Introduction to Java

There are two types of Java programs — Java Stand-Alone Applications and Java Applets.

Java Stand-Alone Applications
A stand-alone Java application refers to a Java program that can run independently on a computer. Acrobat Reader is an excellent example of this type of application. In Java, every stand-alone application begins its execution with the main() method. Java stand-alone applications can be classified into two types:
a. Console based applications
b. Graphical User Interface based applications

Java Applets
Java applets are Java applications that run within a web browser. They are mainly used for internet programming. The applet is capable of performing many tasks on a web page, such as displaying graphics, playing sounds, and accepting user input.

CPU understands Machine Language.

In traditional compilation process, the machine language code generated by the compiler is specific to the platform for which the source code is compiled. Every high-level language has a separate platform specific compiler to generate the machine code that can run on the specific platform. For example, the executable file generated by compiling a C++ program on Windows platform will not run on Macintosh. It needs to be compiled with C++ Macintosh compiler. The below diagram illustrates this:

In Java compilation process, the source code of a Java program is compiled to an intermediate binary code called the Bytecode. This Bytecode cannot be directly executed by the machine. It is understood by a virtual machine known as Java Virtual Machine or JVM. JVM contains a Java interpreter which converts the Bytecode into machine code of the target computer. JVM is platform specific i.e. each platform has its own JVM. But once the proper JVM is installed on the machine, it can run any Java Bytecode program. The below diagram illustrates this:

Program written in an interpreted language is converted into machine code line by line and executed one line at a time whereas program written in a compiled language is converted into machine code all at once and executed.

In traditional compilation process, the machine language code generated by the compiler is specific to the platform for which the source code is compiled. The executable file generated for one platform like Windows will not work on other platforms like Macintosh. In Java compilation process, the source code of a Java program is compiled to an intermediate binary code called the Bytecode. This Bytecode is interpreted and executed by JVM on the target platform. Bytecode is platform independent so the same Bytecode which is generated for one platform like Windows will work on other platforms like Macintosh without any modifications.

JVM
JVM stands for the Java Virtual Machine. It is a virtual machine that runs Java programs and can be installed on several different operating systems such as Windows, UNIX, Macintosh etc. JVMs allow Java programs to run on almost any computer. A JVM processes instructions similar to a physical processor. However, the Java code (with "java" extension) must first be converted into Bytecode that the JVM understands.

JIT
JIT is the Just-In-Time compiler included in the JVM that compiles selected portions of the Bytecode into executable codes to improve the execution performance.

b. Compiler and Interpreter

c. ".java" file and ".class" file

Bytecode and JVM makes Java programs platform independent. This is one of the most important and powerful features of Java. The Bytecode of a Java program can be run on any platform that has a corresponding JVM installed on it. No modifications to this Bytecode are required. Hence, it is said that Java provides "Write once, run anywhere" capabilities.

Four features of Java are:

1. Object Oriented — Java is an object-oriented programming language because it treats everything as an object. The entire program code and data resides within objects and classes. This enables us to easily use and extend the object model.
2. Robust — Java is a robust and reliable programming language. It has strict compile time and runtime checking of data types. Memory allocation and de-allocation is automatic in Java, so there is less for the programmer to worry about.
3. Platform Independent — The concept of "Write once, run anywhere", is one of the most important and powerful features of Java. Java applications can run on any platform that has a corresponding JVM installed on it.
4. Simple — Java is simple and easy to learn programming language. The removal of many unreliable features of C and C++ (its predecessors) has simplified its understanding and implementation.

Java Virtual Machine or JVM is a virtual machine that runs Java programs. It takes the compiled Bytecode as input, converts it into machine code with the help of the Java interpreter. Selected portions of the Bytecode are also compiled by JVM using the Just-In-Time (JIT) compiler for performance. This machine code generated by JVM is then executed by the processor. So JVM along with Bytecode gives Java its "Write once, run anywhere" capabilities.

A platform specific JVM is required to run Java programs on a computer. If a corresponding JVM is installed on the machine then Java can run on that machine.