        assertFalse(result);
+        assertTrue(sut.isSet(Direction.TOP));
+
+        // act / assert: try to harden un-set wall
+        assertThrows(IllegalStateException.class, () -> sut.harden(Direction.LEFT));
+    }
+
+    @Test
+    void testUnharden() {
+        // arrange
+        final Walls sut = new Walls();
+        sut.setAll();
+        sut.harden(Direction.TOP);
+
+        // pre-assert: TOP can't be cleared while hardened
+        assertFalse(sut.clear(Direction.TOP));
+
+        // act
+        sut.unharden(Direction.TOP);
+
+        // assert: TOP can be cleared
+        assertTrue(sut.clear(Direction.TOP));
+    }
+
+    @Test
+    void testGetUnhardenedSet() {
+        // arrange
+        final Walls sut = new Walls();
+
+        // act
+        Stream<Direction> result = sut.getUnhardenedSet();
+
+        // assert
+        assertTrue(result.isEmpty());
+
+        // arrange: set some directions
+        sut.set(Direction.TOP);
+        sut.set(Direction.LEFT);
+
+        // act
+        result = sut.getUnhardenedSet();
+
+        // assert
+        assertEquals(Stream.of(Direction.TOP, Direction.LEFT), result);
+
+        // arrange: harden a direction
+        sut.harden(Direction.TOP);
+
+        // act
+        result = sut.getUnhardenedSet();
+
+        // assert
+        assertEquals(Stream.of(Direction.LEFT), result);
+    }
+}