chapter 4: defining classes 1 1. 1.writing our own classes 1.1 instance variables 1.2 methods 2.some...

Chapter 4: Defining Classes 1

1

1. Writing our own classes1.1 Instance Variables1.2 Methods

2. Some notions of OOP3. Passing and returning objects4. Recap

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programmers can: use pre-defined classes develop their own classes

a class library is a collection of pre-defined classes

the Java standard class library (or Java API - Application Programming Interface) defines many classes ex:

the System class the String class

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So far, our program had only 1 class with 1 method (main)

For large problems… the program becomes large and difficult to understand repetition of instructions, variables are dispersed…

Solution: decompose the problem in sub-problems easier to implement

into methods group related variables and methods into classes

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public class MyProgram {

}

public static void main (String[] args)

{

}

Class: a blueprint/model/pattern to create objects specified by:

state - descriptive characteristics behaviors - what it can do (or what can be done to it)

Object: a specific variable of a class (an instance)

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class objects

Date

Book

PlayingCard

Planet

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Class State Behavior

Date

library book

bank account

A class contains: data declarations

(state) method declarations

(behavior)

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int x, y;char ch;

Data declarations

Method declarations

data (state) day month year

methods (behavior): nextDay isWeekend isMyBirthday printDate …

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in general, we store each class in its own file

the program is now made of at least 2 files:1. the class definition file:

defines the data and methods of a class ex: DateFirstTry.java

2. the driver file: contains the main method declares and uses objects of the class controls the use of other parts of a program often used to test other parts of the program ex: DateFirstTryDemo.java

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public class Date{ private String month; private int day; private int year;

public Date nextDay () { … }

public boolean isWeekEnd () { … }

public void printDate() { ... }

}

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Date.java

Method declarations (behavior)

Data declarations (state)

public class DateFirstTryDemo{ public static void main(String[] args) {

Date date1, date2; date1 = new Date(); date2 = new Date();

if (date1.isWeekEnd()) System.out.print("yuppi");

date1.printDate( );

if (date2.isWeekEnd()) System.out.print("good");

}}

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DateFirstTryDemo.java

usage of the 2 objects

declaration of 2 objects

syntax:nameOfClass nameOfObject = new nameOfClass();

nameOfClass nameOfObject ;

nameOfObject = new nameOfClass();

ornameOfClass nameOfObject = new nameOfClass(parameter list);

nameOfClass nameOfObject;

nameOfObject = new nameOfClass(parameter list);

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Coin myCoin = new Coin();

String aString = new String("the content of the string");

BankAccount account123 = new BankAccount();BankAccount account555 = new BankAccount();

this calls a method called the constructor… will see later

The new operator:A. allocates memoryB. is used to create an object of a classC. associates an object with a variable that names

it.D. All of the above.

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variables/constants declared inside the class but not inside a specific method

also called attributes can be used by any method of the class initialized to 0 (false for booleans) each object (instance) of the class has its own instance data

Example:

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double bal;

class BankAccount

bal 0

ac1

bal 0

ac2

BankAccount ac1=new BankAccount(); BankAccount ac2=new BankAccount();

implement the behavior of all objects of a particular class

all objects of a class share the method definitions some methods are a bit special…

(ex: constructor, toString, equals…)

group of statements that are given a name easier to decompose the problem into sub-

problems that are easier to implement a method is defined once, but can be used

(called/invoked) several times

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method header + method body

syntax:

ex:

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method body

method header

{ statements of the method}

visibility static returnType methodName(listOfParameters)

optional

public void sayHi() { System.out.print("Hi");}

public static void main(String[] args) { …}

to access any members (data or method) within the class

nameOfData nameOfMethod(actualParameters)

from outside the class non-static:

nameOfObject.nameOfData nameOfObject.nameOfMethod(actualParameters)

static: nameOfClass.nameOfData nameOfClass.nameOfMethod(actualParameters)

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public class Coin{ private final int HEADS = 0; private final int TAILS = 1; private int face;

public void flip() { _______= (int)(Math.random()*2); }

public boolean isHeads() { return (_______ == HEADS); }

// flips the coin 5 times public void flip5() { for (int i=1; i<=5; i++)

___________________ }…

}

18class def.


public void flip() { _______= (int)(Math.random()*2); }

public boolean isHeads() { return (_________ == HEADS); }

// flips the coin 5 times public void flip5() { for (int i=1; i<=5; i++)

___________________ }…}

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class def.

public class FlipRace{ public static void main (String[] args) { final int GOAL = 3; int count1 = 0, count2 = 0;

// Create two separate coin objects Coin coin1 = new Coin(); Coin coin2 = new Coin();

// Flip the coins and count heads while (count1<GOAL && count2<GOAL) { ____________________________;

____________________________;

if (coin1.isHeads()) count1++; if (coin2.isHeads()) count2++; } flip(); // 1. ok? coin1.flip; // 2. ok? … }}

driver

When a method is invoked:1. the current method is suspended2. the called method is executed3. when completed, the flow returns to the place where the

method was called and resumes its execution

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doIt

helpMe

helpMe();

obj.doIt();

main

Car.javadriver

Car obj;obj = new Car();

There are two kinds of methods: Methods that compute and return a value

Methods that perform an action does not return a value is called a void method

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similar to a math function must specify the type of that value in its heading: public typeReturned methodName(paramList)

Examples: description: determines if the coin is a tail (1==tail,

0==heads) name: isTail result: boolean

description: returns the value of the face name: getFace result: int

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public boolean isTail(){ if (face == 1) return true; else return false;}

public int getFace() { return (face);}

allows the method to "return" a result syntax: return expression;

1. the expression is evaluated2. the value of the expression is returned as the result of the method

3. the method is exited

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public boolean isTail(){ if (face == 1) return true; else return false;}

public boolean isTail(){ return (________________________);}

public int getFace() { return (face);}

Another way of writing the method isTail() is …. ?

they perform an action performed, but do not “evaluate” to a value ex: they display something, change the value of an attribute, …

they officially return void they use no return expression;

either: return; or nothing…

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public void flip() { face = (int)(Math.random()*2);}

public void flip() { face = (int)(Math.random()*2); return;}

public ___ printFace() { System.out.print(face);}

methods that return a result are expressions that have: a type and a value

methods that do not return a result are not expressions you call them with the statement: objectName.methodName();

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public class Driver { public static void main (String[] args) { Coin coin1 = new Coin(); boolean result; coin1.flip(); // 1. ok? result = coin1.flip(); // 2. ok? if (coin1.flip()) // 3. ok? System.out.print("whatever"); System.out.print(coin1.flip()); // 4. ok? }}

driver

Which one of these statements is syntactically correct?Coin coin1 = new Coin();boolean result;1. coin1.isTail(); 2. result = coin1.isTail(); 3. if (coin1.isTail())

System.out.print("whatever"); 4. System.out.print(coin1.isTail());

A. All are syntactically correctB. 2. and 3. only are syntactically correctC. 2., 3. and 4. only are syntactically correctD. 1. is the only one which is syntactically correctE. 3. is the only one which is syntactically correct

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The body of a method that returns a value must contain at least one _________ statement.

A. voidB. invocation statementC. declarationD. return

Local variables: declared inside a method

variable that is necessary for that method only method parameters are considered local

variables

If 2 methods have a local variable of the same name, they are 2 entirely different variables

Global variables: Java does not have global variables

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Some methods need to receive additional data in order to perform their work allows the function to be more generic

ex. sqrt method works for any doubleMath.sqrt(9), Math.sqrt(15.5), Math.sqrt(75),...

Definitions: formal parameter:

parameter specified in the method header argument or actual/run-time parameter:

parameter specified in the method call / invocation

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public class Account

{

private double balance;

public void deposit (double theAmount)

{

if (theAmount < 0) // deposit value is negative

System.out.println ("Error: Deposit amount is invalid.");

else

balance = balance + theAmount;

}

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public class Banking{ public static void main (String[] args) { Account acct1 = new Account(); acct1.deposit(25.85); acct1.deposit(10); }}

class def.

driver

public class Store{ public void printPrice(String itemName, double p) { System.out.println (“The " + itemName + " costs " + p); }}

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public static void main (String[] args){ Store myStore = new Store(); myStore.printPrice("desk", 300.99);

double special = 195.99; myStore.printPrice("chair", special);

Store yourStore = new Store(); double p = Keyboard.nextDouble(); String s = "desk"; yourStore.printPrice(s, Math.min(100, p));}

class def.

driver

The desk costs 300.99

The chair costs 195.99

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The desk costs 99.0

output

in the class Account…

write the header of the methods: withdraw: getInterestRate: changeName:

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public class Account{ private long acctNumber; private double balance; private String name; private double interestRate; …

public class AccountTest { public static void main(String[] args) { Account A1 = new Account(); A1.widthraw(123456, 5000.0); A1.changeName(“NewName”);

A1.getInterestRate(); } }

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In a driver class… write instructions to create an account and call the previous methods

A variable whose meaning is confined to an object of a class is called:

A. instance variableB. local variableC. global variableD. none of the above

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A variable whose meaning is confined to a method definition is called an/a

A. instance variableB. local variableC. global variableD. none of the above

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when a method is called: the actual parameters are copied into the formal

parameters the method works with a copy of the actual parameters … when we return from the function, the actual

parameters are unchanged

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public void someMethod(int num1, int num2, String message)

{ …}

someMethod(25, count, "Hello");

public class SomeClass{ private double someAttribute;

public void someMethod(int someParam) { System.out.println(someParam); if (someParam < 0) someParam = 0; someAttribute = someParam;

System.out.println(someParam); }}

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public class SomeDriver{ public static void main(String[] args) { SomeClass someObj=new SomeClass(); int someVar = -100;

System.out.println(someVar); someObj.someMethod(someVar); System.out.println(someVar); }}

class def.

driver

-100-1000-100

output

when a method is called: the order of the actual param. must be == to the order of the formal

param. the nb of actual param. must be == to the nb of formal param. the types of the actual param. must be compatible with the types of

the formal param.

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public class Test{ … public void aMethod(String a, long b, double c, char d, boolean e) {…}…

public class Driver {Test myTest = new Test();

myTest.aMethod("hello", 10, 15.5, 'a', 'a' == 'b'); // 1. OK? Yes myTest.aMethod("hello", 10.5, 15, 'a', true); // 2. OK? ===> No because of 10.5 is double…

class def.

driver

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public class Driver {Test myTest = new Test();myTest.aMethod("hello", 10, 15);// invalid : ("hello“, 10, 15, 0, false)

…

class def.

driver

Given the following class definition

The following statement in the driver is

A. ValidB. Invalid, because number of actual & formal

parameters don’t matchC. Invalid, because the type of actual & formal

parameters don’t match

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public class Driver {Test myTest = new Test();myTest.aMethod("hello", 10, 15, 67, true);

…

class def.

driver






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public class Driver {Test myTest = new Test();myTest.aMethod("hello", 10, 15, (char)67, true);

…

class def.

driver






the types of the actual param. must be compatible with the types of the corresponding formal param.

public double myMethod(int p1, int p2, double p3) {…}…int a=1,b=2,c=3;double result = myMethod(a,b,c);

If no exact type match --> automatic type conversion c is casted to a double remember: byteshortintlongfloatdouble char

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All instance variables are understood to have <the calling object>. in front of them

inside a method: myInstanceVariable is identical to this.myInstanceVariable

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public void deposit(int amount){ balance += amount; this.balance += amount;}

this must be used: if a parameter or other local variable has the

same name as an instance variable otherwise, the local variable will be used

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public class Account { private double balance;

public void initialize(double balance) { this.balance = balance; }} localinstance

when an entire object is printed…

the toString method is automatically called if you have not defined a toString method, then

otherwise, define your own version of toString for your object

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public class Banking{ public static void main (String[] args) { Account acct1 = new Account("Ted", 123, 100.0);

acct1.deposit (25.55);

System.out.println(acct1); }}

Account@182f0db

driver

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Defining your own toString()

public class Account{ private long acctNumber; private double balance; private String name;

public Account(String owner, long account, double initial) { … }

// Returns a one-line description of the account as a string.

public String toString(){ return (acctNumber + "\t" + name + "\t" + balance); }…

toString() must: take no parameter return a string object

class def.123 Ted 125.55

to compare two objects, you can use: == operator

compares equality of references returns true:

if the references point to the same object NOT if the objects pointed have the same content

equals() method compares equality of the content of the objects returns true:

if the references point objects that have the same content

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if (myAccount == yourAccount) System.out.print("same object");

if (myAccount.equals(yourAccount)) System.out.print("same content");

The method equals : is defined for all objects unless we redefine it in our class, it has the same semantics as

== we can redefine it to return true under whatever conditions we

think are appropriate (ex. equality of content, not address)

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public class Account { private String name; private double balance; private int acctNumber;

boolean equals(Account anotherAccount) {

}…

EqualsAndToStringDemo.java

DateFourthTry.java

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CONSTRUCTORS Assume:

to initialize an object, we can do:

public class BankAccount{ private String name; private long acctNumber; private double balance; public void initialize(String theOwner, long theAccount, double theInitial) { name = theOwner; acctNumber = theAccount; balance = theInitial; }… BankAccount.java

BankAccount account123 = new BankAccount();account123.initialize("ted", 123, 100);

BankAccount account555 = new BankAccount();account123.initialize("mary", 555, 300);

driver

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a constructor is a special method that: is called automatically called when an object of the class is

declared is usually used to initialize the data of an object

must have the same name as the class name has no return type (not even void) must be public

can have parameters

If you do not define any constructor in your class, a default constructor that initializes instance variables to 0

BankAccount account123 = new BankAccount("ted", 123, 100);

BankAccount account555 = new BankAccount("mary", 555, 300);

public class BankAccount{ private long acctNumber; private double balance; private String name; // a constructor:

public BankAccountBankAccount(String theOwner, long theAccount, double theInitial)

{ name = theOwner; acctNumber = theAccount; balance = theInitial; }

public void deposit(double amount) { … } public void withdraw(double amount) { … } …}

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Example 1

BankAccount.java


public Coin() { flip(); }

public void flip() { face = (int)(Math.random()*2); }

public boolean isHeads() { return (face == HEADS); }…}

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Example 2public class CountFlips{ public static void main (String[]

args) { Coin myCoin = new Coin(); Coin anotherCoin = new Coin(); Coin athirdCoin = new Coin();

… }}

Coin.java

driver

Date.java

ConstructorsDemo.java

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If you do not include any constructors in your class, Java will automatically create a default or no-argument

constructor

The default constructor: takes no arguments initializes all instance variables to zero (null or false) but allows the object to be created

If you include even one constructor in your class Java will not provide this default constructor

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overloading = same name is used to refer to different things "chair" (person or furniture) / (integer or real division)

overloaded methods: several methods have the same name but different definitions

the signature of each overloaded method must be unique signature = name of method + parameter list the compiler determines which version of the method is being

invoked by analyzing the signature the return type of the method is not part of the signature

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public int increment(int x){ return x+1;}

Version 1public int increment(int x, int value){ return x+value;}

Version 2

int result = 1;

result = increment(20, 4); // 1. ok?

result = increment(10); // 2. ok?

result = increment(result); // 3. ok?

result = increment(result, result); // 4. ok?

Invocations

60



Version 2

int result = 1;

result = increment(result, result, result);

Invocations

A. ValidB. Invalid, wrong number of argumentsC. Invalid, wrong type of arguments

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Version 2

int result = 1;

result = increment(result, increment(result, result));

Invocations

A. ValidB. Invalid, wrong number of arguments,C. Invalid, wrong type of arguments.

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Version 2

int result = 1;

result = increment();

Invocations

A. ValidB. Invalid, wrong number of arguments,C. Invalid, wrong type of arguments.

guess what… the println method is overloaded!

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println(String s) {…}println(int i) {…}println(double d) {…}println(char c) {…}…

System.out.println(total);

System.out.println("The total is:");…

practical when the same operation must be done on different types or different numbers of parameters

64

public class Calculator{ public int addem(int op1, int op2) { return op1+op2; } public int addem(int op1, int op2, int op3) { return op1+op2+op3; }

public int opposite(int op) { return -op; } public boolean opposite(boolean op) { return !op; } public char opposite(char op) { if (Character.isUpperCase(op)) return Character.toLowerCase(op); return Character.toUpperCase(op); }}

Calculator myCalc = new Calculator();

System.out.print(myCalc.addem(10,20));

System.out.print(myCalc.addem(10,2,5));

System.out.print(myCalc.opposite(10));

System.out.print(myCalc.opposite('a'=='b'));

System.out.print(myCalc.opposite('a'));

Calculator.java

driver

Constructors are methods… so they can be overloaded

65

public class Account{ private long acctNumber; private double balance; private String name;

public Account(String owner, long nb, double init) { name = owner; acctNumber = nb; balance = init; } public Account(String owner, long nb) { name = owner; acctNumber = nb; balance = 0; } public Account(long nb) { name = ""; acctNumber = nb; balance = 0; }…

public class Banking{ public static void main (String[] args) { Account acct1 = new Account("Jane", 456, 400); // 1. ok? Account acct2 = new Account("Ted", 123); // 2. ok? Account acct3 = new Account("Ted"); // 3. ok? Account acct4 = new Account(); // 4. ok? }}

Banking.javaAccount.java

Information hiding separating how to use a class from how it is implemented Abstraction is another term used

Encapsulation the data and methods of a class are combined into a single

unit (i.e., a class object), which hides the implementation details

Knowing the details is unnecessary because interaction with the object occurs via a well-defined and simple interface

In Java, hiding details is done by marking them private

Interface an object interacts with the rest of the program via an

interface interface = set of methods that allow access to the object the interface hides how a method is implemented

66

An encapsulated object can be thought of as a black box its inner workings are hidden to the client the client can access the object only by invoking the methods

of the interface

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Client Public Methods

Data

(Interface)Private Methods

Java has 4 visibility modifiers: public private protected (involves inheritance, see COMP 249) default or package (COMP 249)

can be applied to all members (data and methods)

68

public members can be directly accessed from anywhere (inside &

outside the object) violate encapsulation

private members can only be accessed from inside the class definition

by default… members can be accessed by any class in the same

package i.e. access is more open than private, but more strict

than public

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Data: should be private public data violate encapsulation constants are OK to be public because they cannot be

modified anyways

Methods: should be public

if the method provides the object's services so that it can be invoked by clients also called service method

should be private if the method is created simply to assist a service method also called a support method

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71

public private

Variables

Methods

NO!Violates

encapsulation

YES!Enforces

encapsulation

Yes, if methodprovides services

to clients

Yes, if methodsupports othermethods in the

class

public class Account { private double rate; private long acctNumber; private double balance; private String name;

public Account(String owner, long nb, double init) {

name = owner; acctNumber = nb; // 1. OK ? balance = init; rate = 0.02; } public double deposit(double amount) { if (amount < 0) // deposit is negative System.out.println("Error"); else balance += amount; //2.OK ? return balance; } private void changeRate(double newRate) {

rate = newRate; }…

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public class Banking{ public static void main (String[] args) { Account acct1; double tedBal;

acct1 = new Account("Ted", 72, 102.56);

acct1.deposit(25.85); // 3. OK ?

acct1.balance = 0; // 4. OK ?

tedBal = acct1.deposit(500.00); //5.OK? acct1.changeRate(5.5); // 6. OK? }}

Account.java driver

public class Printer{ private final int DEFAULT_NB = 10; public void printMany(int nbTimes, String theMessage) {

if (nbTimes < 0 || nbTimes > 100)internalUseOnly(DEFAULT_NB, theMessage);

else internalUseOnly(nbTimes, theMessage); }

private void internalUseOnly(int nbTimes, String AMess) {

for (int i = 1; i <= nbTimes; i++) System.out.println(AMess); }}

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public class PrinterDriver{ public static void main (String[] args) { Printer myPrinter = new Printer();

myPrinter.printMany(5, "hello"); // 1. ok? System.out.println(myPrinter.DEFAULT_NB); // 2. ok? myPrinter.internalUseOnly(5, "hello"); // 3. ok? }}

class def.

driver

data members are usually private so to access them, we usually have set and get methods

mutator (setX): sets the data X and makes sure it stays in a coherent state

accessor (getX): returns the value of data X

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public class Time { private int hour; private int minutes; private int seconds;

public void setHour(_____________) { ________________________________;

}

public void setMinute(___________) { ______________________________;

}

public void setSecond(___________) { _______________________________

}

public int getHour(_______________) {_______________;

} public int getMinute(_______________){

_______________; } public int getSecond(_______________){

_______________; } // constructor public Time(int h, int m, int s) {

________________________;________________________;________________________;

}}

why should the set and get methods be public?

if a mutator is given a value that would make the object in an invalid state, it can:

1. display an error message, and quit the program, OR2. return a boolean (false), and let the calling method

decide what to do

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public setHour(int newHour) {

}

public void setHour(int newHour) {

if (newHour > 24 || newHour < 0){ System.out.println("Error.");

System.exit(0); } else hour = newHour;}

an object can be a parameter to a method ex:

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public void printMany(int nbTimes, String AMess) {…}

public int compareTo(String AMess) {…}

String originalString = "hello";int result = originalString.compareTo("bye bye");

String class

MyPrinters class

MyPrinter p = new MyPrinter();p.printMany(10, "hello");

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public Time(int h, int m, int s) {…} public void setHour(int h) {…} … public int getSecond(){…}

// method to check if a time < another time

________________________________________

{

if (hour < anotherTime.getHour())

return true;

else if (hour > anotherTime.getHour())

return false;

else if (minutes < anotherTime.getMinute())

return true;

else if (minutes > anotherTime.getMinute())

return false;

return (seconds < anotherTime.getSecond());

}

…

public class TimeDriver{ public static void main (String[] args) {

// declare 2 time objects ________________________________;

________________________________;

// check if time1 < time2 if (_________________________________)

System.out.println("ok"); }}

Time.java

driver

an object can be returned by a method ex:

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public String toString() {…}

public String replace(char oldChar, char newChar) {…}

String myString = "hello";String newString = myString.replace('l', 'm');

String class

MyPrinters class

MyPrinters p = new MyPrinters();System.out.print(p.toString()); System.out.print(p);

79


…

// add a time to the current time

public ____________ add(________________)

{

Time result = new Time(0,0,0);

result.hour = hour + t2.getHour(); result.minute = minutes + t2.getMinute(); result.second = seconds + t2.getSecond(); if (result.second >= 60) { result.second -= 60; result.minute += 1; } if (result.minute >= 60) { result.minute -= 60; result.hour += 1; } return result;

}

…

public class TimeDriver{ public static void main (String[] args) { Time t1 = new Time(9,10,50); Time t2 = new Time(19,5,5); Time t3 = new Time(0,0,0);

// t3 is the sum of t1 and t2

_______________________________ ; }}

Time.java

driver

to define a class: data members (attributes)

declared outside any method must decide:

visibility (private? public?)

methods must decide:

visibility (private? public?) type of result (void?, int?, boolean?…) number and types of parameters actual code of the method static or not? (so far… never static)

2 files: the class definition (ex. Coin.java, BankAccount.java) the driver program (ex. CoinFlip.java, Banking.java)

80

A set method is:

A. an accessor methodB. a mutator methodC. a recursive methodD. none of the above

81

Accessor methods:

A. return something equivalent to the value of an instance variable.

B. promotes abstractionC. both A and BD. none of the above

82

The parameter this refers to

A. instance variablesB. local variablesC. global variablesD. the calling object

83

The name of a method and the list of ________ types in the heading of the method definition is called the method signature.

A. parameterB. argumentC. returnD. primitive

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85

name type visibility public class Rational { // data members:

numerator

denominatorname return

typeparameters

visibility

// methods headers

Rational {…}

getNumer {…}

setNumer {…}

reciprocal {…}

add {…}

toString {…}

reduce {…}

}

chapter 4: defining classes 1 1. 1.writing our own classes 1.1 instance variables 1.2 methods 2.some...

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