Object-Oriented Software Development

1
Object-Oriented Software Development

• Object-Oriented models are composed of objects.
• Objects contain data and make computations.
• The decomposition of a complex system is based on
  the structure of classes, objects, and the
  relationship among them. (divide-and-conquer).
• When we divide our problems into sub-problems, we
  will try to design classes to solve these
  sub-problems.
• We will use graphical notation to describe
  object-oriented analysis and design models. This
  notation is based on Unified Language Modelling
  (UML).

2
Classes and Objects

• Objects and classes are two fundamental concepts
  in the object-oriented software development.
• An object has a unique identity, a state, and
  behaviors. In the real life, an object is
  anything that can be distinctly identified.
• A class characterizes the structure of states and
  behaviors that shared by all its instances.
• The terms object and instance are often
  interchangeable.
• The features of an object is the combination of
  the state and behaviors of that object.
• The state of an object is composed of a set of
  attributes (fields) and their current values.
• The behavior of an object is defined by a set of
  methods (operations, functions, procedures).
• A class is a template for its instances. Instead
  of defining the features of objects, we define
  features of the classes to which these objects
  belong.

3
Classes in Java

• A class in Java can be defined as follows
• class Rectangle
• int length, width
• public int area()
• public void changeSizes(int x, int y)
• The name of the class is Rectangle
• Its attributes are length width
• Its methods are area changeSizes
• This Rectangle class is a template for all
  rectangle objects. All instances of this class
  will have same structure.

4
Objects in Java

• An object in Java is created from a class using
  new operator.
• Rectangle r1 new Rectangle()
• length
  r1
• width
• Rectangle r2 new Rectangle()
• length
  r2
• width

5
Graphical Representation of Classes
ClassName is the name of the class
ClassName
field1 field2
method1 methodm
Each field is VisibilityType identifier
initialvalue
Each method is VisibilityType
identifier ( parameter-list )
Rectangle
int length int width
public int area () public void changeSizes (int x, int y)
Ex

• We may not give field,
• methods parts

6
Graphical Representations of Objects

• We may omit ClassName, and just use objectName.
• In this case the class of the object is no
  interest for us.
• We may omit objectName, and just use ClassName.
• In this case, the object is an anonymous object.

objectNameClassName
field1 value1 fieldn valuen
Rectangle r1 new Rectangle() r1.length
20 r1.width 10
r1Rectangle
length 20 width 10
r2Rectangle
length 40 width 30
Rectangle r2 new Rectangle() r2.length
40 r2.width 30
7
Java Execution Model

• Java execution model compromises between
  conventional compilation and interpretation
  approaches.
• Java programs are compiled into Java byte-codes
  (Machine Codes of Java Virtual Machine). These
  Java byte-codes are independent from machine
  codes of any architecture. But they are close to
  machine codes.
• These generated Java byte-codes are interpreted
  by Java interpreters available in different
  platforms.
• So, the generated byte-codes are portable among
  different systems.
• The execution of Java programs are slower because
  we still use the interpretation approach.

8
Class versus Object

• A class is an object template and factory
• It contains the design of an object (class
  instance)
• It contains a way to generate these instances
• An object is an instance of a class
• All objects that are identical except for their
  state during execution are grouped together in a
  class
• All instances of a class have the same members
  but those members take on different values for
  each instance

9
Java Attributes

• Run on many platforms
• Unix, Windows, Mac
• Develop on multiple platforms
• The usual suspects
• Reuse at many levels
• Components
• Classes (families of components)
• Frameworks (collaborating components)

10
How is Java Portable?

• Java source code (.java) is compiled for a JVM
• JVM Java virtual machine
• Byte code files (.class) run on the JVM via
• Machine emulation on local host
• Just-in-time compilation from byte to native
  codes
• An actual silicon implementation of the JVM

11
Applications versus Applets

• Java applications run like any other program,
  with all the rights and privileges thereto
  appertaining
• Applets run in a sandbox, Internet browser or
  applet viewer
• The sand box provides services, but limits what
  applet can do
• No reading or writing files on local machine
  (just from host)
• No network connections, except back to host
• No inter-applet communication except to others
  from same host
• JVM enforces byte-code verification, security
  management
• Java language does not allow pointer manipulation

12
Preparing a Java Program

• We are going to use JDK environment of Sun
  Microsystems.
• JDK environment is simple to use and free.
• You can JDK environment for your own computer
  from
• the sun website http//java.sun.com/j2se/1.3
   
• Editing
• Create a file containing a Java program.
• You may use any text editor.
• This file will be .java extension (Ex
  Test1.java )
• Compiling
• Use Java compiler to compile the Java program. (
  javac Test1.java )
• Java compiler will create a file with .class
  extension. This file will contain the Java
  byte-codes of your Java program ( Test1.class ).
  You can run this file in different platforms.
• The other compilers produce executable files.

13
Preparing a Java Program (cont.)

• Executing
• Execute the byte-codes of your Java program by a
  Java interpreter.
• We will see that there are two types of Java
  programs application, applet
• If our program is an application, we will execute
  it as follows
• java Test1 (will interpret Test1.class
  file )
• If our program is an applet
• First we will create a .html file containing
  a link to our .class
  file.
• Then we will run our applet using
  appletviewer
  appletviewer Test1.html
• We may run our applets under web-browsers
  too.

14
A Simple Console Application Program

• // Author Ilyas Cicekli Date October 9,
  2001
• // A simple application program which prints
  Hello, World
• public class Test1
• public static void main(String args)
• // print Hello, World
• System.out.println("Hello, World)
• // end of main
•  
• // end of class

15
Java data types

• Primitive data types
• integers (byte, short, int, long)
• floating point numbers (float, double)
• boolean (true, false)
• char (any symbol encoded by a 16-bit unicode)
• Objects- everything else
• An object is defined by a class. A class is the
  data type of the object.
• You can define your own objects or
• use predefined classes from library.

16
Structure of an Applet Program

• imported classes
• you should import at least Graphics and Applet
  classes
• public class ltyour class namegt extends Applet
• declarations
• you should declare all variables which will be
  used in your methods
• declarations of methods in your application
• Declarations of your own methods and the methods
  responding to events.
• If a required method is needed but it is not
  declared, it is inherited from Applet class.
  Normally the free versions we get from Applet
  class.

17
Structure of a Console Application Program

• imported classes
• You should at least import classes in java.io
  package.
• public class ltyour application namegt
•  
• public static void main (String args) throws
  IOException
•    declarations of local variables and local
  objects (references)
• executable statements
• other methods if they exist
• Remember the file name should be lt your
  application namegt.java

18
Java Widening Conversions

• In widening conversions, they often go from one
  type to another type uses more space to store the
  value.
• In most widening conversions, we do not loose
  information.
• we may loose information in the following
  widening conversions
• int ? float long ? float long ? double

From To
byte short, int, long, float, double
short int, long, float, double
char int, long, float, double
int long, float, double
long float, double
float double
19
Input and Output (in Console Applications)

• Java I/O is based on input and output streams
• There are pre-defined standard streams
• System.in reading input keyboard
  (InputStream object)
• System.out writing output monitor
  (PrintStream object)
• System.err writing output (for errors) monitor
  (PrintStream object)
• print and println methods (defined in PrintStream
  class) are used to write to the the standard
  output stream (System.out).
• We will get the inputs from the standard input
  stream (System.in).
• To read character strings, we will create a more
  useful object of BufferedReader class from
  System.in.
• BufferedReader stdin
• new BufferedReader(new InputStreamReader(System.i
  n))

20
Reserved Words

• Reserved words are identifiers that have a
  special meaning in a programming language.
• For example,
• public, void, class, static are reserved words
  in our simple programs.
• In Java, all reserved words are lower case
  identifiers (Of course we can use just lower case
  letters for our own identifiers too)
• We cannot use the reserved words as our own
  identifiers (i.e. we cannot use them as
  variables, class names, and method names).

21
Java reserved words

• Data declaration boolean, float, int, char
• Loop keywords for, while, continue
• Conditional keywords if, else, switch
• Exceptional keywords try, throw, catch
• Structure keywords class, extends, implements
• Modifier and access keywords public, private,
  protected
• Miscellaneous true, null, super, this

22
Another Simple Console Application Program

• public class Test2
•  
• public static void main(String args)
• // print the city and its population.
• System.out.println("The name of the city is
  Orlando)
• System.out.println(Its population is
  1000000)
•  
• // Different usage of operator
• System.out.println(Sum of 54 (54))
•  
• // Different output method print
• System.out.print(one..)
• System.out.print(two..)
• System.out.println(three..)
• System.out.print(four..)
• // end of main
• // end of class

23
Life cycle methods for an applet

• init put code here that should be executed only
  once in the applets lifetime this method is
  called when the applet is first encountered
• start put code here to start your applet
  called right after init or when user revisits
  page
• stop put code here to stop your applet called
  when user leaves page containing applet
• destroy put code here to relinquish resources
  many dont even use this method

24
Examples of life cycle code

• init load images
• start start an animation (usually with separate
  thread)
• stop stop animation (by stopping the thread)
• destroy close network connections

25
Methods

• A class contains methods.
• A method is a group of statements that are given
  a name.
• Each method will be associated with a particular
  class (or with an instance of that class).
• We may define methods and invoke them with
  different parameters. When its parameters are
  different, their behavior will be different.

26
Methods

• A method declaration specifies a block of
  statements,
• that gets executed when the method is invoked.
• A method either returns some value (of some data
  type),
• or is declared void ( exception?)
• A method that returns value must have a return
• statement.
• A void method may have a return statement
• without an expression.

27
Creating Objects

• class C
• // fields
• private int x
• private double y
• // constructors
• public C() x1 y2.2
• // methods
• public void m1 (int val) xval
• public void m2 (double val) yval
• The constructor method must have the same name as
  the class, and it does not have any return type
  (not even void).
• Variables x and y are instance-variables, and
  they can be seen only methods of this class.

28
Creating Objects (cont.)

• In some other class, we may create the objects of
  the class C. (If we want, we can also create the
  objects of C in C too).
• public class C2
• .... main (...)
• C obj1, obj2
• obj1 new C()
• obj2 new C()
• .
• .

x
obj1
1
y
2.2
obj2
1
x
y
2.2
29
Accessibility Modifiers for Class Members (cont.)

public private protected package
The class itself yes yes yes yes
Classes in the same package yes no yes yes
Sub-classes in a different package yes no yes no
Non-subclasses in a different package yes no no no