fix bugs for drawing lines and checking box completion

src/main/java/dotsandboxes/DotsAndBoxesGrid.java

@@ -115,7 +115,11 @@ 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).
-        return true;
+        // Check if all four sides are drawn
+        return horizontals[x][y] && // North
+                horizontals[x][y + 1] && // South
+                verticals[x][y] && // West
+                verticals[x + 1][y]; // East
     }
 
     /**
@@ -150,6 +154,10 @@ public class DotsAndBoxesGrid {
         }
 
         // FIXME: You need to throw an exception if the line was already drawn.
+        // Check if the line was already drawn
+        if (this.horizontals[x][y]) {
+            throw new IllegalStateException("Line already drawn");
+        }
 
         this.horizontals[x][y] = true;
 
@@ -185,6 +193,10 @@ public class DotsAndBoxesGrid {
         }
 
         // You need to throw an exception if the line was already drawn.
+        // Check if the line was already drawn
+        if (this.verticals[x][y]) {
+            throw new IllegalStateException("Line already drawn");
+        }
 
         this.verticals[x][y] = true;
         // Try to claim the north or south boxes