CS 112 Introduction to Programming User-Defined Data Types: Examples Yang (Richard) Yang Computer Science Department Yale University 308A Watson, Phone: 432-6400 Email: [email protected]
CS 112 Introduction to Programming User-Defined Data Types: Examples Yang (Richard) Yang Computer Science Department Yale University 308A Watson, Phone:
Jan 02, 2016
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CS 112 Introduction to Programming
User-Defined Data Types: Examples
Yang (Richard) YangComputer Science Department
Yale University308A Watson, Phone: 432-6400
Email: [email protected]
2
Admin
PS6 (Sukoku) questions Friday’s class
Recap: Design and Implementation Methodology: Procedural BasedDesign (goal oriented)
top-down stepwise goal-drivenmethod decomposition
methods designed are those needed for the current goal
verb driven
Program implementation and testing bottom-up
3
Recap: Design and Implementation Methodology: Object-Oriented Design
Identify objects that are part of the problem domain or solution
• Each object has state (variables)• Each object has behaviors (methods)
Often do not consider one specific goal, but rather a context
noun driven
4
Side-by-Side Comparison
5
public class DrawRocket{
public static void main(String args[]){ for (int size = 3; size < 10; size++){ drawRocket(size); } }
public static void drawRocket(int scale){ printCap(scale); ... }
...
public static void printCap(int scale){ ... }}
public class RocketDrawing{ public static void main(String args[]){ for (int size = 3; size < 10; size++){ Rocket curRocket = new Rocket(size); curRocket.draw(); } }}
public class Rocket{ public int rocketSize;
public Rocket(int rocketSize){ this.rocketSize = rocketSize; }
public void draw(){ printCap(); ... } ... public void printCap(){ ... }}
Function-oriented Object-oriented
Recap: Class Definition Components
Variables fields (instance variables per object) static variables (shared by all objects)
Methods static methods (method usable with or without
object)• Can access only static variables
instance methods (can be used only on objects; can access both static and instance variables)
• Constructors• Accessors (do not change object state)• Mutators (modify object state)
6
Point class: Variables
public class Point {
// fields (instance variables, attributes) int x; int y;
// static variables shared by all objects static int numPoints = 0;
Point class: Constructors
public class Point { … // constructors public Point(int x, int y) { this.x = x; this.y = y; numPoints ++; // a shared variable to // keep track of all
// Points created. } public Point() { this(0, 0); }
Point class: Static Methods
public class Point { … // static methods: // cannot access any instance variables // because one may call Point.readpoint(input) // which has no implicit parameter public static Point readPoint(Scanner scan) { Point p = new Point(); p.x = scan.nextInt(); p.y = scan.nextInt(); return p; }
public static int totalPoints() { return numPoints; }
Point class: Instance Methods
// mutators public void translate(int dx, int dy) { x = x + dx; y = y + dy; } public void setLocation(int x, int y) { this.x = x; this.y = y; }
// accessors public double distanceFromOrigin() { return Math.sqrt(x * x + y * y); }
public String toString() { return "(" + x + ", " + y + ")“; }
public void draw() { StdDraw.filledCircle(x, y, 3); StdDraw.textLeft(x, y, toString() ); }}
Point class as blueprint
The class (blueprint) will describe how to create objects. Each object will contain its own data and methods.
Point classstate:
int x, ybehavior:
translate(int dx, int dy)draw()
…
Point object #1
state:x = 5, y = -2
behavior:translate(int dx, int dy)
draw()…
Point object #2
state:x = -245, y = 1897
behavior:translate(int dx, int dy)
draw()…
Point object #3
state:x = 18, y = 42
behavior:translate(int dx, int dy)
draw()…
12
Outline
Admin and recap Defining classes
o Motivation and basic syntax Examples
Design and Implementation Methodology: Object-Oriented Design
Identify objects (nouns) and their behaviors
Often do not consider one specific goal, but rather a context
Often lead to more reusable, extensible software
13
14
Example 1: A Coin Class
We define a Coin class to model a coin in heads-tails games
Design questions: State: what field(s) do we need to represent the state of a
coin?• face, an integer (or boolean) that represents the current face
Operations: what are some common behaviors/operations of a coin in a game context?
• a Coin constructor, to set up the object
• a isHeads method, to return if face is HEADS• a toString method, to return a string description of the current state
• a flip method, to flip the coin• Q: introduce a setFace() method?
See Coin.java, CountFlips.java, FlipRace.java
15
Instance Data: The Two Coins in FlipRace
int face;
class Coin coin1: Coin
face = 0
coin2: Coin
face = 1
16
Example 2: The BankAccount Class
We define an BankAccount class to model a bank account Design questions:
State: what field(s) do we need to represent the state of a bank acct?• acctNumber,an integer • acctName, a string• balance, an integer
Behaviors/operations: what are some common behaviors of a bank account in a simple banking context?
• A constructor, to set up the object
• a getBalance method, to return balance• a toString method, to return a string description of the current state
• a withdraw method, to withdraw from the account• a deposit method, to deposit into the account• a addInterest method, to add interest
See BankAccount.java, Transactions.java
17
Example 2: Account and Transactionspublic class BankAccount { final double RATE = 0.035; long acctNumber; String acctName; double balance;
public BankAccount (String owner, long account, double initial) {
acctName = owner; acctNumber = account; balance = initial; }
public double deposit (double amount) {
balance = balance + amount;
return balance; } …}
public static void main (String[] args) { BankAccount aliceAcct =
new BankAccount (“Alice", 11111, 100.00);
BankAccount bobAcct = new BankAccount (“Bob", 22222, 200.00);
BankAccount charlesAcct = new BankAccount (“Charles", 33333, 300.00);
bobAcct.deposit (30.00); …}
18
Example 2: The Three BankAccount Objects in Transactions
aliceAcct: BankAccount
acctNumber = 11111acctName = “Alice”balance = 100.00
bobAcct: BankAccount
acctNumber = 22222acctName = “Bob”balance = 200.00
charlesAcct: BankAccount
acctNumber = 33333acctName = “Charles”balance = 300.00
After bobAcct.deposit (30.00);
aliceAcct: BankAccount
acctNumber = 11111acctName = “Alice”balance = 100.00
bobAcct: BankAccount
acctNumber = 22222acctName = “Bob”balance = 230.00
charlesAcct: BankAccount
acctNumber = 33333acctName = “Charles”balance = 300.00
19
Example 3: The Ball Class
We define a Ball class to model self-bouncing balls
20
Example 3: The Ball Class
Design questions: State: what field(s) do we need to represent the state of a self-
bouncing ball?• rx, ry, current position• vx, vy, speed• color, current color• left, right, bottom, top:cage (boundaries)
Behaviors/operations: what are some common behaviors of a self-bouncing ball?
• A default constructor, to set up a random ball in unit square• A constructor, to set up a ball with given parameters
• A move method, to update position• A draw method, to display
See Ball.java, BouncingBalls.java
Example 3: Bouncing Ball in Unit Square
public class Ball {
double rx, ry; double vx, vy; double radius;
public Ball() { rx = ry = 0.5; vx = 0.015 - Math.random() * 0.03; vy = 0.015 - Math.random() * 0.03; radius = 0.01 + Math.random() * 0.01; }
public void move() { if ((rx + vx > 1.0) || (rx + vx < 0.0)) vx = -vx; if ((ry + vy > 1.0) || (ry + vy < 0.0)) vy = -vy; rx = rx + vx; ry = ry + vy; }
public void draw() { StdDraw.filledCircle(rx, ry, radius); }}
instance variables
Ball.java
bounce
constructor
methods
Object References
Allow client to manipulate an object as a single entity. Essentially a machine address (pointer).
Ball b1 = new Ball();b1.move();b1.move();
Ball b2 = new Ball();b2.move();
b2 = b1;b2.move();
main memory(64-bit machine)
100 0
101 0
102 0
103 0
104 0
105 0
106 0
107 0
108 0
109 0
110 0
111 0
112 0
addr value
Object References
Allow client to manipulate an object as a single entity. Essentially a machine address (pointer).
Ball b1 = new Ball();b1.move();b1.move();
Ball b2 = new Ball();b2.move();
b2 = b1;b2.move();
main memory(64-bit machine)
100 0
101 0
102 0
103 0
104 0
105 0
106 0
107 0
108 0
109 0
110 0
111 0
112 0
100
registers
0.50
0.50
0.05
0.01
0.03
addr value
b1
Object References
Allow client to manipulate an object as a single entity. Essentially a machine address (pointer). 0.50
0.50
0.05
0.01Ball b1 = new Ball();b1.move();b1.move();
Ball b2 = new Ball();b2.move();
b2 = b1;b2.move();
main memory(64-bit machine)
100
101
102
103
104 0.03
105 0
106 0
107 0
108 0
109 0
110 0
111 0
112 0
100
registers
0.55
0.51
addr value
b1
Object References
Allow client to manipulate an object as a single entity. Essentially a machine address (pointer).
Ball b1 = new Ball();b1.move();b1.move();
Ball b2 = new Ball();b2.move();
b2 = b1;b2.move();
main memory(64-bit machine)
100 0.55
101 0.51
102 0.05
103 0.01
104 0.03
105 0
106 0
107 0
108 0
109 0
110 0
111 0
112 0
100
registers
0.60
0.52
addr value
b1
Object References
Allow client to manipulate an object as a single entity. Essentially a machine address (pointer).
Ball b1 = new Ball();b1.move();b1.move();
Ball b2 = new Ball();b2.move();
b2 = b1;b2.move();
main memory(64-bit machine)
100 0.60
101 0.52
102 0.05
103 0.01
104 0.03
105 0
106 0
107 0
108 0
109 0
110 0
111 0
112 0
100
registers
107 0.50
0.50
0.07
0.04
0.04
addr value
b1
b2
Object References
Allow client to manipulate an object as a single entity. Essentially a machine address (pointer).
Ball b1 = new Ball();b1.move();b1.move();
Ball b2 = new Ball();b2.move();
b2 = b1;b2.move();
main memory(64-bit machine)
100 0.60
101 0.52
102 0.05
103 0.01
104 0.03
105 0
106 0
107 0.50
108 0.50
109 0.07
110 0.04
111 0.04
112 0
107
registers
100
0.57
0.54
addr value
b1
b2
Object References
Allow client to manipulate an object as a single entity. Essentially a machine address (pointer).
Ball b1 = new Ball();b1.move();b1.move();
Ball b2 = new Ball();b2.move();
b2 = b1;b2.move();
main memory(64-bit machine)
100 0.60
101 0.52
102 0.05
103 0.01
104 0.03
105 0
106 0
107 0.57
108 0.54
109 0.07
110 0.04
111 0.04
112 0
100
registers
100
addr value
b1
b2
Data stored in 107 – 111 for bit recycler (garbage collection).
Object References
Allow client to manipulate an object as a single entity. Essentially a machine address (pointer).
Ball b1 = new Ball();b1.move();b1.move();
Ball b2 = new Ball();b2.move();
b2 = b1;b2.move();
main memory(64-bit machine)
100
addr
0.60
value
101 0.52
102 0.05
103 0.01
104 0.03
105 0
106 0
107 0.57
108 0.54
109 0.07
110 0.04
111 0.04
112 0
100
b1
Moving b2 also moves b1 since they are aliases that reference the same object.
registers
100
b2
0.65
0.53
Creating Many Objects Each object is a data type value.
Use new to invoke constructor and create each one.
public class BouncingBalls { public static void main(String[] args) {
int N = Integer.parseInt(args[0]); Ball balls[] = new Ball[N]; for (int i = 0; i < N; i++) balls[i] = new Ball();
while(true) { StdDraw.clear(); for (int i = 0; i < N; i++) { balls[i].move(); balls[i].draw(); } StdDraw.show(20); } }}
create and initializeN objects
animation loop
31
Example 4: Continuous Line Graph What is a base class to allow drawing of
continuous line graphs?
http://en.wikipedia.org/wiki/Turtle_graphics
32
Example: Turtle
We define a Tutle class to keep track of drawing turtle (cursor)
Design questions: State: what field(s) do we need to represent the
state of a drawing turtle?• x,y, current position• angle: the current drawing direction
Operations: what are some common behaviors/operations on a Turle to allow flexible drawing?
• A Turtle constructor, to set up the object
• A goForward( stepSize) method• a turnLeft( angle ) method
Turtle Graphicspublic class Turtle { double x, y; // turtle is at (x, y) double angle; // facing this direction public Turtle(double x0, double y0, double a0) { x = x0; y = y0; angle = a0; }
public void turnLeft(double delta) { angle += delta; }
public void goForward(double d) { double oldx = x; double oldy = y; x += d * Math.cos(Math.toRadians(angle)); y += d * Math.sin(Math.toRadians(angle)); StdDraw.line(oldx, oldy, x, y); } …}
33
N-gon
3 7 1440
34
What is the angle of each turn? angle = 360/ N
What is the initial degree? angle / 2
N-gon
3 7 1440
public class Ngon { public static void main(String[] args) { int N = Integer.parseInt(args[0]); double angle = 360.0 / N; Turtle turtle = new Turtle(0.5, 0, angle/2.0);
double step = Math.sin(Math.toRadians(angle/2.0)); for (int i = 0; i < N; i++) { turtle.goForward(step); turtle.turnLeft(angle); } }}
35
Spira Mirabilis
public class Spiral { public static void main(String[] args) { int N = Integer.parseInt(args[0]); double decay = Double.parseDouble(args[1]); double angle = 360.0 / N; double step = Math.sin(Math.toRadians(angle/2.0)); Turtle turtle = new Turtle(0.5, 0, angle/2.0); for (int i = 0; i < 10 * N; i++) { step /= decay; turtle.goForward(step); turtle.turnLeft(angle); } }}
36
37
Spira Mirabilis
3 1.0 3 1.2 1440 1.00004 1440 1.0004
public class Spiral { public static void main(String[] args) { int N = Integer.parseInt(args[0]); double decay = Double.parseDouble(args[1]); double angle = 360.0 / N; double step = Math.sin(Math.toRadians(angle/2.0)); Turtle turtle = new Turtle(0.5, 0, angle/2.0); for (int i = 0; i < 10 * N; i++) { step /= decay; turtle.goForward(step); turtle.turnLeft(angle); } }}
37
Spira Mirabilis in Nature
38
Caution
René Magritte. "This is not a pipe."
Java. This is not a Turtle.
OOP. Natural vehicle for studying abstract models of the real world.
Turle myTurtle = new Turtle(0.5, 0, 45);myTurle.turnLeft( 90 );
40
Example 5: A Complex Class A complex number (a + bi) is a quintessential
mathematical abstraction (a + bi) + (c + di) = a + c + (b + d)i (a + bi) x (c + di) = ac - bd + (ad + bc)i
Many applications of complex numbers Fractals. Impedance in RLC circuits. Signal processing and Fourier analysis. Control theory and Laplace transforms. Quantum mechanics and Hilbert spaces.
a = 3 + 4i, b = -2 + 3i
a + b = 1 + 7i
a b = -18 + i
| a | = 5
Argos
41
Antennas
42
A Complex Class
Design questions: State: what field(s) do we need to represent the state of a
complex number?• re, a number representing the real part• im, a number representing the imaginary part
Behaviors: what are some common behaviors of a complex number?
• a Complex constructor, to set up the object
• A abs method, to return the magnitude• a toString method, to return a string description of a complex
number
• Mathematical operations such as +, -, *– a plus method to add current complex number with another
complex number– a times method to multiply current complex number with
another complex number– …
43
The Complex Class: Design Question
public class Complex {
double re; double im;
public Complex(double real, double imag) { re = real; im = imag; }
public ?? plus(Complex b) {
… }
…
- What is the return type of plus?- Should plus change the state of the
number
44
The Consistency (Familiarity) Design Principle
Suppose a, b, and c are standard numbers (Complex numbers are numbers after all)Does a + b (think a.+(b) ) change a?
• noWhat is the return of a + b (think a.+
(b)?• The value of a + b so that we can write a + b + c
State (no)change and return type of plus double real = re + b.re;
double imag = im + b.im; return new Complex(real, imag);}
45
Complex.javapublic class Complex {
double re; double im;
public Complex(double real, double imag) { re = real; im = imag; }
public String toString() { return re + " + " + im + "i"; }
public double abs() { return Math.sqrt(re*re + im*im); }
public Complex plus(Complex b) {
double real = re + b.re; double imag = im + b.im; return new Complex(real, imag); }
public Complex times(Complex b) { double real = re * b.re – im * b.im; double imag = re * b.im + im * b.re; return new Complex(real, imag); }}
constructor
instance variables
methods
creates a Complex object,and returns a reference to it
refers to b's instance variable
Immutability: Advantages and Disadvantages Immutable data type. Object's state does not
change once constructed. Complex is an example of Immutable objects. String
defined by Java is another example.
Advantages. Avoid aliasing bugs. Makes program easier to debug. Limits scope of code that can change values. Pass objects around without worrying about modification.
Disadvantage. New object must be created for every value.
47
A Simple Client
public static void main(String[] args) { Complex a = new Complex( 3.0, 4.0); Complex b = new Complex(-2.0, 3.0); Complex c = a.times(b); System.out.println("a = " + a.toString() ); System.out.println("b = " + b.toString() ); System.out.println("c = " + c.toString() );} % java TestClient
a = 3.0 + 4.0ib = -2.0 + 3.0ic = -18.0 + 1.0i
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