Lesson 4 – Classes & Objects

of 87.Lecture D – Classes & Objects

Lesson 4 – Classes & Objects

Unit D1 – Introduction


Container vs. Definition classes

• Generally, classes can be used for two purposes:

Container classes:• a collection of static methods that are not bound to any

particular object (e.g., the main() method).• These static methods usually have something in common

Definition classes:• These classes define new objects, in a way that we will soon

see.


Container vs. Definition Classes - Example

• The Math class is an example of the first kind. It is a container for math utility methods: Math.sqrt(), Math.abs(), Math.max(), ...

• The class Turtle is an example of the second kind. It defines a new type of objects, Turtle objects.

• We will now focus on the second kind.


Class Abstraction

• The most important stage in writing a class is to get an abstraction of the class done first. What does an object of this type represent? What is its interface? What is its internal state?

• This means you have to know what should be the internal state (variables) and also the behavior/interface (methods) of the class before you start to write you class code.


Objects and Classes

• The relationship between Classes and Objects can be seen as the relationship between a blueprint/model and the the actual object built from it. Example: Blueprint of a house (class) and the house (object)

• The class defines: The type of data that will be held in an object The code for the methods of the object

• In order to use a class you must instantiate an object from it. This corresponds to building the house from the blueprint. There may be many objects from a single class


Turtle Class Abstraction

• A Turtle represents a creature moving on the screen with a pen attached to its tail

• The internal state of a Turtle includes: Location on screen; direction of movement; tail up/down

• The interface of a Turtle is:

Turtle();

void moveForward(double units);

void moveBackward(double units);

void turnLeft(double degrees);

// …


Encapsulation

• The internal state of an object is not directly accessible to other parts of the program

• Other parts of the program can only access the object using its interface

• We say that the state is

encapsulated or hidden• This gives modularity

to the program

moveForward()

turnRight()

.

.

.

.

.

.

•tailDown

•x,y

•direction


Clock Class Abstraction

• A Clock represents a 12-hour clock• Its internal state includes: hour, minute, second• Its interface allows telling the clock that a second has

elapsed, and querying the clock for the time:

Clock(int hours, int minutes, int seconds)

int getSeconds()

void secondElapsed()

int getMinutes()

int getHours()

...


Using a Clock class

Clock newYorkTime = new Clock(12,59,59);

for (int j = 0; j < 3; j++) newYorkTime.secondElapsed();

System.out.println(newYorkTime.getHours() + “:” + newYorkTime.getMinutes() + “:” + newYorkTime.getSeconds());


Structure of the Clock class

• The private modifier specifies an identifier that is only visible within the class. Usually used for fields.

• The public modifier specifies an identifier that is accessible to all other classes. Usually used for methods.

public class Clock { private int hours, minutes, seconds; public Clock(int h, int m, int s){ … } public void secondElapsed() { … } public int getHours() { … } public int getMinutes() { … } public int getSeconds() { … } }


Lesson 4 – Classes & ObjectsUnit D2 - Class Elements


Instance Variables

• Recall that a class must define the state of an object and its behavior.

• We declare state variables (variables that hold the state of the object) in a similar way to that of regular variables, only they appear outside methods, inside the class.

• State variables are also called instance variables or fields.• Roughly speaking, the private modifier means that the

variables are not part of the object’s interface.

public class Clock { private int hours, minutes, seconds; // …}


Constructors

• Objects must be initialized before they can be used. • We must specify what is the initial state of the object

before we can use it.• Space for holding the object state data is only allocated

when the object is constructed.• We specify the way an object is initialized using a

constructor, which is a special method which is invoked every time we create a new object

• The name of the constructor is always identical to the class name


Clock Constructor

public Clock(int h, int m, int s){

hours = h;

minutes = m;

seconds = s;}


Clock constructor invoked

public class Clock { private int hours, minutes,seconds; public Clock(int h, int m, int s){ hours = h; minutes = m; seconds = s; } // …

C:

10

45

0

Clock c;c = new Clock(10,45,0);

hours:

Minutes:

Seconds:


Methods

• To make the date object useful, we must provide methods that define its behavior.

• We declare methods in a similar way to the way the method main was declared.

• Only there’s a big difference: the main method was static, which means it wasn’t

bound to a specific object we want to declare instance methods, which operate

on a specific instance of an object


secondElapsed() method

public void secondElapsed() {

if (seconds < 59) seconds++;

else {

seconds=0;

if (minutes < 59) minutes++ ;

else {

minutes = 0;

hours = hours < 12 ? hours+1 : 1;

}

}

}


Return Types

• Methods may return values• The return type of a method indicates the type of value

that the method sends back to the calling client• The return-type of getHours() is int. When a client

ask for the hours read of a clock it gets the answer as an int value.

• A method that does not return a value (such as secondElapsed()) has a void return type

• The return statement specifies the value that should be returned, which must conform with the return type of the method.


Clock accessor methods

public int getHours() {

return hours;

}

public int getMinutes() {

return minutes;

}

public int getSeconds() {

return seconds;

}


Method Context

• The getHours() and secondElapsed() methods are instance methods, which means they act on a particular instance of the class

• They cannot be invoked “out of the blue”. They must act on a particular object:

• An instance method is executed in the context of the object it acts upon.

Clock c = new Clock(1,2,3);getHours(); // Error!! of which clock?c.getHours(); // will return 1


ClockTest example

public class ClockTest {

public static void main(String[] args) { Clock swatch = new Clock(12,59,59); Clock seiko = new Clock(12,59,59);

System.out.println(swatch.getHours()); // 12 System.out.println(seiko.getHours()); // 12

swatch.secondElapsed();

System.out.println(swatch.getHours()); // 1 System.out.println(seiko.getHours()); // 12 }}


Method Parameters

• A method can be defined to accept zero or more parameters

• Each parameter in the parameter list is defined by its type and name

• The parameters in the method definition are called formal parameters

• The values passed to a method when it is invoked are called actual parameters

• The name of the method together with the list of its formal parameters is called the signature of the method

public void setTime(int h, int m, int s)


Method setTime()

public void setTime(int h, int m, int s){ if ((s >= 0) && (s < 60) && (m >= 0) && (m < 60) && (h > 0) && (h <= 12)) { hours = h; minutes = m; seconds = s; } // no effect if input is illegal}


Lesson 4 – Classes & ObjectsUnit D3 - Method writing details


Variable Scope

• Variables may be declared in: Class – state variables Method/constructor – local variables (and parameters) Inner block of a method – also local variables

• A variable is recognized throughout the block in which it was defined. This is called the scope of the variable.

• Local variables are allocated when the method is entered and freed when the method exits.

• The same name may be used in different scopes, and refer to totally different things

• If the same name is used in an outer and inner scope, then the inner scope definition “hides” the outer one.


The this reference

• When appearing inside an instance method, the this keyword denotes a reference to the object that the method is acting upon.

• The following are equivalent:

public int getHours() { return hours;}

public int getHours() { return this.hours;}


Using same names for parameters and fields

• It is usually bad practice to use the same name for a state variable and a local variable or parameter.

• Exception: parameters that correspond exactly to fields

public void setTime(int hours, int minutes, int seconds) { if ((seconds >= 0) && (seconds < 60) && (minutes >= 0) && (minutes < 60) && (hours > 0) && (hours <= 12)) { this.hours = hours; this.minutes = minutes; this.seconds = seconds; } // no effect if input is illegal}


Passing Parameters

• When a parameter is passed, a copy of the value is made and assigned to the formal parameter:

Clock beritling = new Clock(1,2,3);int lunchHour = 12;breitling.setTime(lunchHour,32,14);

hours = lunchHour

minutes = 32

seconds = 14


Parameters are passed by value

Clock c = new Clock(1,2,3);int a= 7;c.funnyGetHour(a); // still 7!!

class Clock {// … public void funnyGetHour(int h){ h = hours; }}


Passing Object parameters

• When an Object is a parameter, only the reference is passed by value. The parameter now is an alias of the object.class XX { // … public void setMidnight(Clock c) { c.setTime(12,0,0); }

class YY {// … XX x = new XX(); Clock myClock = new Clock(1,2,3); x.setMidnight(myClock); // 12:0:0


Example: A Bank Account Object

BankAccountBankAccount

public BankAccount(long accountNumber)

public void deposit(double amount)

public void withdraw(double amount)

public double getBalance()

public void transfer(double amount, BankAccount targetAccount)


Bank Account - example

public class BankAccount {

private long accountNumber; private double balance;

public BankAccount(long accountNumber){ this.accountNumber = accountNumber; balance = 0; }

public double getBalance() { return balance; } // continued in the next slide...}


Bank Account - example

public void deposit(double amount) { balance += amount;}public void withdraw(double amount) { balance -= amount;}public void transfer(double amount,

BankAccount targetAccount){ withdraw(amount); targetAccount.deposit(amount);}


Bank Account – usage example

BankAccount aliceAcc = new BankAccount(1398723);BankAccount bobAcc = new BankAccount(1978394);

aliceAcc.deposit(900); aliceAcc.transfer(700,bobAcc);// Alice’s balance = 200 ; Bob’s balance = 700


Constructor and Method Overloading

• A class can define several constructors -- several ways to initialize an instance of this class

• These constructors differ by the number and/or type of parameters they get.

• When we construct an object, the compiler decides which constructor to invoke according to the number and types of the actual arguments

• A constructor with no parameters is called the default constructor

• Different methods can also use the same name as long as they differ in the number or type of parameters.

• When we invoke a method, the compiler decides which method to invoke according to the number and types of the actual arguments


Constructor Overloading example

public Clock(int h, int m, int s){ hours = h; minutes = m; seconds = s;}public Clock(int h) { this(h, 0 ,0);}public Clock() { this(12);}


Lesson 4 – Classes & ObjectsUnit D4 - Visibility Modifiers


Visibility Modifiers

• We accomplish encapsulation through the appropriate use of visibility modifiers

• Visibility modifiers specify which parts of the program may see and use any particular class/method/field

• Information hiding is good!• A modifier is a Java reserved word that specifies particular

characteristics of a programming construct• We've used the modifier final to define a constant• Java has three visibility modifiers: public, private,

and protected• We will discuss the protected modifier later in the

course


Visibility Modifiers - Classes

• A class can be defined either with the public modifier or without a visibility modifier.

• If a class is declared as public it can be used by any other class

• If a class is declared without a visibility modifier it has a default visibility. This draws a limit to which other classes can use this class (classes in the same package). We will discuss default visibility later in the course.

• Classes that define a new type of objects, that are supposed to be used anywhere, should be declared public.


Visibility Modifiers - Members

• A member is a field, a method or a constructor of the class.

• Members of a class can be declared as private, protected, public or without a visibility modifier:

• Members that are declared without a visibility modifier are said to have default visibility. We will discuss default and protected visibility later in the course.

private int hours; int hours;public int hours;


Public Visibility

• Members that are declared as public can be accessed from any class that can access the class of the member

• We expose methods that are part of the interface of the class by declaring them as public

• Example: the methods getHours(), secondElapsed() and setTime() are part of the interface of class Clock so we define them as public.

• We do not want to reveal the internal representation of the object’s data. So we usually do not declare its state variables as public (encapsulation)


Private Visibility

• A class member that is declared as private, can be accessed only by code that is within the class of this member.

• We hide the internal implementation of the class by declaring its state variables and auxiliary methods as private.

• Data hiding is essential for encapsulation.


Illegal Access - example

// Example of illegal accessclass BankAccountTest { public static void main(String[] args) { BankAccount victim = new BankAccount(2398742); victim.balance = victim.balance - 500; // this will not compile! }}

public class BankAccount { private long accountNumber; private double balance; // …


Encapsulation not Among Instances of Same Class

• Encapsulation is to protect the programmers and is thus between the code of different classes

• Sometimes object instances of the same class need to access each other’s “guts” (e.g., for state copying - if we want to create an identical instance of an object we have)

• Example: from within a BankAccount object, any private member of a

different BankAccount object can be accessed.


Encapsulation - example

public void transfer(double amount, BankAccount targetAccount) { withdraw(amount); targetAccount.deposit(amount);}

// alternative version (valid, but not so nice)public void transfer(double amount, BankAccount targetAccount) { balance -= amount; targetAccount.balance += amount;}


Lesson 4 – Classes & Objects Unit D5 - API documentation


API Documentation

• Your classes are often intended to be used by other programmers

• Programmers that use your class are not interested in the way it is implemented. They want to use it as a whole and are only interested in what it does and how to use it.

• API (Application Programmer Interface) documentation is a description of the interface of the class intended for the application programmer who wants to use it.

• To use the class, we need not (and should not) look at the code. All that is needed is the class API.


API Documentation

• The JDK contains a special tool for the generation of API documentation for your classes, called javadoc.

• Any documentation which is part of the interface begins with /** (double asterick) and ends with */

• javadoc takes as input Java programs and automatically generates documentation using: the public/protected method signatures the documentation comments (enclosed by /** … */).

• The output is an HTML file which can be viewed by an internet browser.


Clock API Documentation

/** * A clock represents a point of time in a 12 * hour period within a precision of seconds. * Its range: 1:00:00 -- 12:59:59. */public class Clock { private int hours; private int minutes; private int seconds; /** * Constructs a Clock: Sets the clock to the * specified time. */ public Clock(int hours, int minutes, int seconds){ //… }


Clock API Documentation – cont. /** * Constructs a Clock: Sets the clock to 12:00:00 */ public Clock(){ this(12,0,0); }

/** * Advances this clock by 1 second. */ public void secondElapsed() { //… }

//…


Javadoc Process

.java file .html filejavadoc

View using a

browser


Clock javadoc - page 1


Clock javadoc - page 2


What should you comment?

• You should put a documentation comment for the class itself and for any member of the class which is part of its interface.

• All public constructors and methods should have documentation comments.

• Private methods are not part of the interface of the class, thus javadoc skips them. (But you should still comment them for internal purposes.)

• In the rare cases that there are public fields, they should have documentation comments.


API Documentation

• Remember that documentation comments are written for programmers who use your class as a whole. They should describe only What the class does, How to use it.

• Documentation comments should not describe how a class is implemented.

• Documentation comments should be Short and descriptive, Written in a simple language (ENGLISH), Accurate.

• Assume that the reader doesn’t know anything about your class


API Documentation Tags

• Documentation comments can also include tagged paragraphs that give a standard way to document several features of the interface such as method parameters, return values, etc.

• A tagged paragraph begins with the symbol @ followed with a tag keywords. Tags: @see, @author, @version, @param, @return, @exception.

• Documentation comments text can include HTML tags.


@param tag

/** * Changes the current time to hour:minute:second * @param hours The new hour value. * @param minutes The new minutes value. * @param seconds The new seconds value. */ public void setTime(int hours, int minutes, int seconds) { this.hours = hours; this.minutes = minutes; this.seconds = seconds; }


setTime generated javadoc


@return

/** * Returns the current hour * @return The current hour (between 1 and 12). */ public void getHour() { return hours; }


getHour javadoc


Naming

• The names you use for your class and for its public methods are part of the class API.

• Good descriptive naming are crucial for a clear API.• General rules about naming:

Follow the Java conventions Use descriptive names Do not use abbreviations! Make names long enough, not unnecessary long Consist of words in English with no abbreviations Use a dictionary

• Read the style guidelines!


Changing the Implementation of a Class

• Encapsulation allows us to change an implementation of a class without affecting other parts of the program.

• Without encapsulation changes to implementation might “break” the program - many changes

• Why would we want to change the implementation? Different implementations have different tradeoffs (e.g., space

conservation, efficiency etc.)

• Example: a time instance in the day can be represented by the number of seconds since midnight

• Assume we want to change the implementation of the clock - what is involved in the process?


Example of new Clock implementation - page 1

/** * A clock represents a point of time in a 12 * hour period within a precision of seconds. * Its range: 01:00:00 -- 12:59:59. */public class Clock { private static final int SECONDS_IN_12H = 12*60*60; private int secondsFromZero; /** * Constructs a Clock: Sets the clock to the * specified time. */ public Clock(int hours, int minutes, int seconds){ secondsFromZero = ((hours%12) * 3600

+ minutes * 60 + seconds); }


Example of new Clock implementation - page 2 /** * Constructs a Clock: Sets the clock to 12:00:00 */ public Clock(){ secondsFromZero=0; }

/** * Advances this clock by 1 second. */ public void secondElapsed() {

secondsFromZero = (secondsFromZero+1) % SECONDS_IN_12H; }

//…


Example of new Clock implementation - page 3 /** * Returns the current hour * @return The current hour (between 1 and 12). */ public void getHour() { int h = secondsFromZero/3600; return h > 0 ? h : 12 ; } // all the others can be written in the same way


Lesson 4 – Classes & ObjectsUnit D6 - Applet Methods & Graphic Objects


Applet Methods

• We saw in earlier applets the use the of the paint method to draw the content of the of the applet on the screen.

• An applet has several other methods that perform specific duties.

• Because the applets are specifically designed to work with web browsers, some particular applet methods are designed around that concept.

public void paint(Graphics g)


Some Applet Methods

• Initializes the applet. Called just after the applet is loaded.

• Starts the applet. Called just after the applet is made active.

• Stops the applet. Called just after the applet is made inactive.

• Destroys the applet. Called when when the browser is exited.

public void init()

public void start()

public void stop()

public void destroy()


Init, start, stop, and destroy.

• The init method is executed once when the applet is first loaded, such as when the browser or the applet viewer initially view the applet. This is the place to to initialize the applet’s environment and

permanent data.

• The start/stop methods of an applet are called when the applet becomes active/inactive. For example, after we use a browser to initially load an applet start is called. When we leave the page, the applet becomes inactive and stop is called. If we return to that page the applet becomes active again and start is called.

initinit startstart stopstop destroydestroy


Objects with Graphic representation

• Many programs are graphic• Many objects in such programs correspond to “graphic

objects” on the screen – this is good programming practice.

• Such a graphic object will know how to draw itself on the screen


LineUp Applet


The Lineup programs

• The LineUp applet, shown in the previous slide shows 4 stick figures of different heights and colors.

• It uses a StickFigure class that represents a stick figure of a certain height, color, and location.

• The applet creates 4 StickFigure objects, of varying color and random height.

• The StickFigure objects are instantiated in the init method of the applet, so they are created once when the applet is loaded.

• The paint method of LineUp simply requests that the stick figures draw themselves.


StickFigure - 1import java.awt.*;public class StickFigure { private int baseX; private int baseY; private Color color; private int height; public StickFigure(int center, int bottom,

Color shade, int size) { baseX = center;

baseY = bottom; color = shade; height = size; }


StickFigure - 2 public void draw(Graphics page) { int top = baseY – height; page.setColor(color); //head page.drawOval(baseX-10,top,20,20); //trunk page.drawLine(baseX,top+20,baseX,baseY-30); page.drawLine(baseX,baseY-30,baseX-15,baseY); //legs page.drawLine(baseX,baseY-30,baseX+15,baseY); //arms page.drawLine(baseX,baseY-70,baseX-25,baseY-70); page.drawLine(baseX,baseY-70,baseX+20,baseY-85); }


LineUp - code

import java.applet.Applet;import java.awt.*;

public class LineUp extends Applet{ private final int APPLET_WIDTH = 400; private final int APPLET_HEIGHT = 150; private final int HEIGHT_MIN = 100; private final int VARIANCE = 30;

private StickFigure figure1, figure2, figure3, figure4;


LineUp - code public void init () { int h1, h2, h3, h4; // heights of figures

h1 = (int)(Math.random() * VARIANCE) + HEIGHT_MIN; h2 = (int)(Math.random() * VARIANCE) + HEIGHT_MIN; h3 = (int)(Math.random() * VARIANCE) + HEIGHT_MIN; h4 = (int)(Math.random() * VARIANCE) + HEIGHT_MIN;

figure1 = new StickFigure(100,150,Color.red,h1); figure2 = new StickFigure(150,150,Color.cyan,h2); figure3 = new StickFigure(200,150,Color.green,h3); figure4 = new StickFigure(250,150,Color.yellow,h4); setBackground(Color.black); setSize(APPLET_WIDTH, APPLET_HEIGHT); }


LineUp - code

public void paint (Graphics page){

figure1.draw(page); figure2.draw(page); figure3.draw(page); figure4.draw(page); }}


Lesson 4 – Classes & ObjectsUnit D7 - Static and Auxiliary Methods


The Static Modifier

• The static modifier can be applied to variables or methods

• It associates a variable or method with the class rather than an object

• Methods that are declared as static do not act upon any particular object. They just encapsulate a given task, a given algorithm.

• We can write a class that is a collection of static methods. Such a class isn’t meant to define new type of objects. It is just used as a library for utilities that are related in some way.


Example - a Math Class

/** * A library of mathematical methods. */public class Math { /** * Computes the trigonometric sine of an angle. */ public static double sin(double x) { … }

/** * Computes the logarithm of a given number. */ public static double log(double x) { … } // ...


Static Variables

• A variable that is declared static is associated with the class itself and not with an instance of it.

• Static variables are also called class variables.• We use static variables to store information that is not

associated with a given object, but is relevant to the class.• We have already seen such usage - constants of a class

(final static)


Static Variables - Examplepublic class BankAccount {

private long accountNumber; private double balance; private static int numberOfAccounts = 0;

public BankAccount() { this.accountNumber = ++numberOfAccounts; this.balance = 0; } public static int getNumberOfAccounts { return numberOfAccounts; }}


Static Methods and Instance Variables

• Static methods: cannot reference instance variables (e.g. - access instance

variable when an object doesn’t exist) can reference static variables or local variables (within the

method)

• this has no meaning inside a static method, thus its use inside a static method is not allowed.

• Instance methods can access static variables.


Division Into Methods

• Complicated tasks or tasks that occur often within a class should be wrapped in a method

• If you can clearly define the task, you should probably make it a method.

• This results in a readable and manageable code• This is very helpful when implementing algorithms, or

when we need “helper” methods in classes.


Division into methods – interest calculation

public class BankAccount { final static int DAYS_IN_YEAR = 365;

public void earnDailyInterest(int days){ double rate = 1 + interestRate()/DAYS_IN_YEAR; for(int j=0 ; j<days ; j++) balance *= rate; }

private double interestRate() { return (balance>100000) ? 0.06 : 0.05; }


Division into Methods – Printing primes

public class PrintPrimes { public static void main(String[] args) { for(j = 2 ; j < 1000 ; j++){ if (isPrime(j)) System.out.println(j); } } private static boolean isPrime(int n){ for(int j=2 ; j < n ; j++) if (n%j == 0) return false; return true; }}


Lesson 4 – Classes & Objects

Lesson 4 – Classes & Objects

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