Updated code for fixing bugs

src/main/java/dotsandboxes/DotsAndBoxesGrid.java

@@ -139,7 +139,10 @@ 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.
+        // throws an exception if the line was already drawn.
+        if (horizontals[x][y]) {
+            throw new IllegalStateException(String.format("Horizontal line at (%d, %d) already drawn", x, y));
+        }
 
         this.horizontals[x][y] = true;
 
@@ -170,7 +173,10 @@ public class DotsAndBoxesGrid {
             throw new IndexOutOfBoundsException(String.format("y was %d, which is out of range. Range is 0 to %d", y, height - 1));
         }
 
-        // You need to throw an exception if the line was already drawn.
+        // throws an exception if the line was already drawn.
+        if (verticals[x][y]) {
+            throw new IllegalStateException(String.format("Vertical line at (%d, %d) already drawn", x, y));
+        }
 
         this.verticals[x][y] = true;
         // Try to claim the north or south boxes