java

• 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
• Examples of Inheritance


  • 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
• 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
• Expressions


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


  • Unary Operators
  • Arithmetic Operators
  • Relational Operators
  • Equality Operators
  • Bitwise Operators
  • Logical Operators
  • Ternary Operator
  • Assignment Operators
• Access Modifiers / Access Specifiers


  private, default, protected, public
• Logical Operators (Logical Short Circuit Operators)


  &&, ||
• Logical Operators


  • if(x != null && x.getValue() < 5) { // Do something }
• Ternary Operator


  • int a=2; int b=5; int min=(a<b)?a:b; System.out.println(min);
• Assignment Operators


  • int a=10; int b=20; a+=4; b-=4; System.out.println(a); System.out.println(b);
• Access Modifiers / Access Specifiers


  They tell us the scope of the variable (where a variable can be accessed)
• Access Modifiers


  • private, default, protected, public
• Instance Data Members


  Each object has a separate copy of value for instance variables, they can be accessed with object only
• Static Data Members


  There is only one copy of value in the memory for a given class, irrespective of how many objects the class has, all objects share this static variable, can be accessed with class name itself
• Static Methods


  Their behavior is common to all objects, can be called directly with the class name, no need of object
• Non-Static (Instance) Methods


  Operate on instance variables of the class, need an object to call
• Wrapper Classes


  Byte, Short, Integer, Long, Float, Double, Character & Boolean, used for data type conversion
• Command Line Arguments

  • class Test { public static void main(String args[]) { int x,y,z; x=Integer.parseInt(args[0]); y=Integer.parseInt(args[1]); z=x+y; System.out.println("The sum="+z); } }
• Taking Input from User at Runtime (Scanner Class)

  • import java.util.Scanner; class ex3 { public static void main(String args[]) { int x,y,z; Scanner sc=new Scanner(System.in); System.out.println("Enter the first no:"); x=sc.nextInt(); System.out.println("Enter the second no:"); y=sc.nextInt(); if(x>y) System.out.println("First no is bigger"); else System.out.println("Second no is bigger"); } }
• while Loop


  Executes a statement or code block repeatedly as long as an expression is true
• while Loop


  • while (expression) { Statement(s) to be executed if expression is true }
• do...while Loop


  Executes the statement(s) first, then checks the condition