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F21SFSoftware Engineering Foundations

2A first

Object-oriented program

Monica Farrow EM G30 email : M.Farrow@hw.ac.ukMaterial available on Vision

Dept ofComputerScience

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Topics

Developing a first object-oriented program, looking at Data types Instance variables Constructors Methods Arithmetic operators

UML class and sequence diagrams are introduced

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A first object-oriented program

We’d like a program to meet this specification:“For any car, we know how much fuel can be held in a full tank, and the average miles per gallon. We’d like to know how far a car should be able to travel on a full tank. British manufacturers provide the tank size in litres but fuel consumption in miles per gallon!”

Sample input Type of car = Ford Ka, Tank size in litres = 40, average miles per gallon = 33.6

Sample output

A Ford Ka can travel 295.68 miles on a full tank

Developing the program In this lecture we see how to design an object-oriented solution

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Object Classes

The idea of a class forms the basis of object-oriented programming in Java

We model real world objects in a computer program

Everything about one type of object is encapsulated in the one place – the class

For this program, a Ford Ka is an object of the Car class Another Car object might be a Volkswagen Sharan

Our program should work for any type of car.

We are aiming for a program to be Readable, Maintainable, well-designed, Robust Should not contain duplicate blocks of code

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Program structure

For a small problem, designing using objects makes a program much larger than it would otherwise be We are laying foundations for much larger

programs

The first time you see an object-oriented program, it looks quite complicated We’ll keep reinforcing class design so that you

absorb the ideas over time

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Classes in a typical program

Typically an application will have A main method, often in a separate class, to start

the application A class for each type of object. The class contains

attributes and functions, for one or more distinct objects of this type. E.g. Car, Person, Course

Possibly a class for each collection of objects (e.g. all cars in a garage, all staff in a company)

A ‘manager’ class. The main functionality of the program is handled here. This often uses a Graphical User Interface.

We show the classes and the associations between them using a UML class diagram

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Example UML class diagram

Classes are shown in rectangles The associations between the classes are shown by

lines The lines may be directional, showing which classes

know about the other classes E.g. Customer class knows about the Account class but not

the CustomerCollection CustomerCollection only knows about Customer objects

Account

Customer

MainClass

ProcessTransactions Account

Collection

CustomerCollection

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Object-oriented solution

With object-oriented design, we look at the problem and see what objects we have, what attributes the objects have and what operations need to be performed on the object

One obvious object in today’s program is a Car Attributes could be : the size of the tank, the

average mpg, the model Operations : find distance travelled on full tank.

i.e. Tank size converted to gallons and multiplied by avg mpg

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CRC Card for Car class

• We can show the Responsibilities of the class using a CRC card.

• CRC stands for Classes, Responsibilities and Collaborators (Collaborators covered later)

Return model details Calculate and return the distance that the

car can travel on a full tank

Maintain data about model, tank size and mgp

Responsibilities

Car

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Today’s program

Will consist of A class containing a main method

This is where the program starts Today it will also handle the main functionality of the

program It will simply create a Car object, obtain that car’s data

and display results

A Car class managing everything about cars. This is a typical class, although very small

MainClass Car

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Objects and classes

We’d like to produce a single template or class which enables us to hold car details and find distance information One Car class

This is the Source file that we write in java A text file named Car.java

Our program will use the Car class to create one or more Car objects, then find the distance information

E.g. Ford Ka, tank size 40 litres, mpg 33.6 E.g. Mercedes Benz E280, tank size 65 litres, mpg 22

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A class consists of

A name, which should start in upper case e.g. Car

Instance variables to describe the data about a particular object Eg. Model, tank size etc

A constructor, to create the object and assign values to the instance variables E.g. tankSize = 66

Methods, to perform operations on the data. These are similar to functions and subroutines in other languages E.g. find out how far the car can travel E.g. tell us what the tank size is

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Outline UML Class diagram

MainCar

main(..)Car

modeltank sizemanufacturer’s mpg create car with initial datareturn the model return the estimated distance

data

operations(doing thingswith the data)

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Outline UML sequence diagram

myCar:Car

main()

Create a car

Get the distance

Get the model of the car

Standard output

Use standard output to display results

the long rectanglerepresents the main method

objects

methods of a class

calling the methods (vertical order is important)

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Class outline

public class Classname {

//instance variables

//constructor(s)

//methods

}

So a Car class starts public class Car

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Instance variables

Instance variables descibe the data about a particular object. They have an access modifier, a data type, a name (identifier), and contain a value

e.g. private String model;

The access modifier is usually private, meaning that other classes can’t use the variable directly

The data type (e.g. String) describes the data (see next slide)

The identifier should be a meaningful name, starting lower case by convention

Declare instance variable all together at the start or the end of the class. I prefer the start, because you can see them easily

The value is stored later.

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Java data types

Java has a set of primitive data types including int for integers double for real numbers char for single characters boolean for true/false values

See http://download.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html for more details

There is a special String class for text, which provides many useful features

Other more complex objects can be represented by classes, such as the Car class

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Car class – instance variables

//model

private String model;

//tank size in litres

private int tankSize;

// miles covered per gallon

//(manufacturer's figures)

private double manfMPG;

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To create an object, use the class constructor

The constructor has the SAME NAME AND CASE as the class (starting with Upper Case)

The constructor should initialise the instance variables (data) either to suitable default values or from data passed to the constructor as

parameters

CONSTRUCTOR

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Creating an object

We will create objects today in the main method, using a statement that looks like this:

Car myCar = new Car (“Ford Ka”, 40, 33.6);

Data type

Variable name

Assign RHS value to LHS variable

Create a new object

Call the Car constructor

Provide the data values as parameters

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Executing this call to the Car constructor

The line below, in the main method, calls the Car constructor and provides the data for this car.Car myCar = new Car (“Ford Ka”, 40, 33.6);

The Car constructor, in the Car.java class, starts with a header like this:

public Car (String model, int tank, double mpg)

Means that any other class can use the constructor

Class name, starts upper case

Type and name used for parameters

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Car class - constructor

The constructor’s task is to create the object, usually assigning values to the instance variables.

The body of the constructor consists of statements

public Car(String model, int tank, double mpg){ //store values supplied in parameters into

//instance variablesthis.model = model;tankSize = tank;manfMPG = mpg;

}

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this

In the constructor We initialise the instance variables, often from a

parameter value

One of the parameters has the same name as an instance variable. We use this to refer to the instance variable

tankSize = tankInstance variable

parameter

this.model = modelInstance variable

parameter

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The program so far : MainCar class

public class MainCar { public static void main (String[] args) { //create a Car

Car myCar = new Car(“Ford Ka”, 40, 33.6);

} //end main method} //end class

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The program so far : Car class

public class Car { //instance variables

private String model; private int tankSize;private double manfMPG;//constructorpublic Car(String model, int tank,

double mpg) {

this.model = model; tankSize = tank; manfMPG = mpg;

}}

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Running the program so far

If we were to run the program as it is The program starts in the main method The first statement

declares a variable of type Car and with name myCar will assign a value to myCar, using = The value is a new Car object, created by executing

the Car constructor Control passes to the Car constructor Each statement in the Car constructor is executed in

turn, assigning values to the instance variables for an object named myCar

Control passes back to the next statement in the main method

At the moment there isn’t one!

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After using the constructor

myCar

model “Ford Ka”

tankSize 40

manfMPG 33.6

We have an object of the Car class called myCar.The instance variables have got values stored in them

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UML Sequence diagram

This diagram shows action starting from the black circle.

The arrow from the black circle represents the main method in the MainCar class being called.

The long rectangle represents the main method. The arrow from the main method shows the Car

constructor being called and a new Car object, called myCar, being created.

The sequence of events is shown starting at the top and moving downwards.

myCar:Car

main()

new Car(“Ka”,40,7.4)

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Using the Car class in the main method

We have created an object of class Car, using the constructor

Then we can use methods of the Car class

To be able to print the line A Ford Ka can travel 295.68 miles on a full tankwe need get the name of the model and the number of estimated miles. The rest of the text is the same for all cars

So the Car class must contain methods to provide this information

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Using the Car class in the main method

Assuming suitable methods exist in the Car class, the main method will look like this:

//create objectCar myCar = new Car ("Ford Ka", 40, 33.6);

//get model name and store in a local variable calle model

String model = myCar.getModel();

//get estimated distance and store in a local variable called distance

double distance = myCar.estimateDistance();

//print the details to standard outputSystem.out.println("A " + model + " can travel " + distance + " miles");

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Points to note on previous code

To call a method from a different class, attach the method name to an object with a period (full stop)

E.g. myCar.getModel() The method runs, using the values of the instance

variables that belong to ‘myCar’ as opposed to any other cars that might have been created

In the main method, we declare temporary local variables of the correct type to store data in. e.g. model, distance

We can concatenate (join) Strings and variable values using the + sign

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The getModel method in the Car class

Any parameters in brackets (none)

public String getModel() {

return model;}

Access modifier

method name, starts lowercase

Return statement

Return type

Curly brackets enclose method body

One of the instance variables

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Car class – methods to get info out

//Return modelpublic String getModel() {return model;

}

//estimate distance car can travelpublic double estimateDistance(){ //there are 0.22 gallons per litrereturn tankSize * manfMPG * 0.22;

}

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ARITHMETIC OPERATORS

Operators are:+: addition, -: subtraction, *: multiplication, /: division (different effect for integers/reals)%: remainder (only for integers, also called mod)++, +=: incrementation operators--, -=: decrementation operators

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What goes in methods

Methods make the program do things Methods can, and frequently do, call other

methods The code inside methods follows the basic

ideas for procedural programs: Sequence (simple statements including

calculations) Selection (branching) Repetition (loops)

Many methods provide an answer. i.e. they return a value

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Running a program with multiple classes

Compile the main method Any associated classes will also be compiled

Run the main method Each statement in the main method is executed

in turn, passing control to constructors and methods when necessary.

At the end of each method, control is passed back to the calling method.

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UML Sequence diagram

In the sequence diagram on the next page The sequence of events is shown starting at

the top and moving downwards The named rectangles represent objects. The empty rectangles represent the methods. The dotted vertical lines are ‘lifelines’ showing

how long the object is in existence for Here, until the end of the program

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A more detailed Sequence diagram

myCar:Car

main()

new Car(“Ka”,40,7.4)

distance=estimateDistance()

model = getModel()

Standard output

println(message incl model & distance)

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UML class diagram

In the UML class diagram on the next slide, each class is divided into 3 sections Class name at the top Instance variable name and type in the middle

section Note that in the diagram the type comes last, in java

you will see that the type comes before the name Methods, including constructors, in the final

section – again, return types are shown after the method name

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Class diagram

MainCar

main(..)Car

- model : String- tankSize : int- manfMPL : double + Car(model:String, tank:int, mpg:double)+ getModel() : String + estimateDistance() : double

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To Do Now

Read over the lecture and look at the code. If you want to install java and eclipse on

your own computer, do so. Or you can wait until after the first lab. Java SE JDK

http://www.oracle.com/technetwork/java/javase/downloads/index.html

Eclipse IDE for Java Developershttp://www.eclipse.org/downloads/

If you’re already familiar with another IDE for running java programs, you’re welcome to continue with it.

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Try running the programs

In the first lab or at home, follow the separate Lab 1 instruction handout, which gives you Experience with running a java program from

the command line Experience with using an ide such as eclipse