1. 1 Inheritance in Java Behind the scenes: new Objects from old

2. 2 Review: MileMover public class MileMover extends UrRobot{ public MileMover(int st, int ave, Direction dir, int beepers) { super(st, ave, dir, beepers); } public void moveMile() { move(); move(); move(); move(); }

3. 3 MileMover: UML Diagrams UrRobot move() turnLeft() pickBeeper() putBeeper() turnOff() Class Name Public Methods

4. 4 MileMover: UML Diagrams MileMover extends UrRobot moveMile() UrRobot move() turnLeft() pickBeeper() putBeeper() turnOff()

5. 5 Tracing method calls MileMover extends UrRobot moveMile() UrRobot turnLeft() pickBeeper() putBeeper() move() turnOff() MileMover foo = new MileMover(1,1,East,5); foo.moveMile(); foo.move(); For every method called on an instance… Check the definition of the class the object was constructed as… If method not found, check the super class If method not found, check the super super class Etc.

6. 6 public class MileMover extends UrRobot{ public MileMover(int st, int ave, Direction dir, int beepers) { super(st, ave, dir, beepers); } public void moveMile() { move(); move(); move(); move(); } } Change MileMover… public void move() This is called “overiding” the move method. To access original move method: make explicit call to parent class using super.method() What would happen if we left it as move() ? super.move(); super.move(); super.move(); super.move();

7. 7 What now?… MileMover extends UrRobot move() 2 UrRobot turnLeft() pickBeeper() putBeeper() move() 1 turnOff() Which one does it use? MileMover foo = new MileMover(1,1,East,5); foo.move();

8. 8 Inheritance: a different view MileMover extends UrRobot move() 2 UrRobot turnLeft() pickBeeper() putBeeper() move() 1 turnOff() Think of it this way: new class is constructed on the fly based on the definitions available starting at the bottom. MileMover foo = new MileMover(1,1,East,5); foo MileMover extends UrRobot move() 2 turnLeft() pickBeeper() putBeeper() turnOff()

9. 9 How to use this advantageously… Baker: go to BlueJ

10. 10 Toward Polymorphism Poly-morph-ism Overriding methods is not polymorphism in and of itself - but it is a big part of it REMEMBER: Each instance interprets a method call in terms of how it was instantiated…

11. 11 …A Quick Step Back - Object references Let’s do original harvesting task with teams of robots -- have 3 robots harvest 2 rows each. HarvesterBot one = new HarvesterBot(2,2,…); HarvesterBot two = new HarvesterBot(4,2,…); HarvesterBot three = new HarvesterBot(6,2,…); one.move(); one.harvestTwoRows(); two.move(); two.harvestTwoRows(); three.move(); three.harvestTwoRows();

12. 12 …A Quick Step Back - Object references Could also intersperse operations like this: one.move(); two.move(); three.move(); one.harvestTwoRows(); two.harvestTwoRows(); three.harvestTwoRows();

13. 13 …A Quick Step Back - Object references How about this? HarvesterBot one; one = new HarvesterBot(2,2,…); one.move(); one.harvestTwoRows(); one = new HarvesterBot(4,2,…); one.move(); one.harvestTwoRows(); one = new HarvesterBot(6,2,…); one.move(); one.harvestTwoRows(); a reference to a HarvesterBot 3 instantiations

14. 14 Object References HarversterBot one; one = new HarversterBot(2,2,…) one = new HarversterBot(4,2,…) one = new HarversterBot(6,2,…) one HarvesterBot Street : 2 Avenue : 2 HarvesterBot Street : 4 Avenue : 2 HarvesterBot Street : 6 Avenue : 2 CODEMemory NOTE: “one” gets re-assigned to point to different, new objects. The old objects, which no longer have a reference to them, are forgotten about and collected as “garbage.” Garbage (no more references to these things)

15. 15 Polymorphism Powerful example: you are all objects - if I tell all of you to “takeABreak()”, you all will hear the same message but will act in different ways (some of you will sleep, some will walk out the door and eat something, some will try to leave school!, some will do work, etc.) - that’s polymorphism sending the same message to different objects - each individual object has a particular way to interpret (implement) the message so, back to code and a Java/Karel example…

16. 16 EXAMPLE: let’s have 3 different types of bots MileWalker owhen move() is invoked, moves 1 mile DropBeeperAndWalker owhen move() is invoked, always drops a beeper and then moves one block forward BackwardWalker (sort of the Michael Jackson of robots!) owhen move() is invoked, moves one block backward for each of these new classes, we will only have to write one method, move() - each, however, will be implemented differently, and, in addition, override the original definition of move() inherited from UrRobot --- let’s see…

17. 17 Remember the Big Picture MileWalker and Harvester extend UrRobot. They are UrRobots. UrRobot MileWalker DropBeeperAndWalker BackwardWalker

18. 18 MileWalker -- definition public class MileWalker extends UrRobot { // constructor same as always public void move() { super.move(); }

19. 19 DropBeeperAndWalker -- DefN public class DropBeeperAndWalker extends UrRobot { // constructor same as always public void move() { putBeeper(); //inherited super.move(); }

20. 20 BackwardWalker -- definition You write it! In addition to writing this class, write a sample “driver” that would demonstrate using one robot each of type MileWalker, DropBeeperAndWalker, and BackwardWalker –We’ll pick someone and put it up in 5 minutes…

21. 21 Sample Driver -- mine vs. yours UrRobot bot; bot = new MileWalker(…); bot.move(); // polymorphic move() bot = new DropBeeperAndWalker(…); bot.move(); // polymorphic move() bot = new BackwardWalker(…); bot.move(); // polymorphic move() a reference can refer to any object as long as the object is of the same type or a type of one of its subclasses somewhere down the Inheritance tree!

22. 22 …now Polymorphism Because these types extend UrRobot they all ARE UrRobots. So these instantiations are legal… UrRobot soren = new UrRobot(…) UrRobot mark = new MileMover(…) UrRobot rebecca = new Harvester(…)

23. 23 Polymorphism UrRobot soren = new UrRobot(…) UrRobot mark = new MileMover(…) UrRobot rebecca = new Harvester(…) An object reference can refer to any object as long as the object is of the same type or a type of one of its subclasses somewhere down the Inheritance tree!

24. 24 Polymorphism UrRobot soren = new UrRobot(…) UrRobot mark = new MileMover(…) UrRobot rebecca = new Harvester(…) rebecca.move(); So, this is a legal call to move() since java can ensure a move method exists somewhere as part of rebecca.

25. 25 Polymorphism UrRobot soren = new UrRobot(…) UrRobot mark = new MileMover(…) UrRobot rebecca = new Harvester(…) rebecca.move(); rebecca.harvestSixRows(); THIS IS NOT legal. Why? Because java cannot ensure that harvestSixRows exists. Why? Because rebecca was declared as a UrRobot.

26. 26 Compile Time vs. Run Time At time of compilation only superficial syntax checking is done. Compiler does not inspect the objects. At Run Time the objects are actually constructed.

27. 27 Compile Time vs. Run Time So Think of things from the compiler’s perspective: UrRobot rebecca = new Harvester(…) Ok, because Harvester extends UrRobot rebecca.move(); Ok, because rebecca is of type UrRobot and UrRobot has a move() method. rebecca.harvestSixRows(); NOT OK -- THIS WILL NOT COMPILE because as far as the compiler knows rebecca is a UrRobot (declared as such) and UrRobot does not have harvestSixRows().

28. 28 How is this helpful? Collections of robots that we can set to work regardless of what type they are. This is very helpful for generic programming. Allows class design to be flexible

29. 29 Recap: Polymorphism UrRobot mark = new MileMover(1,1,East,5); mark.move(); MileMover extends UrRobot move() UrRobot turnLeft() pickBeeper() putBeeper() move() turnOff() Which move() method does mark use? Mark will use the definition of move from MileMover. Why? Because mark was constructed as a MileMover. Now what happens?

30. 30 Polymorphism MileMover extends UrRobot move() 2 UrRobot turnLeft() pickBeeper() putBeeper() move() 1 turnOff() Remember how this will be constructed at run-time… UrRobot mark = new MileMover(1,1,East,5); mark MileMover extends UrRobot move() 2 turnLeft() pickBeeper() putBeeper() turnOff()

31. 31 Blow your mind MileMover extends UrRobot moveMile(){ move();move();move();move(); } UrRobot turnLeft() pickBeeper() putBeeper() move() turnOff() MileTurner extends MileMover move(){ turnLeft(); super.move(); }

32. 32 Polymorphism “It is the robot itself, not the name by which we refer to it, that determines what is done when a message is received.” --Bergin p. 70 “It will act like the class it was constructed as not necessarily as the class it was declared as.” --Franke