Fix box completion logic and IllegalStateException

src/main/java/dotsandboxes/DotsAndBoxesGrid.java

@@ -111,8 +111,8 @@ 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. */
@@ -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.
+        // Throw an exception if the line was already drawn
+        if (horizontals[x][y]) {
+            throw new IllegalStateException("Line already drawn.");
+        }
 
         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.
+        // Throw an exception if the line was already drawn
+        if (verticals[x][y]) {
+            throw new IllegalStateException("Line already drawn.");
+        }
 
         this.verticals[x][y] = true;
         // Try to claim the north or south boxes