Interacting ClassesLecture #6

CMPSCI 121, Spring 2005Introduction to Problem Solving with Computers

Prof. McCallum

No quiz today. It will again be in lab on Thursday. (Read Chapter 3). OWL Written Exercise is due Thursday 9am.

Programming Assignment #5 was due this morning.

Today’s music: Aaron Copland, 1846.Hungarian Rhapsody #2

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Midterm Results Finished grading same day you took it; back to you in labs. On the whole you did very well!

Congratulations! You have learned a lot!

Average: 80 (+11 EC), Median: 81, Min: 27, Max: 100 (x5)

Midterm roughly:A 100-90 (#46)

B 89-80 (#54)

B- 79-75 (#23)

C 74-65 (#22)

D 64-50 (#15)

F 49-0 (#11)

Subject to change!

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Dropping? I hope not, and that you’ll stick with it!

But if you can’t: Last day (without the need to petition the

Dean) is Monday the 30th Can go to Pauline at the front desk in the

CS main office.

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MidtermYou are turning into real programmers now.In future quizzes and exams there will be more questions like this:

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Primitive and Reference Types We’ve seen Java’s 8 primitive types:

int, double, boolean, char

(also byte, short, long, float)

Java also has reference types, for objects Examples of reference variables:

String name;

Counter c1;

They are called references because they refer to a memory location where the object lives

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Memory: Stack and Heap When we use the DrJava interactions pane, and when we

run a standalone Java program or applet, memory is allocated for variables and objects

Understanding how this memory is managed helps us understand how Java works

The JVM uses a stack and a heap. The stack is used to store:

Variables we create in the DrJava interactions pane A “stack frame” for each active method call (we’ll

discuss this later in the course) The heap is used to store objects.

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How the Stack Grows

DrJava Interactions Stack

> int x;

> x = 5;

> double min = 0.5;

> boolean done = false;

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How the Heap GrowsDrJava Interactions Stack and Heap

> int x = 99;

> Counter c1;

> c1

null

> c1 = new Counter();

> c1

Counter@2f996f

> c1.incrementCount();

> Counter c2 = new Counter();

> c2

Counter@4a0ac5

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Value of a Reference Variable The value of are reference variable is either null or a

“heap address” Example:

> Counter c1;

> c1

null

> c1 = new Counter();

> c1

Counter@e05ad6 e05ad6 is a hexadecimal (base 16) number We don’t have to (and can’t) deal with these hex

numbers directly

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“Has a” Relationship Example: An object of type A has an instance variable

which is an object whose type is B. (A “has a” B.) We will create a DormRoom object, and a Freshman object

whose room is that DormRoom UML diagrams that show instance variables and methods:

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DormRoom: Code and UML

> DormRoom room = new DormRoom(208, "Hill");

> room.getLocation()

"208 Hill"

public class DormRoom{ private int num; private String bldgName; public DormRoom(int n, String b){ num = n; bldgName = b; } public String getLocation(){ return num + " " + bldgName; }}

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A DormRoom on the Heap

> DormRoom room = new DormRoom(208, "Hill");

> room.getLocation()

"208 Hill"

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Freshman Code and UML> DormRoom room = new DormRoom(208, "Hill");

> Freshman f = new Freshman("jo", room);

> f.getName()

"jo"

> f.getRoom().getLocation()

"208 Hill"

public class Freshman{ private String name; private DormRoom room; public Freshman(String n, DormRoom r){ name = n; room = r; } public String getName(){ return name;} public DormRoom getRoom(){ return room;}}

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A Freshman on the Heap :)> DormRoom room = new DormRoom(208, "Hill");

> Freshman f = new Freshman("jo", room);

> f.getName()

"jo"

> f.getRoom().getLocation()

"208 Hill"

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Fred Brooks1931-

Computer Scientist of the Week

Ph.D. Harvard, “Applied Math”, 1596 Managed development of OS/390 at IBM. Mythical Man-month

“Assigning more programmers to a project running behind schedule, could actually make it even more late”

No Silver Bullet “No more technologies or practices that will

create a 10-fold improvement in software engineering productivity over 10 years”

Winner of Turing Award in 1999.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

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Another example... A point on the plane is given by its

coordinates x, y in a fixed frame of reference

public class Point { private double x, y; Point(double anX, double aY) { x = anX; y = aY; } public double getX() { return x; } public double getY() { return y; }}

Add a move(double dx, double dy) method to this class.

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Moving a point Add a moving behavior to Point … with a command

public class Point { private double x; private double y; … public void move(double dx, double dy) { x = x + dx; y = y + dy; }}

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Moving points

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Building on Point A circle is defined by its center (a point)

and its radius (a double)public class Circle { private Point center; private double radius; public Circle(Point aCenter, double aRadius) { center = aCenter; radius = aRadius; } public Point getCenter() { return center; } public double getRadius() { return radius; }}

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Complex objectsPoint p = new Point(1, 2);Circle c = new Circle(p, .5);

c.getCenter().getX() 1??

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Moving a Circle

public class Circle { private Point center; private double radius; … public void move(double dx, double dy) { center.move(dx, dy); }}

To move a circle move its center

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Behind the scenes Objects are implemented as chunks of memory Object variables contain the addresses of the

chunks, not the chunks themselves That is, object variables hold references to objects

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The reference to nowhere Uninitialized object type variables contain null It's a value in all object types But it refers to no object Often used to indicate missing or optional data

Point p = null;Circle c = new Circle(p,.5);c.move(1,1);java.lang.NullPointerException: at Circle.move(Circle.java:33) …

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Aliases Several variables can refer to the same object—aliasing

Point p = new Point(1,1);Point p1 = p;

p.move(.5, 1); p.getX() 1.5p1.move(2, 0); p1.getY() 3.5

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The dangers of aliasing…

Point p = new Point(1,1);Circle small = new Circle(p, .5);Circle big = new Circle(p, 5);small.move(.5, 2);big.move(5, 20);

small.getCenter().getX() ?

big.getCenter().getX() ?

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… illustrated

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Another example:A maze game

Maze explorers (just one, representing the player, for now) move around interact with denizens

Maze denizens interact with explorers

Rooms Where explorers and denizens are located

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Explorer knowledge Name: name Location in maze (room): location How much annoyance it can inflict: strength

How much annoyance it can tolerate: tolerance

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Responsibilities and commands

Explorer can be told to... Move around: move (to a new location) Fight a denizen: poke (a denizen) Receive a poke from a denizen, decreasing

tolerance: takeThat Denizen can be told to...

Fight an explorer: poke (an explorer) Receive a poke from an explorer: takeThat

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Interaction diagram

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An explorer gets hit

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Create and query an explorerpublic class Explorer {

private String name; private Room location; private int strength; private int tolerance;

public Explorer(String nm, Room loc, int str, int tol) { name = nm; location = loc; strength = str; tolerance = tol; } public String name() { return name; } public Room location() { return location; } public int strength() { return strength; } public int tolerance() { return tolerance; } ...}

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Command an explorer - fill in ???public class Explorer {

private String name; private Room location; private int strength; private int tolerance; ... public void move(Room newRoom) { ????????????? } public void takeThat(int hitStrength) { ????????????? } public void poke(Denizen opponent) { opponent.takeThat(strength); }}

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Command an explorerpublic class Explorer {

private String name; private Room location; private int strength; private int tolerance; ... public void move(Room newRoom) { location = newRoom; } public void takeThat(int hitStrength) { tolerance -= hitStrength; } public void poke(Denizen opponent) { opponent.takeThat(strength); }}

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MazeWorld in action

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A Movie with a Sequel

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A complete system

Model: represents the objects and relationships of interest characters, rooms, ... in video game your bank account(s)

Interface: allows a user (or another system) to interact with the model graphics, joystick commands ATM

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Interactive system

Controller

Model

View

Interface

Input

Output

input in Java is more complicated and will be discussed later

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Reminder: points and circles Point constructor

public Point(double x, double y) Point queries

public double getX()public double getY()

Circle constructorpublic Circle(Point center, double radius)

Circle queriespublic Point getCenter()public double getRadius()

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Setting up a view Define viewers for different classes of objects

Point viewer PointViewer• Constructor: public PointViewer(Point p)• Viewing command: public void display()

Circle viewer CircleViewer• Constructor: public CircleViewer(Circle c)• Viewing command: public void display()

A viewer keeps a reference to its object If the object changes, the next display call shows the

changed object

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Displaying In a more “real” system, displaying might involve

graphics, for example Here we use the simplest form of output The built-in System.out object provides a channel to

the standard output device (the screen) Output command

public void println(String s)Write a line with text s on the standard output device

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Point viewer

public class PointViewer { private Point point; public PointViewer(Point p) { point = p; } public void display() { System.out.println("A point at " + point.getX() + ", " + point.getY()); }}

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Circle viewer

public class CircleViewer { private Circle circle; public CircleViewer(Circle c) { circle = c; } public void display() { System.out.println("A circle at " + circle.getCenter().getX() + ", " + circle.getCenter().getY() + " with radius " + circle.getRadius()); }}

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Getting it started We’ve been using DrJava to create objects

and interact with them What if we want our Java code to work on its

own? We need a stand-alone way of creating the

appropriate objects and and making them work for us

The main method: the Java virtual machine calls it to start the execution of a Java program

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The main programpublic class PointCircleDisplay { public static void main(String[] args) { Point p = new Point(2, 3); PointViewer pv = new PointViewer(p); pv.display(); p.move(1, -1); pv.display(); Circle c = new Circle(p, 1); CircleViewer cv = new CircleViewer(c); cv.display(); p.move(.5,-2); cv.display(); }}

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Two roles for classes

So far: classes as templates for objects instance variables: object data methods: object functionality

But also: classes as containers for functionality and data that does not need multiple instances

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What goes into a class Static: same independently of how they are

accessed Variables: same value Methods: same computation

Dynamic: different for different class instances Variables: different values Methods: computation may depend on instance

variables

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Putting it all together Using static and dynamic: numbered tickets

public class Ticket { private static int issued; public static int getIssued(){ return issued; } private int number; public Ticket() { number = issued++; } public int getNumber() { return number; }}

static

dynamic

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Issuing tickets

Ticket t1 = new Ticket();t1.getNumber() 0

Ticket t2 = new Ticket();t2.getNumber() 1

Ticket.getIssued() 2

The static variable issued tracks the total number of Ticket objects

The instance (dynamic) variable number stores the number of a particular Ticket object

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Classes as containers

Container for a set of related static methods and variables

To access non-private static members of class X variable: X.var method: X.meth(...)

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Example: the Math class

Defines common mathematical constants and functions

Math.PI 3.141592653589793

Math.sin(Math.PI/2) 1.0

Math.log(Math.E) 1.0

Math.pow(2,3) 8.0

Math.pow(2, 0.5) 1.4142135623730951

Math.sqrt(3*3 + 4*4) 5.0