Ifi7184 lesson6

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IFI7184.DT – Lesson 6

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Lesson 6 - Outline

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Creating Objects

The while Statement

The do while Statement

The for Statement

The for each Statement

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Lesson 5Anatomy of a Method

Anatomy of a Class

Creating Objects

Instantiation

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Recall • Instantiation is creating a object instance

– This is Important to work with more complex objects

• So far we’ve learn to declare a methods – Class methods; or Instance methods.

• learn how to create complex objects– An object instance of a particular class

title = new String (”Let’s learn Java");

This calls the String constructor, which isa special method that sets up the object

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Recall

• We can define – Primitive data type or objects data type

• Objects data type is done when we instantiate

– This object contains a reference variable

• That holds the address of an object

– The object itself must be created separately

• Usually we use the new operator to create an object

"Steve Jobs"name1

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Create 2 classes (Main, Olive)• To create instance methods we will use 2 classes

– named Main and Olive• To create an instance method on the Olive class

– We will write a method named crush and print out “ouch”Public void crush()

• It becomes an instance method if we do not use static– Then in the main application

• We call the Olive classOlive x = new Olive();x.crush();

• To call a instance method we need first to create an instance of Olive class

– Create a new complex Object variable of Olive class

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Classes and Objects

• Recall from our overview of objects – that an object has state and behavior

State Variables (fields)

behaviour functions (methods)

that allow to change the state

System.out.println( “ouch!” );

Olive x ;

Class Olive

Class Main

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Instance Variables

Storing data in instance Variables

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Creating instance Variables

• An object has:

– state (attributes) • descriptive characteristics

– behaviors • what it can do (or what can be done to it)

OliveInstance

Name

num

Print name

Print num

State

behaviours

Main

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Constructor method

• A constructor is used to set up an object when it is initially created

• When you create your own custom class– That class has a constructor method

• Those are special method called when you create a instance of a class

• If you don’t do that the java compiler add a no argument constructor method for you

• A constructor starts with a – Public access modifier; and

– Has the same name as the class.2015

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Constructor method

• Create a custom constructor method– In the OliveInstance

• Create a constructor method that –accept no values and outputs olive name

“Constructor with ” + this.name–Accepts an value (int num) and pass a value

6 to the field defined in the main method

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public OliveInstance(){

System.out.println(this.name);}

public OliveInstance(int num) {this.num = num;

}

Constructor method(OliveInstance)

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package Olive;

public class Main {

public static void main(String[] args) {

Olive x= new Olive();

x.crush();

OliveInstance valuex= new OliveInstance(6);

System.out.println("the value add is: " + valuex.num);

}

}

Main method (Main)

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Examples of Classes

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Lesson 6 - Outline

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Creating Objects

The while Statement


The for Statement


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Bank Account Example

• Let’s look at bank Account example – that demonstrates the implementation details of classes

and methods

• We’ll represent a bank account by a class named Account

• It’s state can include the account number, – the current balance, and the name of the owner

• An account’s behaviors (or services) include– deposits and withdrawals, and adding interest

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Creating Objects• An object has:

– state (attributes) - bankAccount– AccountNumber

– Name

– Balance

• descriptive characteristics

– behaviors (what he can do)– Deposit

– Withraw

– Tranfer

• the ability to make deposits and withdrawals

• Note that the behavior of an object might change its state

BankAcount

AccounNumberNameBalance

DepositWithdrawTransfer

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Classes (Account, Transactions)

• On Account declare:– 3 instance variables

long acctNumber;double balance;String name = "Ted Murphy”;

• Write custom constructors– one that accept no values and outputs the name

“Hello ” + this.name

– Another that accepts two values acctNumber, balance

– And passes those values to the field I defined in the main method

• For acctNumber the value is 72354• For Balance the value is 102.56

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New behavior deposit• Create a method called deposit

– This method should returns the new balance.

– balance = balance + amount

– Output the new balance in the main method• acct1.name + “ balance after deposit: ” + acct1.deposit(25.85)

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public class Transactions {


Account acct1 = new Account(72354);

System.out.println( " final balance after deposit is: " + acct1.acctNumber);

System.out.println(acct1.name + "balance after deposit: " + acct1.deposit(25.85));

}}

continue

Main method (Transactions)

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public class Account {

long acctNumber;double balance;String name = "Ted Murphy";

public Account(){

System.out.print("hello " + this.name);}

public Account( long acctNumber) {

this.acctNumber = acctNumber;}

public double deposit(double amount){

// Deposits the specified amount into the account. Returns the new balance.

balance = balance + amount; return balance; }

}

Constructor and method(Account)

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New behavior Withdraw• Create a method called Withdraw

– This method should returns the new balance.

• balance = balance - amount;– Output the new balance.

acct1.name + " final balance after withdraw is : ” acct1.withdraw(25.85, 0.5)

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Classes

• An object is defined by a class– A class is the blueprint of an object

• The class uses methods

– to define the behaviors of the object

• The class contains the main method of a Java program

– represents the entire program

• A class represents a concept

• An object represents the embodiment of that concept

• Multiple objects can be created from the same class

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Class = Blueprint

• One blueprint to create several similar, but different, houses:

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Objects and Classes

Bank Account

A class(the concept)

John’s Bank AccountBalance: € 5,257

An object(the realization)

Bill’s Bank AccountBalance: € 1,245,069

Mary’s Bank AccountBalance: € 16,833

Multiple objectsfrom the same class

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Multiple objects

• Storing data in instance variables– Write 3 custom constructor for methods in

(Account) that • Accepts 3 values (name, acctNumber and balance)

• and passes the following values to the main field– acct1 = "Ted Murphy", 72354, 102.56– acct2 = "Jane Smith", 69713, 40.00– acct3 = "Edward Demsey", 93757, 759.32

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acct1 72354acctNumber

102.56balance

name "Ted Murphy"

acct2 69713acctNumber

40.00balance

name "Jane Smith"

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class Account// Sets up the account by defining its owner, account number, and initial balance.

public Account (String name, long account, double initial) { this.name = name; this.acctNumber = account; this.balance = initial; }

class Transations// Create some bank accounts and requests various services.

Account acct1 = new Account ("Ted Murphy", 72354, 102.56);

Storing data in instance variables

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Multiple objects

• Add 2 new scanner methods – to prompt user name and transfer amount

– scanningName(), scanningWithdraw(), scanningtransfer(),

– Output the values• Add a new method that calculates the final

balance– Output the name of the account and the final

balance

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New behavior Transfer• Create a method called tranfer

– Ask the use how much he wont to transfer – and offers the option to to transfer to his

account or to another account– This method should returns calculate a new

balance.• Transfer for your account ( balance = balance + amount)• Transfer to another account (balance = balance – amount)

• Output the new balance after transfer

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public class Account {

private double balance;public String name;private int option; // Sets up the account by defining its owner, account

number, // and initial balance.

public Account (String name, long acctNumber, double balance,

int option) { this.name = name; this.balance = balance; this.option = option; }

public double deposit(double amount){

// Deposits the specified amount into the account. Returns the new balance.

balance = balance + amount; return balance; } continue

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public double withdraw (double amount) { balance = balance - amount; return balance; }

public double transfer (double amount) { switch (option) {

case 1:balance = balance + amount;break;

default: balance = balance - amount;break;

}

return balance; }

}

continue

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import account.*;

public class Transations {


Account acct1 = new Account (Input.scanningName(), 72354, 102.56, Input.transferOption());

System.out.println( " final balance after deposit is: " + acct1.deposit(Input.scanningDesposit()));

System.out.println(acct1.name + " final balance after withdraw is : " + acct1.withdraw(Input.scanningWithdraw() ));

System.out.println(acct1.name + " final balance after transfer is : " + acct1.transfer(Input.scanningTransfer()));

}

continue

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Quick CheckHow do we express which Account object's balance is updated when a deposit is made?

Each account is referenced by an object reference variable:

Account myAcct = new Account(…);

and when a method is called, you call it through a particular object:

myAcct.deposit(50);

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Lesson 6 - Outline

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Creating Objects

Conditional and loops

The while Statement


The for Statement


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Conditionals and Loops

• It is time to review Java conditional and repetition statements

• We’ve learn already how to use conditional statements:– the switch statement

– the conditional operator

• We need to how to use repetition statements– the do loop

– the for loop

– The while statement

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Recall• The switch statement helps us to find

– The correct statement case and execute it• Starts by evaluating an expression (case value1 :)

• Then if the result matches (case value3 : execute)

switch ( expression ){ case value1 : statement-list1 case value2 : statement-list2 case value3 : statement-list3 case ...

}

switchandcaseare

reservedwords

If expressionmatches value2,control jumpsto here

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Recall• The conditional operator evaluates

– One of two expressions based on a boolean conditionif (val <= 10)

System.out.println("It is not greater than 10.");

else

System.out.println("It is greater than 10.");

• If the val <= 10 is true, – print("It is not greater than 10.")

• if it is false, – print("It is greater than 10.");

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Loops Statements

• Loops or Repetition statements – allow us to execute a statement multiple times

• Like conditional statements, (if, then, else)– Loops are controlled by boolean expressions

• Java has three kinds of repetition statements: while, do, and for loops

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The while Statement

• A while statement has the following syntax:while ( condition )

statement;

• If the condition is true, the statement is executed

• Then the condition is evaluated again, and if it is still true, the statement is executed again

• The statement is executed repeatedly until the condition becomes false

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Lesson 6 - Outline

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Creating Objects

The while Statement


The for Statement


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Logic of a while Loop

statement

truefalse

conditionevaluated

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The while Statement• An example of a while statement:

int count = 1;

while (count <= 5)

{

System.out.println (count);

count++;

}

• If the condition of a while loop is false initially, the statement is never executed

• Therefore, the body of a while loop will execute zero or more times

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The while Statement

Demonstrating a while loop

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Main class (WhileLoop) inside a package (loop)

• Create a Java application that…– Print a set of values entered by the user.

• In a decreasing order until it reach 0

• we need to use – A temporary variable or counter variable – A variable that save user valuesInt temp = 0; value

– print the results using a While loop with condition• Don’t forget to decrease the value number

Value --;

While(condition){ statement;}

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The while Statement

Demonstrating a while loop with a sentinel value

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Sentinel Values

• Let's practice this time using – A loop that can maintain a running sum

• For that we need to add a sentinel value – is a special input value that represents the end of input

• See WhileLoop2.java

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Main class (WhileLoop2) inside a package (loop)

• Create a Java application that…– Computes the average of a set of values entered by the user. And

prints the running sum as the numbers are entered. – You will also need to add a sentinel value to quit (o to quit)

• Start by – importing Scanner class;– Creating the variables and initialize them int sum = 0, value;– Create a while loop that

• Keeps scanning values from the user and print the sum those valuessum += value;"The sum so far is " + sum

• Until he wont’s to quit"Enter an integer (0 to quit): ”

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Continue

// sentinel value of 0 to terminate loop int sum = 0, value;

Scanner scan = new Scanner (System.in); System.out.print ("Enter an integer (0 to quit): "); value = scan.nextInt(); while (value !=0) {

sum += value; System.out.println ("The sum so far is " +

sum); System.out.print ("Enter an integer (0 to

quit): "); value = scan.nextInt();

}continue

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The while Statement

Demonstrates the use of a while loop for input validation

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Input Validation

• A loop can also be used for input validation– Helping making a program more robust– It's generally a good idea to verify that input is

valid (in whatever sense) when possible

• See WinPercentage.java

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Main class (WinPercentage) package (loop)

• Create a Java application that…– Ask for how many games you played and computes the percentage of

games won by the teacm.• Start by

– Creating the variables int NUM_GAMES, won; double ratio;– Ask from the user ”How many games your team played: “"Enter the number of games won (0 to " + NUM_GAMES + "): "

– Create a while loop condition that• Verify if the number is validwon < 0 || won > NUM_GAMES"Invalid input. Please reenter: ”

• Keep the loop running until user adds the correct number– Then calculate the percentage of games won by the team

ratio = (double)won / NUM_GAMES;– Print the results formatted in percentage

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package loops;// WinPercentage.java Author: Sónia Sousa// Demonstrates the use of a while loop for input validation.import java.text.NumberFormat;import java.util.Scanner;

public class WinPercentage{ // Computes the percentage of games won by a team. public static void main (String[] args) { int NUM_GAMES, won; double ratio;

Scanner scan = new Scanner (System.in); System.out.print ("How many games you played: "); NUM_GAMES = scan.nextInt(); System.out.print (”How many games you won (0 to " + NUM_GAMES + "): "); won = scan.nextInt();

continue

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continue while (won < 0 || won > NUM_GAMES) { System.out.print ("Invalid input. Please reenter: "); won = scan.nextInt(); } ratio = (double)won / NUM_GAMES; NumberFormat fmt = NumberFormat.getPercentInstance(); System.out.println (); System.out.println ("Winning percentage: " + fmt.format(ratio)); }}

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continue while (won < 0 || won > NUM_GAMES) { System.out.print ("Invalid input. Please reenter: "); won = scan.nextInt(); } ratio = (double)won / NUM_GAMES; NumberFormat fmt = NumberFormat.getPercentInstance(); System.out.println (); System.out.println ("Winning percentage: " + fmt.format(ratio)); }}

Sample RunHow many games you played: 12How many games you won (0 to 12): 20Invalid input. Please reenter: 13Invalid input. Please reenter: 12

Winning percentage: 100%

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Infinite Loops• The body of a while loop eventually must

make the condition false

• If not, it is called an infinite loop, which will execute until the user interrupts the program

• This is a common logical error

• You should always double check the logic of a program to ensure that your loops will terminate normally

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An example of an infinite loop:int count = 1;while (count <= 25){ System.out.println (count); count = count - 1;}

• This loop will continue executing until interrupted (Control-C) or until an underflow error occurs

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Lesson 6 - Outline

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Creating Objects

The while Statement

Nested statement


The for Statement


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Nested Loops

• Similar to nested if statements, loops can be nested as well

• That is, the body of a loop can contain another loop

• For each iteration of the outer loop, the inner loop iterates completely

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Quick CheckHow many times will the string "Here" be printed?

count1 = 1;while (count1 <= 3){ count2 = 1; while (count2 < 5) { System.out.println ("Here"); count2++; } count1++;}

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Quick CheckHow many times will the string "Here" be printed?

count1 = 1;while (count1 <= 3){ count2 = 1; while (count2 < 5) { System.out.println ("Here"); count2++; } count1++;}

3* 4 = 12

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Lesson 6 - Outline

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Creating Objects

The while Statement


The for Statement


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Logic of a do while Loop

true

conditionevaluated

statement

false

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Comparing while and do

statement

true false

conditionevaluated

The while Loop

true

conditionevaluated

statement

false

The do Loop

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The do Statement

• An example of a do loop:

• The body of a do loop executes at least once

• See ReverseNumber.java

int count = 0;do{ count++; System.out.println (count);} while (count < 5);

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The do Statement

Demonstrating a do loop

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Main class (doLoop) inside a package (loop)

• Use the WhileLoop.java application and – Change it to a do statement

• same as previous one but this time de condition goes in the end

do {statement;} while (condition)

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Continue

// WhileLoop.java Author: Sónia Sousa//// Demonstrates the use of a while loop//*************************************************************

while ( temp < value ) { System.out.println("The values are: "

+ value); value --;

}

continue

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Lesson 6 - Outline

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Creating Objects

The while Statement


The for Statement


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The for Statement

• A for statement has the following syntax:

for ( initialization ; condition ; increment ) statement;

The initializationis executed once

before the loop begins

The statement isexecuted until the

condition becomes false

The increment portion is executed at the end of each

iteration

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Logic of a for loop

statement

true

conditionevaluated

false

increment

initialization

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The for Statement

• A for loop is functionally equivalent to the following while loop structure:

initialization;while ( condition ){ statement; increment;}

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The for Statement

• The initialization section can be used to declare a variablefor (int count=1; count <= 5; count++) System.out.println (count);

• Like a while loop, the condition of a for loop is tested prior to executing the loop body

• Therefore, the body of a for loop will execute zero or more times

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The for Statement

• The increment section can perform any calculation:for (int num=100; num > 0; num -= 5)

System.out.println (num);

• A for loop is well suited for executing statements a specific number of times that can be calculated or determined in advance

• See ForLoop.java

• See Multiples.java

• See Stars.java

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The For Statement

Demonstrating a for loop using an array of Strings

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Main class (ForLoop) package (loop)

• Start by creating the object array – with the days of the week

static private String[] weekDays= {"Monday", "Tuesday”, "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"};

– Then print the results using a for iterate over an array

• For loop that iterate over an array to print the String[]for (int i = 0; i < weekDays.length; i++) {

System.out.println(weekDays[i]);}


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// ForLoop.java Author: Sónia Sousa

package loops;

public class repeatedLoop {

static private String[] weekDays= {"Monday", "Tuesday", "Wednesday", "Thursday", "Friday",

"Saturday", "Sunday"};


// Demonstrates the use of a For loopfor (int i = 0; i < weekDays.length; i++) {

System.out.println(weekDays[i]);}

}

}

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The For Statement

Demonstrates the use of a for loop to print a sequence of numbers.

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Main class (Multiples) package (loop)

• Create a java application that – Prints multiples of a user-specified number up to a

user-specified limit.• Start by

– Scan a positive number and the upper limit and print them

• Do a for loop that– Print a specific number of values per line of output

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//********************************************************************// Multiples.java Author: Lewis/Loftus//// Demonstrates the use of a for loop.//********************************************************************

import java.util.Scanner;

public class Multiples{ //----------------------------------------------------------------- // Prints multiples of a user-specified number up to a user- // specified limit. //----------------------------------------------------------------- public static void main (String[] args) { final int PER_LINE = 5; int value, limit, mult, count = 0;

Scanner scan = new Scanner (System.in);

System.out.print ("Enter a positive value: "); value = scan.nextInt();

continue

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continue

System.out.print ("Enter an upper limit: "); limit = scan.nextInt();

System.out.println (); System.out.println ("The multiples of " + value + " between " + value + " and " + limit + " (inclusive) are:");

for (mult = value; mult <= limit; mult += value) { System.out.print (mult + "\t");

// Print a specific number of values per line of output count++; if (count % PER_LINE == 0) System.out.println(); } }}

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continue

System.out.print ("Enter an upper limit: "); limit = scan.nextInt();

System.out.println (); System.out.println ("The multiples of " + value + " between " + value + " and " + limit + " (inclusive) are:");

for (mult = value; mult <= limit; mult += value) { System.out.print (mult + "\t");

// Print a specific number of values per line of output count++; if (count % PER_LINE == 0) System.out.println(); } }}

Sample RunEnter a positive value: 7Enter an upper limit: 400

The multiples of 7 between 7 and 400 (inclusive) are:7 14 21 28 3542 49 56 63 7077 84 91 98 105112 119 126 133 140147 154 161 168 175182 189 196 203 210217 224 231 238 245252 259 266 273 280287 294 301 308 315322 329 336 343 350357 364 371 378 385392 399

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The For Statement

Demonstrates the use of nested for loops.

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Main class (Stars) package (loop)

• Create a java application that – Prints a triangle shape using asterisk (star)

characters.• Start by

– Create and initialize MAX_ROWS variable to 10• Do a for loop that

– Print a specific number of values per line of output

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//********************************************************************// Stars.java Author: Lewis/Loftus//// Demonstrates the use of nested for loops.//********************************************************************

public class Stars{ //----------------------------------------------------------------- // Prints a triangle shape using asterisk (star) characters. //----------------------------------------------------------------- public static void main (String[] args) { final int MAX_ROWS = 10;

for (int row = 1; row <= MAX_ROWS; row++) { for (int star = 1; star <= row; star++) System.out.print ("*");

System.out.println(); } }}

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//********************************************************************// Stars.java Author: Lewis/Loftus//// Demonstrates the use of nested for loops.//********************************************************************

public class Stars{ //----------------------------------------------------------------- // Prints a triangle shape using asterisk (star) characters. //----------------------------------------------------------------- public static void main (String[] args) { final int MAX_ROWS = 10;

for (int row = 1; row <= MAX_ROWS; row++) { for (int star = 1; star <= row; star++) System.out.print ("*");

System.out.println(); } }}

Output*******************************************************

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Quick Check

• Write a code fragment that rolls a die 100 times and counts the number of times a 3 comes up.Die die = new Die();

int count = 0;

for (int num=1; num <= 100; num++)

if (die.roll() == 3)

count++;

Sytem.out.println (count);

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The for Statement• Each expression in the header of a for loop

is optional

• If the initialization is left out, no initialization is performed

• If the condition is left out, it is always considered to be true, and therefore creates an infinite loop

• If the increment is left out, no increment operation is performed

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Lesson 6 - Outline

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Creating Objects

The while Statement


The for Statement


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For-each Loops

• A variant of the for loop simplifies the repetitive processing of items in an iterator

• For example, suppose bookList is an ArrayList<Book> object

• The following loop will print each book:for (Book myBook : bookList)

System.out.println (myBook);

• This version of a for loop is often called a for-each loop

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Quick Check

• Write a for-each loop that prints all of the daysOfWeek using the String weekDays array list

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for ( data type ; variable; array that you loop) statement;

for (String daysOfWeek: weekDays) { System.out.println(weekDays[i]);

}

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Recall ForLoop.java

• Using a temporary variable or counter variable – Start by creating an array of stringsstatic private String[] weekDays= {"Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"};

– Then print the results using • For loop that iterate over an array to print the String[]

for (int i=0; i< weekDays.length; i++){System.out.println(weekDays[i]);}


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Anatomy of a Class

Additional examples

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Driver Programs

• A driver program drives the use of other, more interesting parts of a program

• Driver programs are often used to test other parts of the software

• The Transactions class contains a main method that drives the use of the Account class, exercising its services

• See Transactions.java

• See Account.java

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// Transactions.java Author: Lewis/Loftus// Demonstrates the creation and use of multiple Account objects.

public class Transactions{

// Creates some bank accounts and requests various services. public static void main (String[] args) { Account acct1 = new Account ("Ted Murphy", 72354, 102.56); Account acct2 = new Account ("Jane Smith", 69713, 40.00); Account acct3 = new Account ("Edward Demsey", 93757, 759.32);

acct1.deposit (25.85);

double smithBalance = acct2.deposit (500.00); System.out.println ("Smith balance after deposit: " + smithBalance);

continue

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continue

System.out.println ("Smith balance after withdrawal: " + acct2.withdraw (430.75, 1.50));

acct1.addInterest(); acct2.addInterest(); acct3.addInterest();

System.out.println (); System.out.println (acct1); System.out.println (acct2); System.out.println (acct3); }}

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System.out.println ("Smith balance after withdrawal: " + acct2.withdraw (430.75, 1.50));

acct1.addInterest(); acct2.addInterest(); acct3.addInterest();

System.out.println (); System.out.println (acct1); System.out.println (acct2); System.out.println (acct3); }}

OutputSmith balance after deposit: 540.0Smith balance after withdrawal: 107.55

72354 Ted Murphy $132.9069713 Jane Smith $111.5293757 Edward Demsey $785.90

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// Account.java Author: Lewis/Loftus//// Represents a bank account with basic services such as deposit// and withdraw.

import java.text.NumberFormat;

public class Account{ private final double RATE = 0.035; // interest rate of 3.5%

private long acctNumber; private double balance; private String name;

// Sets up the account by defining its owner, account number, // and initial balance. public Account (String owner, long account, double initial) { name = owner; acctNumber = account; balance = initial; }

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// Deposits the specified amount into the account. Returns the // new balance. public double deposit (double amount) { balance = balance + amount; return balance; }

// Withdraws the specified amount from the account and applies // the fee. Returns the new balance. public double withdraw (double amount, double fee) { balance = balance - amount - fee; return balance; }

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// Adds interest to the account and returns the new balance.

public double addInterest () { balance += (balance * RATE); return balance; }

// Returns the current balance of the account.

public double getBalance () { return balance; }

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

public String toString () { NumberFormat fmt = NumberFormat.getCurrencyInstance(); return (acctNumber + "\t" + name + "\t" + fmt.format(balance)); }}2015

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• There are some improvements that can be made to the Account class

• Formal getters and setters could have been defined for all data

• The design of some methods could also be more robust, such as verifying that the amount parameter to the withdraw method is positive

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Ifi7184 lesson6

Education

Transcript of Ifi7184 lesson6