inheritance in java

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Inheritance in Java

Behind the scenes: new Objects from old

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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(); move(); move(); move(); move(); }}

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MileMover: UML Diagrams

UrRobot

move()turnLeft()pickBeeper()putBeeper()turnOff()

Class Name

Public Methods

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MileMover: UML Diagrams

MileMover extends UrRobot

moveMile()

UrRobot

move()turnLeft()pickBeeper()putBeeper()turnOff()

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Tracing method calls


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.

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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(); 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(); super.move(); super.move(); super.move(); super.move();

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What now?…


move()2

UrRobot

turnLeft()pickBeeper()putBeeper()move()1

turnOff()

Which one does it use?


foo.move();

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Inheritance: a different view


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.


foo MileMover extends UrRobot

move()2

turnLeft()

pickBeeper()

putBeeper()

turnOff()

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How to use this advantageously…

Baker: go to BlueJ

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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…

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…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();

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Could also intersperse operations like this:

one.move();

two.move();

three.move();


two.harvestTwoRows();

three.harvestTwoRows();

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How about this?

HarvesterBot one;

one = new HarvesterBot(2,2,…);

one.move();



one.move();



one.move();


a reference to a HarvesterBot

3 instantiations

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Object References

HarversterBot one;

one = new HarversterBot(2,2,…)



oneHarvesterBot

Street : 2

Avenue : 2

HarvesterBot

Street : 4

Avenue : 2

HarvesterBot

Street : 6

Avenue : 2

CODE Memory

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)

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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…

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EXAMPLE:

let’s have 3 different types of bots• MileWalker

o when move() is invoked, moves 1 mile

• DropBeeperAndWalker o when move() is invoked, always drops a beeper and then

moves one block forward

• BackwardWalker (sort of the Michael Jackson of robots!)

o when 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…

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Remember the Big Picture

MileWalker and Harvester extend UrRobot. They are UrRobots.

UrRobot

MileWalker

DropBeeperAndWalker

BackwardWalker

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MileWalker -- definition

public class MileWalker extends UrRobot

{

// constructor same as always

public void move() {

super.move(); super.move();




}

}

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DropBeeperAndWalker -- DefN

public class DropBeeperAndWalker

extends UrRobot

{

// constructor same as always

public void move() {

putBeeper(); //inherited

super.move();

}

}

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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…

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Sample Driver -- mine vs. yours

UrRobot bot;

bot = new MileWalker(…);

bot.move(); // polymorphic move()

bot = new DropBeeperAndWalker(…);


bot = new BackwardWalker(…);


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!

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…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(…)

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Polymorphism



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!

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Polymorphism



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.

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Polymorphism



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.

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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.

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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().

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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

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Recap: Polymorphism

UrRobot mark = new MileMover(1,1,East,5);

mark.move();


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?

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Polymorphism


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()

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Blow your mind


moveMile(){ move();move();move();move();

}

UrRobot

turnLeft()

pickBeeper()

putBeeper()

move()

turnOff()

MileTurner extends MileMover

move(){ turnLeft(); super.move();

}

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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

inheritance in java

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