1. Chair of Software Engineering 1 Introduction to Programming Bertrand Meyer Exercise Session 5 6 October 2008

2. Chair of Software Engineering 2 Assignments  Deadline for assignment 3 on Thursday

3. Chair of Software Engineering 3 Today’s learning goals  Declaring the type of an object  Reading class interfaces and understanding feature calls

4. Chair of Software Engineering Declaring the type of an object  The type of any object you use in your program must be declared somewhere  Where can such declarations appear in a program?  in feature declarations:  argument types  return type for queries  in the local clauses of routines where you declare any objects that only the routine needs and knows

5. Chair of Software Engineering 5 Queries and commands Command: Query (attribute): Query (function): commandname (a1: T1; a2, a3: T2) is -- Comment require … do … ensure … end attributename: TYPE -- Comment functionname (a1: T1; a2, a3: T2): TYPE is -- Comment require … do … ensure … end optional mandatory

6. Chair of Software Engineering 6 Feature declaration vs. feature call Feature declaration feature_name_1 (arg1: A; arg2: B; …) is -- Comment do … end feature_name_2 (arg1: C; arg2: D; …): SOME_RETURN_TYPE is -- Comment do … end Feature call object1.feature_name_1 (a1, b1) object2 := object3.feature_name_2 (c1, d1) The underlined entities denote objects The red entities are type names

7. Chair of Software Engineering 7 Objects or classes? (example) ‏ class game... feature -- Access map_name: string -- Name of the map to be loaded for the game last_player: player -- Last player that moved... (continued on next slide) ‏ Hands-On

8. Chair of Software Engineering 8 Objects or classes? (example) ‏... feature -- Status report is_occupied (a_location: traffic_place): boolean is -- Check if `a_location' is occupied by some flat hunter. require a_location_exists: a_location /= Void local old_cursor: cursor do Result := False -- Remember old cursor position. old_cursor := players.cursor... Hands-On

9. Chair of Software Engineering 9 Objects or classes? (example) ‏ -- Loop over all players to check if one occupies -- `a_location'. from players.start -- do not consider estate agent, hence skip the first -- entry in `players'. players.forth until players.after or Result loop if players.item.location = a_location then Result := True end players.forth end -- Restore old cursor position. players.go_to(old_cursor) end Hands-On

10. Chair of Software Engineering 10 Objects or classes?  Terms “class” and “type” used interchangeably for now  Queries (attributes and functions) have a return type. However, when executing the query, you get an object.  You call features on objects.  Arguments given to a routine are objects, they all have certain types.  Local variables declared in a routine are objects, but they all have certain types.  Result and Current are objects.  Inheritance is a relation defined between classes.

11. Chair of Software Engineering 11 Queries and commands Command: Query (attribute): Query (function): commandname (a1: T1; a2, a3: T2) is -- Comment require … do … ensure … end attributename: TYPE -- Comment functionname (a1: T1; a2, a3: T2): TYPE is -- Comment require … do … ensure … end optional mandatory

12. Chair of Software Engineering 12 Query or command? (example) ‏ class GAME... feature -- Access map_name: STRING -- Name of the map to be loaded for the game last_player: PLAYER -- Last player that moved... (continued on next slide) ‏ Hands-On

13. Chair of Software Engineering 13 Query or command? (example)... feature -- Status report is_occupied (a_location: TRAFFIC_PLACE): BOOLEAN is -- Check if `a_location' is occupied by some flat hunter. require a_location_exists: a_location /= Void local old_cursor: CURSOR do Result := False -- Remember old cursor position. old_cursor := players.cursor... Hands-On

14. Chair of Software Engineering 14 Query or command? (example) -- Loop over all players to check if one occupies -- `a_location'. from players.start -- do not consider estate agent, hence skip the first -- entry in `players'. players.forth until players.after or Result loop if players.item.location = a_location then Result := True end players.forth end -- Restore old cursor position. players.go_to(old_cursor) end Hands-On