Fix bugs in the code

src/main/java/dotsandboxes/DotsAndBoxesGrid.java

@@ -112,7 +112,8 @@ 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. */
@@ -141,6 +142,10 @@ public class DotsAndBoxesGrid {
 
         // 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) is already drawn", x, y));
+        }
+
         this.horizontals[x][y] = true;
 
         // Try to claim the north or south boxes
@@ -172,6 +177,10 @@ public class DotsAndBoxesGrid {
 
         // You need to throw an exception if the line was already drawn.
 
+        if (this.verticals[x][y]) {
+            throw new IllegalStateException(String.format("Line at (%d, %d) is already drawn", x, y));
+        }
+        
         this.verticals[x][y] = true;
         // Try to claim the north or south boxes
         boolean claimE = claimBox(x, y, player);