There are the following features in Java Programming Language.

• Simple: Java is easy to learn. The syntax of Java is based on C++ which makes easier to write the program in it.
• Object-Oriented: Java follows the object-oriented paradigm which allows us to maintain our code as the combination of different type of objects that incorporates both data and behavior.
• Portable: Java supports read-once-write-anywhere approach. We can execute the Java program on every machine. Java program (.java) is converted to bytecode (.class) which can be easily run on every machine.
• Platform Independent: Java is a platform independent programming language. It is different from other programming languages like C and C++ which needs a platform to be executed. Java comes with its platform on which its code is executed. Java doesn’t depend upon the operating system to be executed.
• Secured: Java is secured because it doesn’t use explicit pointers. Java also provides the concept of ByteCode and Exception handling which makes it more secured.
• Robust: Java is a strong programming language as it uses strong memory management. The concepts like Automatic garbage collection, Exception handling, etc. make it more robust.
• Architecture Neutral: Java is architectural neutral as it is not dependent on the architecture. In C, the size of data types may vary according to the architecture (32 bit or 64 bit) which doesn’t exist in Java.
• Interpreted: Java uses the Just-in-time (JIT) interpreter along with the compiler for the program execution.
• High Performance: Java is faster than other traditional interpreted programming languages because Java bytecode is “close” to native code. It is still a little bit slower than a compiled language (e.g., C++).
• Multithreaded: We can write Java programs that deal with many tasks at once by defining multiple threads. The main advantage of multi-threading is that it doesn’t occupy memory for each thread. It shares a common memory area. Threads are important for multi-media, Web applications, etc.
• Distributed: Java is distributed because it facilitates users to create distributed applications in Java. RMI and EJB are used for creating distributed applications. This feature of Java makes us able to access files by calling the methods from any machine on the internet.
• Dynamic: Java is a dynamic language. It supports dynamic loading of classes. It means classes are loaded on demand. It also supports functions from its native languages, i.e., C and C++.

Java Virtual Machine is a virtual machine that enables the computer to run the Java program. JVM acts like a run-time engine which calls the main method present in the Java code. JVM is the specification which must be implemented in the computer system. The Java code is compiled by JVM to be a Bytecode which is machine independent and close to the native code.

JVM

JVM is an acronym for Java Virtual Machine; it is an abstract machine which provides the runtime environment in which Java bytecode can be executed. It is a specification which specifies the working of Java Virtual Machine. Its implementation has been provided by Oracle and other companies. Its implementation is known as JRE.

JVMs are available for many hardware and software platforms (so JVM is platform dependent). It is a runtime instance which is created when we run the Java class. There are three notions of the JVM: specification, implementation, and instance.

JRE

JRE stands for Java Runtime Environment. It is the implementation of JVM. 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

JDK is an acronym for Java Development Kit. It 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

Many types:

• Class (Method) Area: Class Area stores per-class structures such as the runtime constant pool, field, method data, and the code for methods.
• Heap: It is the runtime data area in which the memory is allocated to the objects
• Stack: Java Stack stores frames. It holds local variables and partial results, and plays a part in method invocation and return. Each thread has a private JVM stack, created at the same time as the thread. A new frame is created each time a method is invoked. A frame is destroyed when its method invocation completes.
• Program Counter Register: PC (program counter) register contains the address of the Java virtual machine instruction currently being executed.
• Native Method Stack: It contains all the native methods used in the application.

Just-In-Time (JIT) compiler: It is used to improve the performance. JIT compiles parts of the bytecode that have similar functionality at the same time, and hence reduces the amount of time needed for compilation. Here the term “compiler” refers to a translator from the instruction set of a Java virtual machine (JVM) to the instruction set of a specific CPU.

A platform is the hardware or software environment in which a piece of software is executed. There are two types of platforms, software-based and hardware-based. Java provides the software-based platform.

There are the following differences between the Java platform and other platforms.

Java is the software-based platform whereas other platforms may be the hardware platforms or software-based platforms.

Java is executed on the top of other hardware platforms whereas other platforms can only have the hardware components.

The bytecode. Java compiler converts the Java programs into the class file (Byte Code) which is the intermediate language between source code and machine code. This bytecode is not platform specific and can be executed on any computer.

Classloader is a subsystem of JVM which is used to load class files. Whenever we run the java program, it is loaded first by the classloader. There are three built-in classloaders in Java.

• Bootstrap ClassLoader: This is the first classloader which is the superclass of Extension classloader. It loads the rt.jar file which contains all class files of Java Standard Edition like java.lang package classes, java.net package classes, java.util package classes, java.io package classes, java.sql package classes, etc.
• Extension ClassLoader: This is the child classloader of Bootstrap and parent classloader of System classloader. It loads the jar files located inside $JAVA_HOME/jre/lib/ext directory.
• System/Application ClassLoader: This is the child classloader of Extension classloader. It loads the class files from the classpath. By default, the classpath is set to the current directory. You can change the classpath using “-cp” or “-classpath” switch. It is also known as Application classloader.

Yes, Java allows to save our java file by .java only, we need to compile it by javac .java and run by java classname Let’s take a simple example:

//save by .java only  

class A{  

public static void main(String args[]){  

System.out.println(“Hello java”);  

}  

}  

//compile by javac .java  

//run by     java A

• compile it by javac .java
• run it by java A

The program compiles and runs correctly because the order of specifiers doesn’t matter in Java.

In Java, access specifiers are the keywords which are used to define the access scope of the method, class, or a variable. In Java, there are four access specifiers given below.

• Public The classes, methods, or variables which are defined as public, can be accessed by any class or method.
• Protected Protected can be accessed by the class of the same package, or by the sub-class of this class, or within the same class.
• Default Default are accessible within the package only. By default, all the classes, methods, and variables are of default scope.

Private The private class, methods, or variables defined as private can be accessed within the class only.

The methods or variables defined as static are shared among all the objects of the class. The static is the part of the class and not of the object. The static variables are stored in the class area, and we do not need to create the object to access such variables. Therefore, static is used in the case, where we need to define variables or methods which are common to all the objects of the class.

For example, In the class simulating the collection of the students in a college, the name of the college is the common attribute to all the students. Therefore, the college name will be defined as static.

• There are various advantages of defining packages in Java.
• Packages avoid the name clashes.
• The Package provides easier access control.
• We can also have the hidden classes that are not visible outside and used by the package.
• It is easier to locate the related classes.

It is a programming paradigm based on objects having data and methods defined in the class to which it belongs. Object-oriented paradigm aims to incorporate the advantages of modularity and reusability. Objects are the instances of classes which interacts with one another to design applications and programs. There are the following features of the object-oriented paradigm.

• Follows the bottom-up approach in program design.
• Focus on data with methods to operate upon the object’s data
• Includes the concept like Encapsulation and abstraction which hides the complexities from the user and show only functionality.
• Implements the real-time approach like inheritance, abstraction, etc.
• The examples of the object-oriented paradigm are C++, Simula, Smalltalk, Python, C#, etc.

The Object is the real-time entity having some state and behaviour. In Java, Object is an instance of the class having the instance variables as the state of the object and the methods as the behaviour of the object. The object of a class can be created by using the new keyword.

The Object is the real-time entity having some state and behaviour. In Java, Object is an instance of the class having the instance variables as the state of the object and the methods as the behaviour of the object. The object of a class can be created by using the new keyword.

All object references are initialized to null in Java.

The constructor can be defined as the special type of method that is used to initialize the state of an object. It is invoked when the class is instantiated, and the memory is allocated for the object. Every time, an object is created using the new keyword, the default constructor of the class is called. The name of the constructor must be similar to the class name. The constructor must not have an explicit return type.

Based on the parameters passed in the constructors, there are two types of constructors in Java.

• Default Constructor: default constructor is the one which does not accept any value. The default constructor is mainly used to initialize the instance variable with the default values. It can also be used for performing some useful task on object creation. A default constructor is invoked implicitly by the compiler if there is no constructor defined in the class.
• Parameterized Constructor: The parameterized constructor is the one which can initialize the instance variables with the given values. In other words, we can say that the constructors which can accept the arguments are called parameterized constructors.