Java introduction

WHAT IS JAVA? • Java is an object oriented programming language. • Java is platform independent. • Java is used in mobile phones and other embedded devices.

HISTORY OF JAVA • Java was developed by James Gosling from sun microsystems in the year 1995. • In January 1996, JDK1.0 was released in the name of OAK. • The JDK 1.0.2 was the first stable version. • James Gosling, Mike Sheridan and Patrick Naughton initiated the green project in June 1991.

Features of java • Simple • Secure • Portable • Object-Oriented • Robust • Multithreaded • Architecture neutral • Interpreted • High Performance • Distributed • Dynamic

Types of SDK’s in java • Java SE • Java EE • Java ME • Java Embedded • Java DB • Web Tier • Java Card • Java TV

Java virtual machine • Java Virtual Machine is used for converting the bytecode into the machine understandable format. • JVM provides an abstraction between the java program and the operating system. • Key components in JVM are • Class loader • Bytecode verifier • Just In Time (JIT) compiler

How to compile and run java program • The java program should be saved with an extension “.java” • The command to compile the java program is “javac name.java ” • The command to run the java program is “java name”

Sample Java program – Hello World

Main Concepts of OOP • Encapsulation • Inheritance • Polymorphism • Abstraction

Encapsulation • Encapsulation is the technique of making the fields in a class private and providing access to the fields via public methods. • The main benefit of encapsulation is the ability to modify our implemented code without breaking the code of others who use our code. • Encapsulation gives maintainability, flexibility and extensibility to our code.

Access Specifiers in java • public • private • protected • Default

Public access specifier • A class, method, constructor, interface etc., declared public can be accessed from any other class. • Therefore fields, methods, blocks declared inside a public class can be accessed from any class belonging to the Java Universe. • If the public class we are trying to access is in a different package, then the public class still need to be imported.

Private access specifiers • Methods, Variables and Constructors that are declared private can only be accessed within the declared class itself. • Private access modifier is the most restrictive access level. Class and interfaces cannot be private. • Variables that are declared private can be accessed outside the class if public getter methods are present in the class. • Using the private modifier is the main way that an object encapsulates itself and hide data from the outside world.

Sample code for private access specifier

Protected access specifier • Variables, methods and constructors which are declared protected in a superclass can be accessed only by the subclasses in other package or any class within the package of the protected members' class. • The protected access modifier cannot be applied to class and interfaces. Methods, fields can be declared protected, however methods and fields in a interface cannot be declared protected.

Sample code for protected access specifier

Default specifier • The default specifier is used to make a class visible to all the other classes in its package but not visible to classes from other packages

Inheritance • Inheritance is a compile-time mechanism in Java that allows you to extend a class with another class • The keyword extends is used for inheriting a Class. • The class which extends some other class is called as derived class • The base class is called as the super class or parent class.

Types of inheritance • Simple inheritance • Multilevel inheritance • Hierarchical inheritance • Hybrid inheritance

Simple inheritance • There are only 1 base class and 1 derived class • Syntax: Class Parentclass { } Class Dclass extendsParentclass { }

Simple inheritance sample code and output

Multilevel inheritance • It contains derived classes which are in turn base class to another class. class Base { } class Derived1 extends Base { } class Derived2 extends Derived1 { }

Multilevel inheritance sample code and output

Hierarchical inheritance • It contains one base class and more than one derived class. class Base { } class Derived1 extends Base { } class Derived2 extends Base{ }

Sample code and output for Hierarchical inheritance

Hybrid inheritance • It is a combination of any two or more inheritances.

Sample code for hybrid inheritance

Interface • An interface in Java is similar to a class, but the body of an interface can include only abstract methods and final fields (constants). • A class implements an interface by providing code for each method declared by the interface. • The keyword used in interface concepts are interface and implements

Syntax for interface interface interfacename { method_declaration(); } class A implements interfacename { method_declaration(){ } }

Java introduction

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