the bug was fixed and the unit tests were passed

src/main/java/dotsandboxes/DotsAndBoxesGrid.java

@@ -112,8 +112,13 @@ 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 (getHorizontal(0, 0) && getHorizontal(0, 1) && getVertical(0, 0) && getVertical(1, 0)) {
             return true;
         }
+        else
+            return false;
+    }
+
 
     /** Tries to claim a box for a player. If the box is complete, sets the ownership and returns true. */
     private boolean claimBox(int x, int y, int p) {
@@ -140,6 +145,7 @@ public class DotsAndBoxesGrid {
         }
 
         // FIXME: You need to throw an exception if the line was already drawn.
+        if (getHorizontal(x, y)) throw new IllegalStateException();
 
         this.horizontals[x][y] = true;