Classes Chapter 6 Spring 2007 CS 101 Aaron Bloomfield The Car class More on classes vs.
Download ReportTranscript Classes Chapter 6 Spring 2007 CS 101 Aaron Bloomfield The Car class More on classes vs.
Classes
Chapter 6 Spring 2007 CS 101 Aaron Bloomfield 1
The Car class
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More on classes vs. objects
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A new example: creating a Car class
What properties does a car have in the real world?
Color Position (x,y) Fuel in tank We will implement these properties in our Car class public class Car { private Color color; private int xpos; private int ypos; private int fuel; //...
} 4
Car’s instance variables
public class Car { private Color color; private int xpos; private int ypos; private int fuel; //...
} - color - fuel + … Car - xpos - ypos 5
Instance variables and attributes
Default initialization If the variable is within a method, Java does NOT initialize it If the variable is within a class, Java initializes it as follows: Car Numeric instance variables initialized to 0 - color = null - xpos = 0 Logical instance variables initialized to false - fuel = 0 + … - ypos = 0 Object instance variables initialized to null 6
Car behaviors or methods
What can a car do? And what can you do to a car?
Move it Change it’s x and y positions Change it’s color Fill it up with fuel For our computer simulation, what else do we want the Car class to do?
Create a new Car Plot itself on the screen Each of these behaviors will be written as a method 7
Creating a new car
To create a new Car, we call: Car c = new Car(); Notice this looks like a method You are calling a special method called a constructor A constructor is used to create (or construct) an object It sets the instance variables to initial values The constructor: public Car() { fuel = 1000; color = Color.BLUE; } 8
Constructors
No return type!
For now, all constructors are public EXACT same name as class
public Car() { fuel = 1000; color = Color.BLUE; } 9
Our Car class so far
public class Car { private Color color; private int xpos; private int ypos; private int fuel; public Car() { fuel = 1000; color = Color.BLUE; } } public class Car { private Color color = Color.BLUE
; private int xpos; private int ypos; private int fuel = 1000 ; public Car() { } } 10
Our Car class so far
public class Car { private Color color = Color.BLUE; private int xpos = 0 ; private int ypos = 0 ; private int fuel = 1000; Car - color = Color.BLUE
- fuel = 1000
+ Car()
+ … - xpos = 0 - ypos = 0 public Car() { } } Called the default constructor The default constructor has no parameters If you don’t include one, Java will SOMETIMES put one there automatically 11
Alien Song
AlienSong.mpg
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Another constructor
Another constructor: public Car (Color c, int x, int y, int f) { color = c; xpos = x; ypos = y; fuel = f; } This constructor takes in four parameters The instance variables in the object are set to those parameters This is called a specific constructor An constructor you provide that takes in parameters is called a specific constructor 13
}
Our Car class so far
public class Car { private Color color = Color.BLUE; private int xpos = 0; private int ypos = 0; private int fuel = 1000; public Car() { } Car - color = Color.BLUE
- fuel = 1000 - xpos = 0 - ypos = 0 + Car()
+ Car (Color, int, int, int)
+ … public Car (Color c, int x, int y, int f) { color = c; xpos = x; ypos = y; fuel = f; } 14
Using our Car class
Now we can use both our constructors: Car c1 = new Car(); Car c2 = new Car (Color.BLACK, 1, 2, 500); c1 c2 Car - color = Color.BLUE
- fuel = 1000 + Car() + Car (Color, int, int, int) + … - xpos = 0 - ypos = 0 Car - color = Color.BLACK
- fuel = 500 - xpos = 1 - ypos = 2 + Car() + Car (Color, int, int, int) + … 15
So what does private mean?
Consider the following code
Note that it’s a different class!
public class CarSimulation { public static void main (String[] args) { Car c = new Car(); System.out.println (c.fuel); } } Recall that fuel is a private instance variable in the Car class Private means that code outside the class CANNOT access the variable For either reading or writing Java will not compile the above code If fuel were public, the above code would work 16
So how do we get the fuel of a Car?
Via accessor methods in the Car class: public int getFuel() { } return fuel; public Color getColor() { return color; } public int getYPos() { return ypos; } public int getXPos() { } return xpos; As these methods are within the Car class, they can read the private instance variables As the methods are public, anybody can call them 18
So how do we
set
the fuel of a Car?
Via mutator methods in the Car class: public void setFuel (int f) { } fuel = f; public void setColor (Color c) { color = c; } public void setXPos (int x) { xpos = x; } public void setYPos (int y) { ypos = y; } As these methods are within the Car class, they can read the private instance variables As the methods are public, anybody can call them 19
Why use all this?
These methods are called a get/set pair Used with private variables We’ll see why one uses these later in this slide set Our Car so far: Car - color = Color.BLUE
- fuel = 1000 - xpos = 0 - ypos = 0 + Car() + Car (Color, int, int, int)
+ void setXPos (int x) + void setYPos (int y) + void setColor (Color c) + void setFuel (int f) + int getFuel() + int getXPos() + int getYPos() + Color getColor()
+ … 20
Back to our specific constructor
public class Car { private Color color = Color.BLUE; private int xpos = 0; private int ypos = 0; private int fuel = 1000; public class Car { private Color color = Color.BLUE; private int xpos = 0; private int ypos = 0; private int fuel = 1000; } } public Car (Color c, int x, int y, int f) { color = c; xpos = x; ypos = y; fuel = f; } } public Car (Color c, int x, int y, int f) { setColor (c); setXPos (x); setYPos (y); setFuel (f); 21
Back to our specific constructor
Using the mutator methods (i.e. the ‘set’ methods) is the preferred way to modify instance variables in a constructor We’ll see why later 22
Today’s demotivators
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So what’s left to add to our Car class?
What else we should add: A mutator that sets both the x and y positions at the same time A means to “use” the Car’s fuel A method to paint itself on the screen Let’s do the first: public void setPos (int x, int y) { setXPos (x); setYPos (y); } Notice that it calls the mutator methods 24
Using the Car’s fuel
Whenever the Car moves, it should burn some of the fuel For each pixel it moves, it uses one unit of fuel We could make this more realistic, but this is simpler Math.abs() returns the absolute value public void setXPos (int x) { xpos = x; } public void setXPos (int x) { fuel -= Math.abs
(getXPos()-x); xpos = x; } public void setYPos (int y) { ypos = y; } public void setYPos (int y) { fuel -= Math.abs
(getYPos()-y); 25 ypos = y; }
Setting both positions at once
public void setPos (int x, int y) { setXPos(x); setYPos(y); } Notice that to modify the instance variables, the mutator methods are used 26
Drawing the Car
The simple way to have the Car draw itself: } public void paint (Graphics g) { g.setColor (color); g.fillRect (xpos-50, ypos-100, 100, 200); This draws a single rectangle that is 100 by 200 pixels in size Lets use constants for the car’s height and width...
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Drawing the Car
A better version: private final int CAR_WIDTH = 100; private final int CAR_HEIGHT = 200; public void paint (Graphics g) { g.setColor (color); g.fillRect (getXPos() CAR_WIDTH/2 , getYPos() CAR_HEIGHT/2 , CAR_WIDTH , CAR_HEIGHT ); } This makes it easier to change the car size We could have made the car size instance variables and set them via mutators 28 Lets add tires!
Drawing the Car
private final int CAR_WIDTH = 100; private final int CAR_HEIGHT = 200; private final int TIRE_WIDTH = 20; private final int TIRE_HEIGHT = 40; private final int TIRE_OFFSET = 20; public void paint (Graphics g) { g.setColor (color); g.fillRect (getXPos()-CAR_WIDTH/2, getYPos()-CAR_HEIGHT/2, CAR_WIDTH, CAR_HEIGHT); // Draw the tires g.setColor (Color.BLACK); g.fillRect (getXPos()-(CAR_WIDTH/2+TIRE_WIDTH), getYPos()-(CAR_HEIGHT/2-TIRE_OFFSET), TIRE_WIDTH, TIRE_HEIGHT); g.fillRect (getXPos()-(CAR_WIDTH/2+TIRE_WIDTH), getYPos()+(CAR_HEIGHT/2-TIRE_OFFSET-TIRE_HEIGHT), TIRE_WIDTH, TIRE_HEIGHT); g.fillRect (getXPos()+(CAR_WIDTH/2), getYPos()+(CAR_HEIGHT/2-TIRE_OFFSET-TIRE_HEIGHT), TIRE_WIDTH, TIRE_HEIGHT); g.fillRect (getXPos()+(CAR_WIDTH/2),
Don’t worry about this – just know that it draws four tires
getYPos()-(CAR_HEIGHT/2-TIRE_OFFSET), TIRE_WIDTH, TIRE_HEIGHT); } 29
What happens when the car runs out of fuel?
We could do a number of things: Not allow the car to move anymore Print out a message saying, “fill me up!” We’ll color the car red We’ll insert the following code at the beginning of the paint() method: if ( fuel < 0 ) { color = Color.RED; } 30
Drawing the Car
private final int CAR_WIDTH = 100; private final int CAR_HEIGHT = 200; private final int TIRE_WIDTH = 20; private final int TIRE_HEIGHT = 40; private final int TIRE_OFFSET = 20; public void paint (Graphics g) { if ( fuel < 0 ) { color = Color.RED; } g.setColor (color); g.fillRect (getXPos()-CAR_WIDTH/2, getYPos()-CAR_HEIGHT/2, CAR_WIDTH, CAR_HEIGHT); // Draw the tires g.setColor (Color.BLACK); g.fillRect (getXPos()-(CAR_WIDTH/2+TIRE_WIDTH), getYPos()-(CAR_HEIGHT/2-TIRE_OFFSET), TIRE_WIDTH, TIRE_HEIGHT); g.fillRect (getXPos()-(CAR_WIDTH/2+TIRE_WIDTH), getYPos()+(CAR_HEIGHT/2-TIRE_OFFSET-TIRE_HEIGHT), TIRE_WIDTH, TIRE_HEIGHT); g.fillRect (getXPos()+(CAR_WIDTH/2), getYPos()+(CAR_HEIGHT/2-TIRE_OFFSET-TIRE_HEIGHT), TIRE_WIDTH, TIRE_HEIGHT); g.fillRect (getXPos()+(CAR_WIDTH/2), getYPos()-(CAR_HEIGHT/2-TIRE_OFFSET), TIRE_WIDTH, TIRE_HEIGHT); } 31
Our car in action
CarGUI.java
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How well do you feel you understand the Car class?
Very well! This stuff is easy!
Fairly well – with a little review, I’ll be good Okay. It’s not great, but it’s not horrible, either Not well. I’m kinda confused Not at all. I’m soooooo lost.
20% 20% 20% 20% 20% V er y w el l! T hi F s ai st rly uf w f i el ...
l – O w ka y.
ith I a t’s li n tt.
ot N .
g ot re w at el , b l. I’ u.
m N ..
k ot in a da t a c ll.
...
I ’m s oo oo o.
..
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Cubic Tragedy
Cubic_tragedy_m640.mov
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Miscellaneous Stuff
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What I’m not expecting you to know yet…
What the static keyword means And why the main() method is And why other methods are not Why you should always call the mutator methods, instead of setting the field directly Just know that it’s a good programming practice, and follow it We’ll see why later Why instance variables are supposed to be private Just know that it’s a good programming practice, and follow it Again, we’ll see why soon 36
Terminology
An attribute of a class can be called: Instance variable or class variable We’ll see the difference later Static variable (or static field) Synonymous with class variable Field Generally means either type Variable Also means either type Attribute Property Argh!
I will generally use the terms variable or field when I am not differentiating between the two And instance variable/field and class variable/field when I am 37
The main() method
Consider a class with many methods: public class WhereToStart { public static void foo (int x) { // ...
} public static void bar () { // ...
} public static void main (String[] args) { // ...
} } Where does Java start executing the program?
Always at the beginning of the main() method!
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Running a class without a main() method
Consider the Car class It had no main() method!
The main() method was in the CarSimulation (or CarGUI) class So let’s try running it… 39
Car.java
Program Demo
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Variable initialization
A local variable is NOT initialized to a default value This is any variable declared within a method Or within a block within a method This is pretty stupid, in my opinion Parameters are initialized to whatever value they are passed Instance and class variables are initialized to default values Numbers to zero, booleans to false, references to null This means any field in a class Either class variables or instance variables 41
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How well do you feel you understand creating classes so far?
Very well! This stuff is easy!
Fairly well – with a little review, I’ll be good Okay. It’s not great, but it’s not horrible, either Not well. I’m kinda confused Not at all. I’m soooooo lost.
20% 20% 20% 20% 20% V er y w el l! T hi F s ai st rly uf w f i el ...
l – O w ka y.
ith I a t’s li n tt.
ot N .
g ot re w at el , b l. I’ u.
m N ..
k ot in a da t a c ll.
...
I ’m s oo oo o.
..
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The Circle class
Introducing static-ness, visibilities, etc.
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A Circle class
We are going to develop a Circle class Perhaps for use in a graphics program Why?
Partly to review creating classes Go over some topics that were a bit fuzzy Constructors and creating objects Show why one uses the get/set methods instead of directly modifying the instance variables Discuss visibilities (public, private, etc.) Discuss the static keyword 45
Circle class properties
What properties does a circle have?
Radius PI = 3.141592653589793234
Color (if plotting in a graphics program) (x,y) location These properties will become instance variables We are only going to play with the first two (radius and PI) in this example Thus, we are ignoring the color and location 46
Our Circle class
Circle c = new Circle(); c public class Circle { double radius; Note the radius field is not initialized by us double PI = 3.1415926536; } Circle - radius = 0.0
PI = 3.14159… … + … We’re ignoring the public for now Note the fields are not static 47
Accessing our Circle object
Any variable or method in an object can be accessed by using a period The period means ‘follow the reference’ c Example: System.in
Example: System.out.println
(c.radius); Example: c.PI = 4; This is bad – PI should have been declared final (this will be done later) Circle - radius = 0.0
PI = 3.14159… … + … 48
What’s the output?
public class Circle { double radius; double PI = 3.1415926536; } public class CircleTest { public static void main (String[] args) { int x; Circle c = new Circle(); System.out.println (x); } } Java will give a “variable not initialized” error When a variable is declared as part of a method, Java does not initialize it to a default value 49
What’s the output now?
public class Circle { double radius; double PI = 3.1415926536; } public class CircleTest { public static void main (String[] args) { int x; Circle c = new Circle(); System.out.println (c.radius); Java outputs 0.0!
} } When a variable is declared as part of a class, Java does initialize it to a default value 50
What’s going on?
A (method) variable needs to be initialized before it is used Usually called a local variable A instance variable is automatically initialized by Java All numbers are initialized to 0, booleans to false, etc.
This is a bit counter-intuitive… 51
Circle class behaviors
What do we want to do with (and to) our Circle class?
Create circles Modify circles (mutators) Find out about our circles’ properties (accessors) Find the area of the circle Plot it on the screen (or printer) A few others… These will be implemented as methods 52
Calling the Circle constructor
To create a Circle object: c1 Circle c1 = new Circle(); This does four things: Creates the c1 reference Creates the Circle object Makes the c1 reference point to the Circle object Calls the constructor with no parameters (the ‘default’ constructor) Circle - radius = 0.0
PI = 3.14159… … + Circle() + Circle (double r) + … The constructor is always the first method called when creating (or ‘constructing’) an object 53
Calling the Circle constructor
To create a Circle object: c1 Circle c1 = new Circle( 2.0
); This does four things: Creates the c1 reference Creates the Circle object Makes the c1 reference point to the Circle object Calls the constructor with 1 double parameters (the ‘specific’ constructor) Circle - radius = 0.0
PI = 3.14159… … + Circle() + Circle (double r) + … The constructor is always the first method called when creating (or ‘constructing’) an object 54
Constructors
Remember, the purpose of the constructor is to initialize the instance variables PI is already set, so only radius needs setting public Circle() { } radius = 1.0; Note there is no return type for constructors public Circle (double r) { radius = r; } Note that the constructor name is the EXACT same as the class name Note that there are two “methods” with the same name!
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What happens in memory
Consider: Circle c = new Circle(); A double takes up 8 bytes in memory Thus, a Circle object takes up 16 bytes of memory As it contains two doubles Circle Shorthand representation c c - radius = 1.0
- PI = 3.1415926536
… + Circle() + Circle (double r) + … Circle - radius = 1.0
- PI = 3.14159
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}
Circle class: our class so far
public class Circle { double radius; double PI = 3.1415926536; public Circle() { radius = 1.0; } public Circle (double r) { radius = r; } 57
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How well do you feel you understand the constructors?
Very well! This stuff is easy!
Fairly well – with a little review, I’ll be good Okay. It’s not great, but it’s not horrible, either Not well. I’m kinda confused Not at all. I’m soooooo lost.
20% 20% 20% 20% 20% V er y w el l! T hi F s ai st rly uf w f i el ...
l – O w ka y.
ith I a t’s li n tt.
ot N .
g ot re w at el , b l. I’ u.
m N ..
k ot in a da t a c ll.
...
I ’m s oo oo o.
..
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New 2005 demotivatiors!
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}
Circle class: our class so far
public class Circle { double radius; double PI = 3.1415926536; public Circle() { radius = 1.0; } public Circle (double r) { radius = r; } 60
Consider the following code
public class CircleTest { public static void main (String[] args) { Circle c1 = new Circle(); Circle c2 = new Circle(); Circle c3 = new Circle(); Circle c4 = new Circle(); } } 61
What happens in memory
There are 4 Circle objects in memory Taking up a total of 4*16 = 64 bytes of memory c1 Circle - radius = 1.0
- PI = 3.14159
c2 Circle - radius = 1.0
- PI = 3.14159
c3 Circle - radius = 1.0
- PI = 3.14159
c4 Circle - radius = 1.0
- PI = 3.14159
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Consider the following code
public class CircleTest { public static void main (String[] args) { Circle c1 = new Circle(); //...
Circle c1000000 = new Circle(); } } public class CircleTest { public static void main (String[] args) { Vector v = new Vector(); for ( int i = 0; i < 1000000; i++ ) v.add (new Circle()); } } These programs create 1 million Circle objects!
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What happens in memory
There are 1 million Circle objects in memory Taking up a total of 1,000,000*16 ≈ 16 Mb of memory c1 Circle - radius = 1.0
- PI = 3.14159
c2 Circle - radius = 1.0
- PI = 3.14159
… c1000000 Circle - radius = 1.0
- PI = 3.14159
Note that the final PI field is repeated 1 million times 64
The use of static for fields
If a variable is static , then there is only ONE of that variable for ALL the objects That variable is shared by all the objects c1 Circle - radius = 1.0
c2 Circle - radius = 1.0
c3 … - radius = 1.0
Circle - radius = 1.0
PI 3.1415926536
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More on static fields
What does the following print Note that PI is not final Circle c1 = new Circle(); Circle c2 = new Circle(); Circle c3 = new Circle(); Circle c4 = new Circle(); c1.PI = 4.3; System.out.println (c2.PI); It prints 4.3
Note you can refer to static fields by object.variable
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Even more on static fields
There is only one copy of a static many objects are declared in memory field no matter how Even if there are zero objects declared!
The one field is “common” to all the objects Static variables are called class variables As there is one such variable for all the objects of the class Whereas non-static variables are called instance variables Thus, you can refer to a static field by using the class name: Circle.PI
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Even even more on static fields
This program also prints 4.3: Circle c1 = new Circle(); Circle c2 = new Circle(); Circle c3 = new Circle(); Circle c4 = new Circle(); Circle.PI = 4.3; System.out.println (c2.PI); 68
Even even even more on static fields
We’ve seen static fields used with their class names: System.in
(type: InputStream) System.out
Math.PI
Integer.MAX_VALUE
Game.BOARD_X_COORD
(type: OutputStream) (type: double) (type: int) (in HW J6) 69
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How well do you feel you understand static-ness?
Very well! This stuff is easy!
Fairly well – with a little review, I’ll be good Okay. It’s not great, but it’s not horrible, either Not well. I’m kinda confused Not at all. I’m soooooo lost.
20% 20% 20% 20% 20% V er y w el l! T hi F s ai st rly uf w f i el ...
l – O w ka y.
ith I a t’s li n tt.
ot N .
g ot re w at el , b l. I’ u.
m N ..
k ot in a da t a c ll.
...
I ’m s oo oo o.
..
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Hand Paintings
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Back to our Circle class
public class Circle { double radius; final static double PI = 3.1415926536; public Circle() { radius = 1.0; } Note that PI is now final and static public Circle (double r) { radius = r; } } But it doesn’t do much!
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Adding a method
public class Circle { double radius; final static double PI = 3.1415926536; // Constructors...
double computeArea () { return PI*radius*radius; } } Note that a (non-static) method can use both instance and class variables 73
Using that method
public class CircleTest { public static void main (String[] args) { Circle c = new Circle(); c.radius = 2.0; double area = c.computeArea(); System.out.println (area); } } Prints 12.566370614356
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What happens when that method is called
public class Circle { double radius; final static double PI = 3.1415926536; public Circle() { radius = 1.0; } area 12.566
// other constructor double computeArea () { return PI*radius*radius; } } public class CircleTest { public static void main (String[] args) { Circle c = new Circle(); c.radius = 2.0; double area = c.computeArea(); System.out.println (area); } } c - radius = 2.0
PI = 3.14159… … + Circle() + Circle (double r) + computeArea() + … 75
}
Review of our Circle class
public class Circle { double radius; final static double PI = 3.1415926536; public Circle() { } Slight change from before public Circle (double r) { radius = r; } double computeArea () { return PI*radius*radius; } 76
A note about methods/variable order
Within a method, a variable must be declared before it is used In a class, methods and variables can be declared in any order This is different than C++ 77
Review of static for fields
If a variable is static , then there is only ONE of that variable for ALL the objects That variable is shared by all the objects c1 Circle - radius = 1.0
c2 Circle - radius = 1.0
c3 … - radius = 1.0
Circle - radius = 1.0
PI 3.1415926536
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Adding another method
double oneOverRadius() { return 1.0/radius; } I couldn’t think of a good reason to divide something by the radius… 80
What happens now?
Code in class CircleTest’s main() method Circle c = new Circle(); // c.radius is now 0.0
System.out.println (c.oneOverRadius()); Java won’t crash, but many other programming languages (C and C++, in particular) will So we’ll call this a ‘crash’ for the sake of this lecture Java prints “Infinity” Not what we wanted, though!
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One way to fix this…
public class Circle { double radius = 1.0; final static double PI = 3.1415926536; // Constructors...
double computeArea () { return PI*radius*radius; } double oneOverRadius() { return 1.0/radius; } } Note that the radius variable is now initialized to 1.0
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Back to our program…
This code will now run properly: Circle c = new Circle(); // c.radius = 1.0
System.out.println (c.oneOverRadius()); But this code will “crash”: Circle c = new Circle(); // c.radius = 1.0
c.radius = 0.0; System.out.println (c.oneOverRadius()); 83
Where the “crash” occurs
public class CircleTest { public static void main (String[] args) { Circle c = new Circle(); // c.radius = 1.0
public class Circle { double radius = 1.0; final static double PI = 3.1415926536; c.radius = 0.0; } double computeArea () { return PI*radius*radius; } } System.out.println
(c.oneOverRadius()); double oneOverRadius() { } return 1.0/radius; Here is the badly written code Here is where the “crash” occurs 84
Motivation for private fields
Problem: We do not want people using our Circle class to be able to modify the fields on their own Solution: Don’t allow other code to modify the radius field Give it private visibility private field means that only code within the class can modify the 85
One way to fix this…
public class Circle { private double radius = 1.0; final static double PI = 3.1415926536; // Constructors...
double computeArea () { return PI*radius*radius; } double oneOverRadius() { return 1.0/radius; } } Note that the radius variable is now private 86
Today’s demotivators
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Back to our program…
This code will now not compile: Circle c = new Circle(); // c.radius = 1.0
c.radius = 0.0; System.out.println (c.oneOverRadius()); Java will give a compile-time error: radius has private access in Circle 88
Back to our program…
This code will also not compile: Circle c = new Circle(); // c.radius = 1.0
System.out.println (c.radius); Java will give the same compile-time error: radius has private access in Circle 89
The problem now…
But now you can’t modify a Circle’s radius!
Or find out what it is Solution: Use a get/set methods in Circle: A mutator method: void setRadius (double r) { radius = r; } An accessor method: double getRadius () { return radius; } 90
Our Circle class so far
public class Circle { private double radius = 1.0; final static double PI = 3.1415926536; // Constructors...
} double computeArea () { return PI*radius*radius; } double oneOverRadius() { return 1.0/radius; } void setRadius (double r) { radius = r; } double getRadius () { return radius; } 91
Using the get/set methods
public class CircleTest { public static void main (String[] args) { Circle c = new Circle(); c.setRadius (1.0); System.out.println
(c.computeArea()); System.out.println
(c.getRadius()); } } Here a method is invoked public class Circle { private double radius = 1.0; final static double PI = 3.1415926536; double computeArea () { return PI*radius*radius; } double oneOverRadius() { return 1.0/radius; } void setRadius (double r) { radius = r; } } double getRadius () { return radius; }
Wait! Another problem!
public class CircleTest { public static void main (String[] args) { Circle c = new Circle(); c.setRadius (0.0); Here is the problem now… System.out.println (c.oneOverRadius()); } } 93
This problem is easily fixed
Change the setRadius() method to the following void setRadius (double r) { if ( r > 0.0 ) radius = r; else radius = 1.0; } Now there is (almost) no way for code outside the Circle class to change the radius to zero This is the purpose of mutators To prevent changing the fields to a “bad” value We’ll see another motivation in a bit 94
Visibilities in Java
There are four visibilities: private : Only code within the same class can access the field or method Note: “access” means reading or writing the field, or invoking the method public : Any code, anywhere, can access the field or method protected : Used with inheritance We won’t get to that this semester default : Almost the same as public This is the default (duh!) Note that it can’t be specified like the others Also called ‘package’ 95
A few notes on visibilities
You can NOT specify visibilities for method variables Any method variable can only be accessed within that method Think of it as public within the method (after it’s defined) and private outside the method You can also specify visibilities for methods and classes We will see this a bit in this course 96
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3.
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How well do you feel you understand visibilities?
Very well! This stuff is easy!
Fairly well – with a little review, I’ll be good Okay. It’s not great, but it’s not horrible, either Not well. I’m kinda confused Not at all. I’m soooooo lost.
20% 20% 20% 20% 20% V er y w el l! T hi F s ai st rly uf w f i el ...
l – O w ka y.
ith I a t’s li n tt.
ot N .
g ot re w at el , b l. I’ u.
m N ..
k ot in a da t a c ll.
...
I ’m s oo oo o.
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DeCSS: The program
#include
DeCSS: The shirt (and tie!)
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How to decrypt a DVD: in haiku form.
(Thanks, Prof. D. S. T.) ----------------------- (I abandon my exclusive rights to make or perform copies of this work, U. S. Code Title Seventeen, section One Hundred and Six.) Muse! When we learned to count, little did we know all the things we could do some day by shuffling those numbers: Pythagoras said "All is number" long before he saw computers and their effects, or what they could do
DeCSS: The poem
Table Zero is: Five, zero, one, two, three, four, oh, one, two, three, four.
Table One is long: two to the eighth power bytes.
Ready? Here they are: Fifty one; then one hundred fifteen; fifty nine; thirty eight; ninety nine; thirty five; one hundred seven; one hundred eighteen; sixty two; one hundred twenty six; fifty four; forty three; one hundred ten; then 100
DeCSS: The number
The world’s first illegal prime number:
4856507896573978293098418946942861377074420873513579240196520736686985134010472 3744696879743992611751097377770102744752804905883138403754970998790965395522701 1712157025974666993240226834596619606034851742497735846851885567457025712547499 9648219418465571008411908625971694797079915200486670997592359606132072597379799 3618860631691447358830024533697278181391479795551339994939488289984691783610018 2597890103160196183503434489568705384520853804584241565482488933380474758711283 3959896852232544608408971119771276941207958624405471613210050064598201769617718 0947811362200272344827224932325954723468800292777649790614812984042834572014634 8968547169082354737835661972186224969431622716663939055430241564732924855248991 2257394665486271404821171381243882177176029841255244647445055834628144883356319 0272531959043928387376407391689125792405501562088978716337599910788708490815909 7548019285768451988596305323823490558092032999603234471140776019847163531161713 0785760848622363702835701049612595681846785965333100770179916146744725492728334 8691600064758591746278121269007351830924153010630289329566584366200080047677896 7984382090797619859493646309380586336721469695975027968771205724996666980561453 3820741203159337703099491527469183565937621022200681267982734457609380203044791 2277498091795593838712100058876668925844870047077255249706044465212713040432118 2610103591186476662963858495087448497373476861420880529443 101
DeCSS: The images
102
DeCSS: The recordings
All this info from http://www-2.cs.cmu.edu/~dst/DeCSS/Gallery/ Or do a Google search for “decss gallery” 103
DeCSS: The movie
104
Overriding methods (and constructors)
Consider the following code: Creates a Circle of radius 1.0
Circle c1 = new Circle (); Circle c2 = new Circle (2.0); Creates a Circle of radius 2.0
Java knows which constructor to call by the list of parameters This is called “overloading” Meaning it means multiple things, depending on the context We’ve seen overloading before: 3+4 Performs integer addition 3.0+4.0
“3”+”4” Performs floating-point addition Performs string concatenation The ‘+’ operator is overloaded 105
Overriding methods (and constructors), take 2
The following Circle constructors would not be allowed: We are assuming PI is not final for this example public Circle() { radius = 1.0; } public Circle (double r) { radius = r; } public Circle (double p) { PI = p; } When Circle(1.0) is called, which one is meant?
106
Using mutators in the constructor
Our second constructor has a problem: public Circle (double r) { radius = r; } Consider the following code: Circle c = new Circle (0.0); System.out.println ( c.oneOverRadius() ); The method is dividing by zero (again) 107
Using mutators in the constructor
This is easily fixed!
Our revised constructors: public Circle() { setRadius (1.0); } public Circle (double r) { setRadius (r); } The mutator will properly set the radius (and won’t set it to zero) 108
Why we always use the mutators
Consider a modified version of our circle class: class Circle { double radius; double diameter; String size; // ...
Our mutator now looks like this: That’s a lot of code to copy if you decide not to call the mutator!
void setRadius (double r) { if ( radius <= 0.0 ) radius = 1.0; else radius = r; diameter = 2*radius; if ( radius < 1.0 ) size = “small”; else if ( radius < 5.0 ) size = “medium”; else if ( radius < 10.0 ) size = “large”; else size = “huge”; } 109
Google April Fools Day Jokes (2007)
http://www.google.com/tisp/
http://mail.google.com/mail/help/paper/
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An aside: scoping issues
Consider this class: class Foo { int x = 0; public Foo (int x) { x = x; } } Which x is being referred to in the constructor?
Remember that Java will use the most recent x So this doesn’t set the instance variable!
Instead, it sets the parameter equal to itself 113
An aside: scoping issues
Let’s modify that class: Another solution: class Foo { int x = 0; public Foo (int x) { this.x = x; } } class Foo { int x = 0; public Foo ( int y ) { x = y; } } The ‘this.x’ means instance variable x the And the ‘x’ means the parameter Now this does the right thing By renaming the parameter, we achieve the same effect This very relevant to HW 7!
114
Back to the static discussion
Remember that there is one (and only one) static PI field, regardless of how many objects are declared Consider the following method: double getPI() { return PI; } It doesn’t read or modify the “state” of any object In this example, it doesn’t read/write the radius In fact, that particular method doesn’t care anything about the objects declared It’s only accessing a static field 115
Make getPI() static
Consider the following: static double getPI() { return PI; } As the method is static, it can ONLY access static fields A static method does not care about the “state” of an object Examples: Math.sin(), Math.tan(), Math.cos() They don’t care about the state of any Math object They only perform the computation 116
Invoking static methods
As with static fields, they can be called using either an object or the class name: Circle c = new Circle(); System.out.println (c.getPI()); System.out.println (Circle.getPI()); Static methods are also called class methods 117
static methods and non static fields
Consider the following (illegal) Circle method: static double getRadius() { return radius; } And the code to invoke it: public static void main (String[] args) { Circle c1 = new Circle(); Circle c2 = new Circle(); Circle c3 = new Circle(); Circle c4 = new Circle(); System.out.println (Circle.getRadius()); } 118
What happening in memory
Which radius field does Circle.getRadius() want?
c1 Circle - radius = 1.0
c2 Circle - radius = 1.0
c3 … - radius = 1.0
Circle - radius = 1.0
PI 3.1415926536
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The main static lesson
A static method cannot access or modify the state of the object it is a part of If you remember nothing else about static methods, remember this!
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static and non static rules
Non-static fields and methods can object name Static fields and methods can be accessed by class name or the object name Non-static methods can refer to fields ONLY BOTH be accessed by the EITHER the static and non-static Static methods can are part of ONLY access static fields of the class they 121
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How well do you feel you understand static-ness?
Very well! This stuff is easy!
Fairly well – with a little review, I’ll be good Okay. It’s not great, but it’s not horrible, either Not well. I’m kinda confused Not at all. I’m soooooo lost.
20% 20% 20% 20% 20% V er y w el l! T hi F s ai st rly uf w f i el ...
l – O w ka y.
ith I a t’s li n tt.
ot N .
g ot re w at el , b l. I’ u.
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k ot in a da t a c ll.
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Very unofficial demotivators
123
Back to our main() method
public static void main (String[] args) We’ll learn about arrays in chapter 8 The method does not return a value Any code anywhere can call this method It’s a static method: •Can’t access non-static fields or methods directly •Can be called only by the class name 124
Implications of main() being static
It can call other static methods within the same class class StaticMethods { static void method1() { System.out.println
(“hi!”) ; } public static void main (String args[]) { method1(); } } Note that we didn’t have to prefix method1() with a object Java assumes that it is in the same class 125
Another use of static methods
Let’s say we want each Circle object to have a unique identifier The identifier will be a positive integer So in the Circle class, we add a int id = 0; instance variable: Thus, each Circle object will have it’s own id number To keep track of the last assigned id number, we will use a class variable static int lastAssignedID = 0; Thus, for all Circle objects created, we will have a lastAssignedID field single 126
Circle ID numbers
We can then create a method to obtain a unique ID number: public static int getID() { return ++lastAssignedID; } This method is static, and only can access static fields In our Circle constructors, we put the following line: id = getID(); 127
Today’s demotivators
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Debugging Java Code
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Debugging Java code
In Main.java: In Error1.java: public class Main { public static void main (String args[]) { Error1 e1 = new Error1(); e1.foo(); e1.bar(); } }
This will cause a null pointer exception!
public class Error1 { public Error1() { } public void foo() { String s = null; System.out.println
(s.substring(5)); } } public void bar() { foo(); } 130
Program Demo
Errors/Main.java
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What the error output means
The error output: Exception in thread "main" java.lang.NullPointerException
at Error1.foo(Error1.java:6) at Main.main(Main.java:4) This means that: A null point exception happened In the Error1.foo() method on line 6 of Error1.java
And that was called from Main.main() on line 4 of Main.java
Note that the top two lines tell the most useful information So we comment out the e1.foo() call in main() 132
Debugging Java code
In Main.java: In Error1.java: public class Main { public static void main (String args[]) { Error1 e1 = new Error1(); //e1.foo(); e1.bar(); } } public class Error1 { public Error1() { } public void foo() { String s = null; System.out.println
(s.substring(5)); } } public void bar() { foo(); } 133
Program Demo
Errors/Main.java
134
What the error output means
The error output: Exception in thread "main" java.lang.NullPointerException
at Error1.foo(Error1.java:6) at Error1.bar(Error1.java:10) at Main.main(Main.java:5) This means that: A null point exception happened In the Error1.foo() method on line 6 of Error1.java
And that was called from Error1.bar() on line 10 of Error.java
And that was called from Main.main() on line 5 of Main.java
Again, note that the top two lines tell the most useful information 135
Honda’s best commercial
cog.mov
136
More on methods
137
Calling a method
Consider two Strings: String s = String t = “foo” ; “bar” ; Calling s.substring(2) is different than calling t.substring(2) Why?
Because of the object it is being called out of The method works the same in both cases (returns the substring) But it returns different results Whenever we are calling a method, we also need to know which object we are calling it out of 138
Return values
Many methods return a value Math.cos() String.valueOf() Consider: double d = Math.cos (90 * Math.PI/180.0); Let’s consider the Math.cos() method public double cos (double a) { double c = 0.0; // compute cos somehow into a variable c return c ; } The value c in the cos() method is copied into d 139
The return keyword
The return keyword does a few things: Immediately terminate the current method Pass the value back to whoever called the method You can have a return anywhere you want Inside loops, ifs, etc.
You can have as may returns as you want as well: public String foo (int x) { if ( x == 1 ) return “one” ; else if ( x == 2 ) return else return “other” ; “two” ; } 140
More on returns
Consider this class: public class Foo { // Default constructor omitted on this slide public String bar (String s) { String t = return t; “CS 101 ” + “ ” + s; } } And the code to invoke it: Foo w = new Foo(); String x = “rules” ; String y = foo.bar (x); System.out.println (y); What happens in memory?
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“rules” ; w this Foo + Foo() + bar (String s): String + … } t y x s 101 ” “rules" “CS 101 rules" 142
Returning an object from a method
We could rewrite our bar() method a number of ways: public String bar (String s) { String t = “CS 101 ” + “ ” + s; return t; } public String bar (String s) { return new String (“CS 101 ” + } “ ” + s); public String bar (String s) { return “CS 101 ” + “ ” + s; } 143
Returning a non-object from a method
In other words, returning a primitive type from a method public foo () { // ...
return x + y; } This method evaluates x+y, then returns that value to the caller 144
Chapter 2: Computer bugs
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Off to arrays….
146
Yale vs. Harvard
147
Rational class
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What we’ve seen so far
An example of creating a class Car Up next: another example Rational Represents rational numbers A rational number is any number that can be expressed as a fraction Both the numerator and denominator must be integers!
Discussed in section 4.8 of the textbook 149
What properties should our Rational class have?
The numerator (top part of the fraction) The denominator (bottom part of the fraction) Not much else… 150
What do we want our Rational class to do?
Obviously, the ability to create new Rational objects Setting the numerator and denominator Getting the values of the numerator and denominator Perform basic operations with rational numbers: + - * / Ability to print to the screen 151
Our first take at our Rational class
Our first take Rational - numerator = 0 - denominator = 0 public class Rational { private int numerator; private int denominator; //...
} + … This does not represent a valid Rational number!
Why not?
Java initializes instance variables to zero Both the numerator and denominator are thus set to zero 0/0 is not a valid number!
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Our next take at our Rational class
Our next take public class Rational { private int numerator = 0 ; private int denominator = 1 ; //...
} Rational - numerator = 0 - denominator = 1 We’ve defined the attributes of our class + … Next up: the behaviors 153
The default constructor
Ready?
public Rational() { } Rational - numerator = 0 - denominator = 1 + Rational() + … Yawn!
Note that we could have initialized the instance variables here instead The default constructor is called that because, if you don’t specify ANY constructors, then Java includes one by default Default constructors do not take parameters 154
The specific constructor
Called the specific constructor because it is one that the user specifies They take one or more parameters public Rational (int num, int denom) { setNumerator (num); setDenominator (denom); Rational } - numerator = 0 - denominator = 1 Note that the specific constructor calls the mutator methods instead of setting the instance variables directly We’ll see why later + Rational() + Rational (int num, int denom) + … 155
Accessor methods
Our two accessor methods: public int getNumerator () { return numerator; } public int getDenominator () { return denominator; } Rational - numerator = 0 - denominator = 1 + Rational() + Rational (int num, int denom) + int getNumerator() + int getDemonimator() + … 156
Mutator methods
Our two mutator methods: public void setNumerator (int towhat) { numerator = towhat; } public void setDenominator (int towhat) { denominator = towhat; } 157
Rational addition
How to do Rational addition:
a b
c d
ad
bc bd
Our add() method: public Rational add (Rational other) { } 158
The this keyword
this
Returns:
Rational - numerator = 5 - denominator = 6 + Rational () + Rational (int n, int d) + Rational add (Rational other) + … + Rational add (Rational other) + … Rational - numerator = 1 - denominator = 3 + Rational () + Rational (int n, int d) + Rational add (Rational other) + … 159
The this keyword
this is a reference to whatever object we are currently in Will not work in static methods We’ll see why later Note that the main() method is a static method While we’re at it, when defining a class, note that NONE of the methods so far were static 160
Rational addition
How to do Rational addition:
a b
c d
ad
bc bd
Our add() method: public Rational add (Rational other) { int a = this.getNumerator(); int b = this.getDenominator(); int c = other.getNumerator(); int d = other.getDenominator(); return new Rational (a*d+b*c, b*d); } 161
Rational addition
The following method is equivalent: Our add() method: public Rational add (Rational other) { int a = getNumerator(); int b = getDenominator(); int c = other.getNumerator(); int d = other.getDenominator(); return new Rational (a*d+b*c, b*d); } 162
Rational addition
The following method is equivalent, but not preferred: Our add() method: public Rational add (Rational other) { int a = numerator; int b = denominator; int c = other.numerator; int d = other.nenominator; return new Rational (a*d+b*c, b*d); } 163
Rational addition
The following method is equivalent, but not preferred: Our add() method: public Rational add (Rational other) { int a = this.
numerator; int b = this.
denominator; int c = other.numerator; int d = other.nenominator; return new Rational (a*d+b*c, b*d); } 164
Rational subtraction
How to do Rational subtraction:
a b
c d
ad
bc bd
Our subtract() method: public Rational subtract (Rational other) { int a = this.getNumerator(); int b = this.getDenominator(); int c = other.getNumerator(); int d = other.getDenominator(); return new Rational ( a*d-b*c, b*d ); } 165
Rational multiplication
How to do Rational multiplication:
a b c
*
d
ac bd
Our multiply() method: public Rational multiply (Rational other) { int a = this.getNumerator(); int b = this.getDenominator(); int c = other.getNumerator(); int d = other.getDenominator(); return new Rational ( a*c, b*d ); } 166
Rational division
How to do Rational division:
a b
c d
ad bc
Our divide() method: public Rational divide (Rational other) { int a = this.getNumerator(); int b = this.getDenominator(); int c = other.getNumerator(); int d = other.getDenominator(); return new Rational ( a*d, b*c ); } 167
Printing it to the screen
If we try printing a Rational object to the screen: Rational r = new Rational (1,2); System.out.println (r); We get the following: Rational@82ba41 Ideally, we’d like something more informative printed to the screen The question is: how does Java know how to print a custom class to the screen?
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The toString() method
When an object is put into a print statement: Rational r = new Rational (1,2); System.out.println (r); Java will try to call the toString() method to covert the object to a String If the toString() method is not found, a default one is included Hence the Rational@82ba41 from the previous slide So let’s include our own toString() method 169
The toString() method
Our toString() method is defined as follows: public String toString () { return getNumerator() + "/" + getDenominator(); } Note that the prototype must ALWAYS be defined as shown The prototype is the ‘ public String toString() ’ 170
Printing it to the screen
Now, when we try printing a Rational object to the screen: Rational r = new Rational (1,2); System.out.println (r); We get the following: 1/2 Which is what we wanted!
Note that the following two lines are (mostly) equivalent: System.out.println (r); System.out.println (r.toString()); 171
Our full Rational class
Rational - numerator = 0 - denominator = 1 + Rational() + Rational (int num, int denom) + int getNumerator() + int getDemonimator() + void setNumerator (int num) + void setDenominator (int denom) + Rational add (Rational other) + Rational subtract (Rational other) + Rational multiply (Rational other) + Rational divide (Rational other) + String toString() 172
Our Rational class in use, part 1 of 4
This code is in a main() method of a RationalDemo class First, we extract the values for our first Rational object: Scanner stdin = new Scanner(System.in); System.out.println(); // extract values for rationals r and s Rational r = new Rational(); System.out.print("Enter numerator of a rational number: "); int a = stdin.nextInt(); System.out.print("Enter denominator of a rational number: "); int b = stdin.nextInt(); r.setNumerator(a); r.setDenominator(b); 173
Our Rational class in use, part 2 of 4
Next, we extract the values for our second Rational object: Rational s = new Rational(); System.out.print("Enter numerator of a rational number: "); int c = stdin.nextInt(); System.out.print("Enter denominator of a rational number: “) ; int d = stdin.nextInt(); s.setNumerator(c); s.setDenominator(d); Notice that I didn’t create another Scanner object!
Doing so would be bad I used the same one 174
Our Rational class in use, part 3 of 4
Next, we do the arithmetic: // operate on r and s Rational sum = r.add(s); Rational difference = r.subtract(s); Rational product = r.multiply(s); Rational quotient = r.divide(s); 175
Our Rational class in use, part 4 of 4
Lastly, we print the results // display operation results System.out.println("For r = " + r.toString() + " and s = " System.out.println(" + s.toString()); r + s = " + sum.toString()); System.out.println(" System.out.println(" System.out.println(" System.out.println(); r - s = " + difference.toString()); r * s = " + product.toString()); r / s = " + quotient.toString()); 176
A demo of our Rational class
RationalDemo.java
177
Other things we might want to add to our Rational class
The ability to reduce the fraction So that 2/4 becomes 1/2 Not as easy as it sounds!
More complicated arithmetic Such as exponents, etc.
Invert Switches the numerator and denominator Negate Changes the rational number into its (additive) negation We won’t see any of that here 178
a) b) c) d) e)
How well do you feel you understand the Rational class?
Very well! This stuff is so easy.
With a little review, I’ll be good.
Not very well at all.
I’m so lost. What’s a class again?
I’d rather not answer this question, thanks.
179