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Java Virtual Machine
and
Runtime Environment

Basic Concept
 When you write a program in C++ it is known as
source code.
 The C++ compiler converts this source code into the
machine code of underlying system (e.g. Windows)
 If you want to run that code on Linux you need to
recompile it with a Linux based compiler.
 Due to the difference in compilers, sometimes you
need to modify your code.

The Concept of WORA
 Java introduced WORA: Write Once Run Anywhere
 When you write a java program it is known as the source code of
java.
 The java compiler compiles this source code for a software system
known as JVM
 This compiled code is known as ‘Byte Code’
 We have different JVMs for different systems (such as JVM for
Windows , JVM for Linux etc).
 When we run our program the JVM interprets translates) the
compiled program into the language understood by the underlying
system.
 So we write our code once and the JVM runs it everywhere
according to the underlying system.

Byte Code
 Java programs (Source code) are compiled
into a form called Java bytecodes.
 The Java compiler reads Java language
source (.java) files, translates the source into
Java bytecodes, and places the bytecodes
into class (.class) files.
 The compiler generates one class file for
each class contained in java source file.

Java Virtual Machine
 The central part of java platform is java virtual
machine
 Java bytecode executes by special software known
as a "virtual machine".
 Most programming languages compile source code
directly into machine code, suitable for execution
 The difference with Java is that it uses bytecode - a
special type of machine code.
 The JVM executes Java bytecodes, so Java
bytecodes can be thought of as the machine
language of the JVM.

Java Runtime Environment(JRE)
 The Java Virtual Machine is a part of a large
system i.e. Java Runtime Environment(JRE).
 Each operating system and CPU architecture
requires different JRE.
 The JRE consists of set of built- in classes,
as well as a JVM.
 Without an available JRE for a given
environment, it is impossible to run Java
software.

Data Types
 The Java programming language is strongly-typed, which
means that all variables must first be declared before they can
be used.
int gear = 1;
 The eight primitive data types supported by the Java
programming language are:
– int, byte, short, long, float, double, boolean, char

Naming Conventions
 Java uses identifiers to name
– variables
– methods
– classes
– packages
 Syntax rules
– Must begin with a letter (upper- or lower-case)
– May be followed by any number (including 0) of letters and
digits
– The characters $ and _ are considered letters
– Java identifier names are case sensitive
– May not duplicate a Java keyword (e.g. class or main)

Control Structures
 Loops: for, while, do while
 If, if else, if else if, switch case

Class and object
 Class is a user defined data type.
 Object is variable of a class
– Data Members and Methods
 Constructor
 Method Overloading
 Inheritance
 Method Overriding

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