Abstract class in Java:
Abstraction defines the ability to make a class as "abstract" declaring with keyword "abstract" which is considered as incomplete.
A class which is declared with the abstract keyword is known as an abstract class in Java. It can have abstract and non-abstract methods (method with the body).
abstract keyword is used to create a abstract class and method. Abstract class in java can't be instantiated. An abstract class is mostly used to provide a base for subclasses to extend and implement the abstract methods and override or use the implemented methods in abstract class
Rules or Conditions of Abstract Class in Java:
1) Once we declare a class "abstract" than object can not created or instantiated for the abstract class. if we try, this will give compile time error.
2) Abstract class can have fields(properties), abstract methods.
3) This can have non-abstract methods.
4) If we declare a method as abstract than no need to provide method body. Whoever is extending this class, they have to provide implementation for the abstract methods.
5) This does not have much importance unless it has sub classes.
6) Abstract methods should be public or protected.
What is Java and why need to choose:
Java is a programming language to develop the standalone application, web based, enterprise applications. Every Java developer needs obviously to be installed in their system for their needs.
As well, who does not develop Java applications and hate Java also need to install at least JRE because some of the software's are developed in Java. So to execute or run applications Java runtime environment is required.
Java is free to download and free license. Java available in 30+ languages.
When we download JDK and install it. Than we get development tools and JRE. JRE means Java Runtime Environment which consists of Java Virtual Machine(JVM), core classes along with the required core libraries. We can understand that JRE is a part of Java installation. Or individually we can install only JRE. Java plugin is required to run web browser.
History of Java, Versions, Releases, principles
Every system or computer uses and relys on Java to work most of the software's properly. Java has evolved by James Gosling, King of programming Java at Sun Micro systems which is now acquired by Oracle Corporation.
Initially James Gosling and his team has started developing Java in 1991 and was named earlier as "Oak" tree which is in front of his office. Later named as Green, but finally called as Java.
Made it first available public implementation in 1995 version 1.0 which is platform independent. "Write Once, Run Anywhere".
Java introduces automatic garbage collection implementation to free unused objects and maintain memory utilization sufficiently. Once object has no references or unreachable memory than object becomes eligible for automatic garbage collection. This is low priority thread.
