unit 2

• Object-Oriented Thinking


  A way of viewing the World
• Agents and Communities


  An object-oriented program is structured as a community of interacting agents, called objects
• Scenario


  • I (stays at Hyderabad) wants to send a flower bouquet to my friend, who is at Chennai
• Objects


  • Each object has a role to play
  • Each object provides a service, or performs an action, that is used by other members of the community
• Messages & Methods


  • Actions are initiated in object-oriented programming by the transmission of a message to an agent (an object) responsible for the action
  • The message encodes the request for an action
  • It is accompanied by any additional information (arguments) needed to carry out the request
  • The receiver is the object to whom the message is sent
  • In response to a message, the receiver will perform some method to satisfy the request
• Responsibilities


  A fundamental concept in object-oriented programming is to describe the behavior in terms of responsibilities
• Object


  • It is the basic unit of Object Oriented Programming and it represents the real life entities
  • Objects have two characteristics: they all have state and behavior
• Examples of Objects


  • student
  • teacher
  • fan
  • dog
  • pen
• State


  It is represented by attributes of an object
• Behavior


  It is represented by methods of an object
• Identity


  It gives a unique name to an object
• Class


  • A class is a user defined blueprint or prototype (template) from which objects are created
  • It represents the set of properties or methods that are common to all objects of one type
  • It has definitions of methods and data
• Data Hiding & Encapsulation


  • Using private access modifiers(access specifiers), we can hide data from other classes
  • Binding data and its operations together into a single unit is known as encapsulation
  • A java class is the example of encapsulation
• Abstraction


  • Hiding internal details (implementation details) and showing functionality is known as abstraction
  • In Java, we use abstract class and interface to achieve abstraction
• Inheritance


  • Inheritance in Java is a mechanism in which one object acquires all the properties and behaviors of a parent object
  • Existing class: Super Class / Base Class / Parent Class
  • New class: Sub Class / Derived Class / Child Class
  • The idea behind inheritance in Java is that you can create new classes that are built upon existing classes
  • When you inherit from an existing class, you can reuse methods and fields of the parent class
  • Moreover, you can add new methods and fields in your current class also
  • Inheritance represents the IS-A relationship which is also known as a parent-child relationship
• Inheritance Examples


  • Triangle IS-A Shape
  • Rectangle IS-A Shape
• Why use inheritance in java


  • For Method Overriding (so runtime polymorphism can be achieved)
  • For Code Reusability
• Polymorphism


  • Polymorphism is the ability of an object (methods) to take on many forms
  • Generally it occurs when we have many classes that are related to each other by inheritance
  • In Java, we use method overloading and method overriding to achieve polymorphism
• Java Buzzwords/Features


  • Simple
  • Object Oriented
  • Platform Independent
  • Compiled & Interpreted but High performance
  • Architecture-neutral
  • Robust
  • Multithreaded
  • Distributed
  • Secure
• Simple


  • Java is easy to learn and its syntax is quite simple, clean and easy to understand
  • Java inherits the C/C++ syntax and many of the object-oriented features of C++
  • Java has removed many complicated and rarely-used features, for example, explicit pointers, operator overloading, etc.
• Object Oriented


  • In java everything is an Object which has some data and behavior
  • Un like C++, in java even the function main() has to be part of some class only
  • We can't have any method outside of the class
  • Java can be easily extended as it is based on Object Model
• Platform Independent


  • C/CPP compiler produces ".exe" file which is interpreted by OS and the instructions in the ".exe" file are specific to OS(ie platform dependent)
  • Java Compiler produces ".class" file (byte code), which is interpreted on any platform with corresponding JVM
  • The instructions in the ".class" file (byte code) is platform independent
  • JVM(Java Virtual Machine) is a engine that enable the computer to run java programs
  • JVM is the java interpreter, which converts java byte code into machine language
  • It starts the java program execution by calling main() method
  • It is part of JRE(Java Runtime Environment)
• Compiled & Interpreted but High performance


  • Java is compiled and interpreted language
  • Java Compiler generates '.class' file (Byte code)
  • Java Interpreter interprets this byte code, converts into an executable file (exe file)
  • Most previous attempts at cross-platform solutions have done so at the expense of performance
  • Java byte code was carefully designed so that it would be easy to translate directly into native machine code for very high performance by using a just-in-time compiler
• Architecture-neutral


  • Java is "write once run anywhere anytime" language
  • Operating system upgrades, processor upgrades, and changes in any core system resources does not make a program to malfunction
• Robust


  • Java uses strong memory management
  • There is no pointers, avoids security problems
  • Manages memory allocation and de allocation (In fact, de allocation is completely automatic, because Java provides garbage collection for unused objects)
  • Java makes an effort to eliminate error-prone situations by emphasizing mainly on compile time error checking and runtime checking(exception handling & type checking)
• Multithreaded


  • With Java's multithreaded feature it is possible to write programs that can perform many tasks simultaneously
  • Multithreading is a special type of Multitasking
• Multitasking


  • Running multiple tasks simultaneously
  • Each task is a separate program
  • If we run programs sequentially one after the other, if the currently running program requires an I/O, the processor will be idle until the i/o operation is completed
  • The aim of multi tasking is to reduce this idle time of the processor
  • With the help of CPU Scheduling algorithms(Example- Round Robin), we can assign processor another job when the currently running program requires an i/o
• Multitasking Advantages


  • Throughput(no.of jobs that can be completed in a unit time) will be increased
  • Quick response time for jobs(i.e no job needs to wait for a long time for the processor)
• Thread


  • Part of a program
  • Sequence of instructions, which has independent path of execution
• Multithreading


  Running multiple threads of a program simultaneously
• Distributed


  • Java is distributed because it facilitates users to create distributed applications in Java
  • RMI (Remote Method Invocation) and EJB (Enterprise Java Bean) are used for creating distributed applications
  • This feature of Java makes us able to access resources(objects/files) by calling the methods from any machine on the internet
• Secure


  • No explicit pointer
  • Java Programs run inside a virtual machine sandbox. Java Security Manager determines what resources a java class can access such as reading and writing to the local disk
• Primitive Data Types


  • boolean
  • byte
  • short
  • int
  • long
  • float
  • double
  • char
• Variables


  • Variables are containers for storing data values
  • The value depends on the data type of the variable
  • The value can be altered during program exam execution
  • There are three types of variables in java: local, instance and static
  • Syntax: datatype variable_name[=default_value]
• Variable Declaration


  • int x=50
• Expressions


  Any unit of code that can be evaluated to a value is an expression
• Expressions


  • 10+15
  • (x*y)/z
• Operators


  • Unary Operators
  • Arithmetic Operators
  • Relational Operators
  • Equality Operators
  • Bitwise Operators
  • Logical Operators
  • Ternary Operator
  • Assignment Operators
• Unary Operators


  • expr++
  • expr--
  • ++expr
  • --expr
• Arithmetic Operators


  • *
  • /
  • %
  • +
  • -