Software Engineering Foundations

1
Software Engineering Foundations

• 2
• A first
• Object-oriented program

Monica Farrow EM G30 email
monica_at_macs.hw.ac.uk Material available on Vision
and my website
2
Topics this lecture

• A definition of UML
• Developing a first object-oriented program,
  looking at
• Data types
• Instance variables
• Constructors
• Methods
• Parameters
• Scope

3
What is (the) UML?

• Unified Modeling Language
• A language for specifying, visualising and
  constructing the descriptive diagrams and
  documentation of software system
• A family of mainly graphical notations
• a picture is worth 1000 words
• Helps in designing and describing software
  systems, particularly object-oriented systems
• Industry standard for object-oriented modelling
• This module includes an introduction to UML
• Initially UML diagrams will be used occasionally
  where appropriate

4
Websites

• There is also a lot of UML material on the web
• I recommend
• http//bdn.borland.com/article/0,1410,31863,00.htm
  l
• http//www.agilemodeling.com/essays/umlDiagrams.ht
  m

5
A first object-oriented program

• Wed like a program to meet this
  specificationFor any car, we know how much
  fuel can be held in a full tank, and the average
  miles per gallon. Wed like to know how far a car
  might travel on a full tank. British
  manufacturers provide the tank size in litres but
  fuel consumption in miles per gallon!
• Sample output
• A Ford Ka can travel 296 miles on a full tank

6
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
• Because a good program must be
• Readable, Maintainable, well-designed, Robust
• Should not contain duplicate code

7
Program structure

• For a small problem, using objects makes the
  program much larger than they would otherwise be
• We are laying foundations for much larger
  programs
• The first time you see a class, it looks quite
  complicated
• Well keep reinforcing class design so that the
  ideas sink in over time

8
A Program may contain these classes

• A class containing the main method
• Essential. The program starts here. We will use
  this in a class by itself. It could be used
  within an object class.
• Object classes that
• Either we write ourselves e.g. Person, Car, Name
• Or are supplied as part of the java API e.g. for
  collections, for GUIs, for random numbers, etc.
• Static classes (a later lecture)
• These are independent of an individual object
  E.g. Math a collection of useful functions
  (rounding, squaring..)

9
Example class diagram

• Typically a program will have
• A main method alone in a class to start the
  application
• A class which handles the main functions of the
  program, often interacting through a GUI or text
  interface
• Classes for distinct objects
• Classes for collections of similar objects

MainClass
Customer
Customer Collection
Process Transactions
Account
Account Collection
10
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 here 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

11
Todays program

• Will consist of
• a class containing a main method
• This is where the program starts
• It will simply create a Car object and obtain
  that cars data to display results
• A Car class containing everything about cars.
  This is a typical class, although very small

12
Objects and classes

• Wed 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
• And one or more Car objects
• e.g. Ford Ka, tank size 40 litres, mpg 33.6
• E.g. Mercedes Benz E280, tank size 65 litres, mpg
  22

13
A class consists of

• 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.
  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

14
Class outline

• public class Classname
• //instance variables
• //constructor(s)
• //methods
• So a Car class starts
• public class Car

15
Instance variables

• Instance variables have an access modifier, a
  data type and a name (identifier)
• The access modifier is usually private, meaning
  that other classes cant use the variable
  directly
• The data type describes the data
• E.g. String for text, int for integer etc
• The identifier should be a meaningful name,
  starting lower case by convention
• Declare them at the start or the end of the
  class. I prefer the start, because you can see
  them easily
• e.g. private String model

16
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

17

CONSTRUCTOR

• First an object must be created
• 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 is passed to the constructor as
  parameters

18
Creating an object

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

19
Executing this call to the Car constructor

• The line below 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

Class name, starts upper case
public Car (String model, int tank, double mpg)
Means that any other class can use the constructor
Type and name of parameters
20
Car class - constructor

• The constructors 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 parameter details into
• //instance variables
• this.model model
• tankSize tank
• manfMPG mpg

21
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

22
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
23
The program so far Car class
public class Car //instance variables
private String model private int tankSize
private double manfMPG //constructor public
Car(String model, int tank,
double mpg)
this.model model tankSize tank manfMPG
mpg
24
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
• assigns 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 isnt one!

25
After using the constructor
26
UML Sequence diagram
MainCar
main()
myCarCar
new Car(Ka,40,7.4)
27
Using the Car class in the main method

• First we create 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 296 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

28
Using the Car class in the main method

• Assuming suitable methods exist in the Car class,
  the main method will look like this
• //create object
• Car myCar new Car ("Ford Ka", 40, 33.6)
• //get model
• String model myCar.getModel()
• //get estimated distance
• double distance myCar.estimateDistance()
• //print the details to standard output
• System.out.println("A " model " can travel "
  distance " miles")

29
Points to note on previous code

• When calling the constructor, we provide the same
  types as specified in the constructor signature
• 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 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
• We can concatenate Strings using the sign

30
Whats in a method?
Access modifier
method name, starts lowercase
Return type
Any parameters in brackets (none)
public String getModel() return model
Return statement
One of the instance variables
Curly brackets enclose method body
31
Car class methods to get info out

• //Return model
• public String getModel()
• return model
• //estimate distance car can travel
• public double estimateDistance()
• //there are 0.22 gallons per litre
• return tankSize manfMPG 0.22

32
ARITHMETIC OPERATORS

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

33
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)
• Selection (branching)
• Repetition (loops)
• Its your job, as a coder, to get things in the
  right order

34
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.

35
Sequence diagram
MainCar
Standard output
main()
myCarCar
new Car(Ka,40,7.4)
model getModel()
distance estimateDistance()
println(message incl model distance)
36
Class diagram
MainCar main(..)
Car String model int tankSize double manfMPL
String getModel() double estimateDistance()
37
Lab One

• See separate handout
• Familiarity with running a java program from the
  command line
• Familiarity with using an ide such as eclipse
• Run and experiment with the First and the CarMain
  programs

38
Next

• The next few lectures reinforce the ideas of
  classes and objects, while also introducing
  selection and repetition