IFI7184.DT – Lesson 5

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Review - Lesson 4Encapsulation

Anatomy of a Method

Anatomy of a Class

Creating Objects

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Writing methods

• The programs we’ve written used classes– defined in the Java standard class library

• Java.lang; java.math; java.util

– And were written on the public main method

• Now we will begin to design programs that – rely on classes that we write ourselves

• The class that contains – the main method is just the starting point of a program

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Encapsulation• But first you need to understand

– The principle of object oriented language concept depended on a few concepts

• One is Encapsulation

• It means – Packaging complex functionality into small pieces of

functions• You can see it as your application functions wrapped in a box

• Why – Hiding it complexity make it easier to use

• Easier o debug• Easier to add more code

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Encapsulation

• We’ve learn how to create your program by putting all your code in the main method – But when your application get larger It becomes

very difficult to manage – So instead you need to break your code into

individual classes• Grouping it functionality for your application

– When you do this you can for instance • Restrict access to one part of the application

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Java application

• A java application – consists of more than one class– The starting class is the main method– Then,

• you have all sort of supporting classes• Those are called customize methods

– Can be either encapsulate data or functionality, or both

– to understand better you need to understand • What is a Method Declaration, control flow

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Method Declarations

• A method declaration Specifies – the code that will be executed when the method

is invoked (called)• In object oriented vocabulary

– A method, as you know is a function or• A piece of code with a given name

– that can be called (invoked) in parts of an application. • A function in Java is a member of a class.

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Method Control Flow

• When you run an java application– First thing it looks is for

• The main method public static void main (String[] args)

– But a java application is grouped with many other• Customized Methods and classes

– They can be define by you or important from existing libraries

– When you customize your own method. • You are declaring a method in a class.

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Invoking Methods

• When a method is invoked (called), – The flow control jumps to the called method or

class and starts executing its code

• When complete, – The flow returns to the place where the method

was called and continues

• The invocation may or may not return a value, – depending on how the method is defined

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Method Header

private static void calc (int num1, int num2)

methodnamereturn

typeparameter list

The parameter list specifies the type and name of each parameter

The name of a parameter in the method declaration is called a formal parameter

Visibility Modifiers

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Visibility Modifiers

• In Java, we accomplish encapsulation through the appropriate use of visibility modifiers– A modifier is a Java reserved word that

specifies particular characteristics of a method or data

• Java has three visibility modifiers: – public, protected, and private

– But first we need to understand the anatomy of the method

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Visibility Modifiers• Public methods

– Are methods that are declared with public visibility

• They are available in the entire application. • They can be invoked/called by everyone and

everywhere in the application. – In Object oriented words

• Can be referenced anywhere

• A Public methods are also called service methods

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Visibility Modifiers• Private methods

– Is the opposite of public. Is only available in that class.• can be referenced only within that class

– Are method created simply to assist a service method

• They are not intended to be called by a client,

• So it should not be declared with public visibility

– Private methods are also called is called a support method

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Visibility Modifiers

• Additional visibility methods– Without a visibility modifier

• Have default visibility and • Can be referenced by any class in the same

package– Protected visibility

• Is an inherence. • Is available to current class and to any subclasses• But, we will not we will not cover in this course

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Method Header

• Next characteristics of a method is – if it is static or not.

• When you wont method to be static. – This means that

• The method is a class method or an instance of that class.

– It can be called direct from the class definition• If you don't know if to add static or not just

– try not to add static and see if you call it in the class or not.

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The return Statement• The return type.

• A return statement specifies the value that will be returned– return expression return result;

!!! The expression must conform to the return type

• The return type of a method

– indicates the type of value that the method sends back to the calling location

• void: means the method is not returning anything• int: means the method is returning an integer• char: means the method is returning a character

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Method Header

• In end of the method Header you should – open and close parenthesis ()

private static void getInput(){ method body }

– To name your method • use same rules assigned to variables

– In between parenthesis you add the received parameters or values

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Method Bodyprivate static int calc (int num1, int num2)

{ int sum = num1 + num2; char result = sum;

return result;}

The return expressionmust be consistent withthe return type

sum and resultare local data

They are created each time the method is called, and are destroyed when it finishes executing

Input parameters or arguments

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Passing parameters• In java variable are always passed by copy

– When a method is called, the actual parameters in the invocation are copied into the formal parameters in the method header

Int calc (int s1, int s2){ int sum = num1 + num2; return sum;}

The sum is = calc (s1, s2);

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Local Data• As we’ve seen, local variables can be declared

inside a method– The formal parameters of a method create automatic

local variables when the method is invoked

• When the method finishes, – all local variables are destroyed (including the formal

parameters)

• Keep in mind that instance variables, declared at the class level, exists as long as the object exists

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Anatomy of a Method

• Creating reusable code with Methods– Java program Wages

• Declaring Methods with arguments– Java program Aritemetic– Java program AddValues– Java program testMethod– Java program Calculator2

Anatomy of a Method

Declaring Methods with arguments

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Java program QuarterOfthMonth

• Let’s design two individual methods called – scanning and condition

• This method should receive a number of the month (between 1 and 12) and print out which quarter of the month the number of the month belongs to.

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//********************************************************************// ConditionIf.java Author: Sónia Sousa//// Demonstrates the use of if-then-else statement//********************************************************************public class ConditionIf {

public static void main(String[] args) {

Scanner scan = new Scanner(System.in);System.out.print("Enter a number of the month (between 1 and 12)");

int i = scan.nextInt();

// structure of a if-then-else statement and the condition codeif ( i>=1 && i<=3) {

System.out.println("You have enter " + i + ", this month belongs to the first quarter of the year");

}else if (i>=4 && i <=6) {

System.out.println("You have enter " + i + ", this month belongs to the second quarter of the year");

}else {

System.out.println("You have enter " + i + ", this month does not belongs to the first half of the year");

}}

}

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Java program SquareRoot

• Write a Java program that assign two int values (num1 and num2) and computes the square root of the sum of num1 and num2 and prints the result on the screen.– Customize it in a method called AddValues

• This method should receive the two values• Compute the square root • and return the results

– Call the method in the main method and print out the results

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Lesson 5 - OutlineEncapsulation

Anatomy of a Method

Anatomy of a Class

Creating Objects

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Sum up

• So far we’ve learn how to– build your own method

• We’ve Learn how to – encapsulate functionality inside of methods

• that are part of classes called from a definition of a class• That are called inside of the class

– a instance of the class or an object

• We‘ve seen, that– Parameter and values need to declared; and – Values can be returned

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Sum up

• Keep in mind, that – when the method finishes,

• all local variables are destroyed (including the formal parameters)

– instance variables, are declared at the class level,– exists as long as the object exists

• Now we are going to address – The anatomy of a class

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The anatomy of a class

• Complete applications typically consist of more than one class– The starting class contains the main void method– Other code is separated in different classes or

methods• Encapsulate data or functionality or both• It is up to you to decide how to separate• The goal is to create classes that you can reuse it

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The anatomy of a class

• We can take one of two views of an object:– internal

• the details of the variables and methods of the class that defines it

– external • the services that an object provides and how the

object interacts with the rest of the system

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Invoking Methods

• We've seen that once an object has been instantiated, – we can use the dot operator to invoke its methods

numChars = title.length()• A method may return a value,

– which can be used in an assignment or expression

• A method invocation can be thought of as asking an object to perform a service

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myMethod();

myMethodcompute

Method Control Flow• Method called in the same class

– You only need the method name

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doIt

helpMe

helpMe();

obj.doIt();

main

Method Control Flow• Method called as part of another class or object

Anatomy of a class

How to extract the code into separate classes

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Java program Calculator3

• Start by open the calculator2 application– Customize 3 more methods calcSub, calcDiv, calcMult and

uses a switch statement to see which case the user choose and print the results.

• We will separate into – two main external classes

• One called SimpleMath.java• Another called scanHelper.java

• Then call them in the main class– Change the name of SimpleMath class to MathHelper

• (using Refactor in eclipse)

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Anatomy of a class

• How Organize classes into packages– Until now we add every application in the default package

• Most classes in java applications are placed in packages or subfolder of the project

– It is very import to organize your classes into groups

– Create a new package name helpers• Move helper classes into helper package

– Use refactor -> move• Declare the new package in the main application

– Import helpers.MathHelper;– Import helpers.ScanHelper;

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Sum up

• Let’s now examine methods in more detail• A method declaration specifies the code that will be executed

– when the method is invoked (called)

• When a method is invoked,– the flow of control jumps to the method and executes its code

• When complete, – the flow returns to the place where the method was called and

continues

• The invocation may or may not return a value, depending on how the method is defined

Anatomy of a class

How to organize classes into packages

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Java program positiveNumber

• Write a external class called numbers that verifies whether a number is positive or not

• Write also a main method class that asks the user for a number (int) between -10 and 10 and inform the user if the number is positive or not.– Use scanHelper.java and numbers.java classes

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Lesson 5Encapsulation

Anatomy of a Method

Anatomy of a Class

Creating Objects

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Instantiate

• You can declare a method in Java application into 2 ways– Class method

• Called from the definition of the class– Instance method

• Called from the instance of the class or an object

• So far we passed the values as arguments– But to work with objects that has data in it we need to

instantiate– This is Important when we need to work with more

complex objects

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Instantiate

• Instantiation is creating a object instance– An object is an 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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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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Recall

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

• Objects data type is done when we instanciate– Can be used to represent real-world entities– For instance, an object

• represent a particular employee in a company– Each employee object

» handles specific information• related to that employee

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References

• An object reference variable holds the address of an object

– The object itself must be created separately

• Usually we use the new operator to create an object

• Note that a primitive variable

– contains the value itself,

– but an object variable contains the address of the object

"Steve Jobs"name1

num1 38

Instance Variables

Storing data in instance Variables

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Create a new classe (Olive)

• Declare 3 instance variables on the class Olive definition– Variables names are: name, flavor, oil

• When declaring a variable you need to assign a– Access modifier, data type and an name. You can also assign a value to itpublic String name ="Kalamata";public String flavor ="Grassy";public int oil = 3;

– Call the method in the main method and print out the resultsOlive name1 = new Olive();System.out.println(name1.name);

– Instance variables are known as fields and are available in the entire application

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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

Nameflavoroil

Print namePrint flavorPrint oil

State

behaviours

Main