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Java is a high-level programming language originally developed by Sun Microsystems and released in 1995. Java runs on a variety of platforms, such as Windows, Mac OS, and the various versions of UNIX. This tutorial gives a complete understanding of Java. This reference will take you through simple and practical approaches while learning Java Programming language. It is owned by Oracle, and more than 3 billion devices run Java.
It is used for:
1.Mobile applications (specially Android apps) 2.Desktop applications 3.Web applications 4.Web servers and application servers
5.Games
6.Database connection
And much, much more!
Why Use Java? 1.Java works on different platforms (Windows, Mac, Linux, Raspberry Pi, etc.)
2.It is one of the most popular programming language in the world
3.It is easy to learn and simple to use
4.It is open-source and free
5.It is secure, fast and powerful
6.It has a huge community support (tens of millions of developers)
7.Java is an object oriented language which gives a clear structure to programs and allows code to be reused, lowering development costs
8.As Java is close to C++ and C#, it makes it easy for programmers to switch to Java or vice versa

When we consider a Java program, it can be defined as a collection of objects that communicate via invoking each other's methods. Let us now briefly look into what do class, object, methods, and instance variables mean.
Object − Objects have states and behaviors. Example: A dog has states - color, name, breed as well as behavior such as wagging their tail, barking, eating. An object is an instance of a class.
Class − A class can be defined as a template/blueprint that describes the behavior/state that the object of its type supports.
Methods − A method is basically a behavior. A class can contain many methods. It is in methods where the logics are written, data is manipulated and all the actions are executed.
Instance Variables − Each object has its unique set of instance variables. An object's state is created by the values assigned to these instance variables. First Java Program Let us look at a simple code that will print the words Hello World.
Example Live Demo
public class MyFirstJavaProgram {

/* This is my first java program. * This will print 'Hello World' as the output */

public static void main(String []args) {

System.out.println("Hello World"); // prints Hello World
} }

JVM (Java Virtual Machine) is an abstract machine. It is called a virtual machine because it doesn't physically exist. It is a specification that provides a runtime environment in which Java bytecode can be executed. It can also run those programs which are written in other languages and compiled to Java bytecode. JVMs are available for many hardware and software platforms. JVM, JRE, and JDK are platform dependent because the configuration of each OS is different from each other. However, Java is platform independent. There are three notions of the JVM: specification, implementation, and instance. The JVM performs the following main tasks: Loads code, Verifies code, Executes code, Provides runtime environment.

JRE is an acronym for Java Runtime Environment. It is also written as Java RTE. The Java Runtime Environment is a set of software tools which are used for developing Java applications. It is used to provide the runtime environment. It is the implementation of JVM. It physically exists. It contains a set of libraries + other files that JVM uses at runtime.

JDK is an acronym for Java Development Kit. The Java Development Kit (JDK) is a software development environment which is used to develop Java applications and applets. It physically exists. It contains JRE + development tools. JDK is an implementation of any one of the below given Java Platforms released by Oracle Corporation: Standard Edition Java Platform Enterprise Edition Java Platform Micro Edition Java Platform The JDK contains a private Java Virtual Machine (JVM) and a few other resources such as an interpreter/loader (java), a compiler (javac), an archiver (jar), a documentation generator (Javadoc), etc. to complete the development of a Java Application.

Java is an Object-Oriented Language. As a language that has the Object-Oriented feature, Java supports the following fundamental concepts − Polymorphism , Inheritance , Encapsulation , Abstraction , Classes , Objects , Instance , Method , Message Passing , In this chapter, we will look into the concepts - Classes and Objects.

Object − Objects have states and behaviors. Example: A dog has states - color, name, breed as well as behaviors – wagging the tail, barking, eating. An object is an instance of a class.

Class − A class can be defined as a template/blueprint that describes the behavior/state that the object of its type support.

Objects in Java Let us now look deep into what are objects. If we consider the real-world, we can find many objects around us, cars, dogs, humans, etc. All these objects have a state and a behavior. If we consider a dog, then its state is - name, breed, color, and the behavior is - barking, wagging the tail, running. If you compare the software object with a real-world object, they have very similar characteristics. Software objects also have a state and a behavior. A software object's state is stored in fields and behavior is shown via methods. So in software development, methods operate on the internal state of an object and the object-to-object communication is done via methods.

Classes in Java A class is a blueprint from which individual objects are created. Following is a sample of a class.
Example
public class Dog {
String breed; int age; String color; void barking() {
}
void hungry() {
}
void sleeping() {
}
}

A class can contain any of the following variable types.

Local variables − Variables defined inside methods, constructors or blocks are called local variables. The variable will be declared and initialized within the method and the variable will be destroyed when the method has completed.

Instance variables − Instance variables are variables within a class but outside any method. These variables are initialized when the class is instantiated. Instance variables can be accessed from inside any method, constructor or blocks of that particular class.

Class variables − Class variables are variables declared within a class, outside any method, with the static keyword.

A class can have any number of methods to access the value of various kinds of methods. In the above example, barking(), hungry() and sleeping() are methods.

Following are some of the important topics that need to be discussed when looking into classes of the Java Language.