public class TileId
{
    public static final String WALL = "#";
    public static final String UNEXPLORED = " ";
    public static final String DROID = ".";

    private TileId ()  // prevent instantiation!
    {
    }
}
public class TileId
{
    public static final String WALL = "#";
    public static final String UNEXPLORED = " ";
    public static final String TRAVERSE = ".";
    public static final String DROID = "D";

    private TileId ()  // prevent instantiation!
    {
    }
}
Update TileId.java