This document provides an introduction to Java, including descriptions of the Java ecosystem, the Java Development Kit (JDK), Java Runtime Environment (JRE), and key features of Java such as portability, object orientation, and security. The JDK contains development tools like compilers and debuggers as well as the JRE, which runs Java programs. The JRE includes the Java Virtual Machine (JVM) and class libraries. The JVM interprets bytecode into machine code and includes a just-in-time (JIT) compiler to improve performance. Java's portability comes from writing bytecode rather than platform-specific machine code.

1. Introduction to Java
Sujit Kumar
Zenolocity LLC

2. Java Eco System
• Java Language
• Java Development Kit (JDK)
• Java Runtime Environment (JRE)

3. What is JDK?
• Java Development Kit (Development Tools + JRE )
• Contains tools to develop java programs and a
JRE to run the java programs.
• Tools include:
1. Compiler (javac)
2. Java Application launcher (java)
3. Debugger (jdb)
4. Generate Java Documentation (javadoc)

4. Most Important Features of Java
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Portable (Write Once, Run Anywhere)
Interpreted
Object Oriented
Distributed
Secure
Scalable

5. Portable – Write Once, Run Anywhere
• In Java, compilation of source code produces
machine independent byte code that can run
on any OS or hardware that has a JVM.
• JVM interprets this byte code and converts to
machine dependent executable code.
• In C/C++, compilation of source code produces
machine dependent executable code. Source
code needs to be recompiled for every OS or
hardware.

6. What is JRE?
• JRE = JVM (includes JIT) + class libraries
• No development tools like compiler, debugger.
• JVM actually runs the program. JVM is called virtual
because it provides an interface that does not depend
on the underlying operating system and hardware.
• Every vendor of a hardware platform provides a
platform specific JRE.
• JRE is platform dependent, byte code is platform
independent.
• Java application launcher (java) opens a JRE, loads a
class and invokes its main method.

7. JIT Compiler
• The byte code is compiled when it is about to
be executed (hence the name "just-intime"), and then cached and reused later
without needing to be recompiled.
• In contrast, a traditional interpreted virtual
machine will simply interpret the byte
code, generally with much lower performance.

8. JDK, JRE, JVM, JIT

9. Compare Java to C/C++