Background information

1
LESSON 1
Introduction and a Taste of Java

• Background information
• Introduction to Java

2
Object-Oriented Concepts

• What is an Object?
• A software bundle of related variable methods.
  Often used to model real-world objects.
• What is a Method?
• A function (subroutine) associated with an object.

3
OO Concepts continued.

• What is Modularity?
• The source code for an object can be written
  maintained independently of the source code for
  other objects.
• Object can be easily passed around in the system.

4
OO Concepts continued.

• What is Encapsulation?
• Data actions are combined into a single item (a
  class object) the details of the implementation
  are hidden.
• An object has a public interface that other
  objects use to communicate with it.
• Information hiding, Abstract data types
  Encapsulation refer to basically the same idea.

5
OO Concepts continued.

• What is a Message?
• Software objects interact communicate with each
  other using messages.
• Message components are
• object to which message is addressed
• name of the method to perform
• any parameters needed by the method

6
OO Concepts continued.

• What is a Class?
• A blueprint, or prototype, that defines the
  variables the methods common to all objects of
  a certain kind.
• What is Inheritance?
• A class inherits state behaviour from its
  superclass. Allows code reuse.

7
OO Concepts continued.

• What is an Interface?
• A contract in the form of a collection of method
  constant declarations.
• A device or a system that unrelated entities use
  to interact.
• When a class implements an interface, it promises
  to implement all of the methods declared in that
  interface.

8
OO Concepts continued.

• What is Polymorphism?
• Ability of one thing to have several forms, or
  one form to apply to several things.
• E.g. two different classes can have methods with
  the same name.

9
Object-Oriented Programming

• OOP
• A design and programming technique
• Some terminology
• object - usually a person, place or thing (a
  noun)
• method - an action performed by an object (a
  verb)
• type or class - a category of similar objects
  (such as automobiles)
• Objects have both data and methods
• Objects of the same class have the same data
  elements and methods
• Objects send and receive messages to invoke
  actions

10
Example of an Object Class
Class automobile

• Data Items
• manufacturers name
• model name
• year made
• color
• number of doors
• size of engine
• etc.

• Methods
• Define/Assign data items (specify manufacturers
  name, model, year, etc.)
• Change a data item (color, engine, etc.)
• Display data items
• Calculate cost
• etc.

11
Why OOP?

• Save development time (and cost) by reusing code
• once an object class is created it can be used in
  other applications
• Easier debugging
• classes can be tested independently
• reused objects have already been tested

12
Java Characteristics

• Platform independent
• Virtual machine architecture
• Object-oriented
• Full GUI multimedia support
• Also platform independent
• Multi-threading capabilities
• Parallel programs
• Applications (locally) Applets (remotely)

13
Compilers vs. Assemblers vs. Interpreters

• Compilers and Assemblers
• translation is a separate user step
• translation is off-line, i.e. not at run time
• Interpreters - another way to translate source to
  object code
• interpretation (from source to object code) is
  not a separate user step
• translation is on-line, i.e. at run time

Compiler, Assembler, or Interpreter
Object Code
Source Code
14
Java Program Translation
Data for Java Program
Java Program

• Both Compilation and Interpretation
• Intermediate CodeByte Code
• portable low-level code
• similar to assembly code,but hardware
  independent
• invisible to Java programmer
• Interpreter translates from generic byte code to
  hardware-specific machine code

JavaVirtual Machine
Java Compiler
Byte-Code Program
Byte-Code Interpreter
Machine-Language Instructions
Computer Execution of Machine-Language
Instructions
Output of Java Program
15
Java Program Translation Including Linker
Data for Java Program
Previously Compiled Helper Programs
Java Program
Java Compiler
JavaVirtual Machine
Byte-Code Program
Byte-Code Interpreter
Machine-Language Instructions
Linker
Computer Execution of Machine-Language
Instructions
Output of Java Program
16
Java Virtual Machine

• The JVM executes a special machine code
  ("bytecode")
• Java compiles to .class files
• Imaginary computer
• Simulated by an Interpreter
• Advantage
• Enables platform independence
• Disadvantage
• Takes longer to interpret than the norm
  (compiled)

17
Applets vs. Java Applications

• Applets
• Java programs intended to be downloaded via the
  WWW and run immediately
• little applications
• requires a web browser
• Applications
• Java programs intended to be installed then run
• often larger applications
• Slightly different programming for each, but both
  are easy to do