Resolve "Fix errors in assignment"

Closes #1 (closed)

src/main/java/dotsandboxes/DotsAndBoxesGrid.java

@@ -110,9 +110,13 @@ public class DotsAndBoxesGrid {
             return false;
         }
 
-        // 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;
+        // Could be shorter but this is more readable
+        boolean north = horizontals[x][y];
+        boolean south = horizontals[x][y + 1];
+        boolean west = verticals[x][y];
+        boolean east = verticals[x + 1][y];
+
+        return north && south && west && east;
     }
 
     /** Tries to claim a box for a player. If the box is complete, sets the ownership and returns true. */
@@ -139,7 +143,8 @@ 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.
+        // You need to throw an exception if the line was already drawn.
+        if (horizontals[x][y]) throw new IllegalArgumentException("Line is already taken.");
 
         this.horizontals[x][y] = true;
 
@@ -171,6 +176,7 @@ public class DotsAndBoxesGrid {
         }
 
         // You need to throw an exception if the line was already drawn.
+        if (verticals[x][y]) throw new IllegalArgumentException("Line is already taken.");
 
         this.verticals[x][y] = true;
         // Try to claim the north or south boxes