src/main/java/dotsandboxes/DotsAndBoxesGrid.java

@@ -111,8 +111,7 @@ public class DotsAndBoxesGrid {
         }
 
         // A box is complete if the north and south horizontals and the east and west verticals have all been drawn.
-        // FIXME: You'll need to fix this code (after writing a test first).
-        return true;
+        return horizontals[x][y] && horizontals[x][y+1] && verticals[x][y] && verticals[x+1][y];
     }
 
     /** Tries to claim a box for a player. If the box is complete, sets the ownership and returns true. */
@@ -139,7 +138,9 @@ public class DotsAndBoxesGrid {
             throw new IndexOutOfBoundsException(String.format("y was %d, which is out of range. Range is 0 to %d", y, height));
         }
 
-        // FIXME: You need to throw an exception if the line was already drawn.
+        if (this.horizontals[x][y]) {
+            throw new IllegalStateException(String.format("line at %d, %d was already drawn and cannot be redrawn.", x, y));
+        }
 
         this.horizontals[x][y] = true;
 

src/test/java/dotsandboxes/DotsAndBoxesGridTest.java

@@ -27,5 +27,34 @@ public class DotsAndBoxesGridTest {
         assertTrue(true);
     }
 
-    // FIXME: You need to write tests for the two known bugs in the code.
+    @Test
+    public void boxCompleteDetectsCompletedBoxes() {
+        logger.info("Test to show that boxComplete returns true if all sides have been drawn");
+        DotsAndBoxesGrid case1 = new DotsAndBoxesGrid(4,4,1);
+        case1.drawHorizontal(0,0,0);
+        case1.drawVertical(0,0,1);
+        case1.drawHorizontal(0,1,0);
+        case1.drawVertical(1,0,1);
+        assertTrue(case1.boxComplete(0,0));
+    }
+
+    @Test
+    public void boxCompleteDetectsIncompleteBoxes() {
+        logger.info("Test to show that boxComplete returns false if not all sides have been drawn");
+        DotsAndBoxesGrid case1 = new DotsAndBoxesGrid(4,4,1);
+        case1.drawHorizontal(0,0,0);
+        case1.drawVertical(0,0,1);
+        case1.drawHorizontal(0,1,0);
+        assertFalse(case1.boxComplete(0,0));
+    }
+
+    @Test
+    public void drawMethodsDetectRedrawnLines() {
+        logger.info("Test to show that drawHorizontal throws an exception if the line was already drawn");
+        DotsAndBoxesGrid case1 = new DotsAndBoxesGrid(4,4,1);
+        case1.drawHorizontal(0,0,0);
+        assertThrows(IllegalStateException.class, () -> {
+            case1.drawHorizontal(0,0,0);
+        });
+    }
 }