src/main/java/dotsandboxes/DotsAndBoxesGrid.java

@@ -112,7 +112,11 @@ 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).
+        if (horizontals[x][y] && horizontals[x][y+1] && verticals[x][y] && verticals[x+1][y]) {
             return true;
+        } else {
+            return false;
+        }
     }
 
     /** Tries to claim a box for a player. If the box is complete, sets the ownership and returns true. */
@@ -141,6 +145,10 @@ public class DotsAndBoxesGrid {
 
         // FIXME: You need to throw an exception if the line was already drawn.
 
+        if (horizontals[x][y]) {
+            throw new IllegalArgumentException("Horizontal line already drawn");
+        }
+
         this.horizontals[x][y] = true;
 
         // Try to claim the north or south boxes
@@ -172,6 +180,10 @@ public class DotsAndBoxesGrid {
 
         // You need to throw an exception if the line was already drawn.
 
+        if (verticals[x][y]) {
+            throw new IllegalArgumentException("Vertical line already drawn");
+        }
+
         this.verticals[x][y] = true;
         // Try to claim the north or south boxes
         boolean claimE = claimBox(x, y, player);

src/test/java/dotsandboxes/DotsAndBoxesGridTest.java

@@ -28,4 +28,31 @@ public class DotsAndBoxesGridTest {
     }
 
     // FIXME: You need to write tests for the two known bugs in the code.
+    
+    @Test
+    public void boxCompleteDetectsCompletedBoxes() {
+        DotsAndBoxesGrid case1 = new DotsAndBoxesGrid(5, 5, 2);
+        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() {
+        DotsAndBoxesGrid case2 = new DotsAndBoxesGrid(5, 5, 2);
+        case2.drawHorizontal(0, 0, 0);
+        case2.drawVertical(0, 0, 1);
+        assertFalse(case2.boxComplete(0, 0));
+    }
+
+    @Test
+    public void checkDrawLineException() {
+        DotsAndBoxesGrid case3 = new DotsAndBoxesGrid(5, 5, 2);
+        case3.drawHorizontal(0, 0, 0);
+        case3.drawVertical(0, 0, 1);
+        assertThrows(IllegalArgumentException.class, () -> case3.drawHorizontal(0, 0, 0));
+        assertThrows(IllegalArgumentException.class, () -> case3.drawVertical(0, 0, 1));
+    }
 }