Java Basics

What type of code is a .java file?

human-readable source code

What type of code is a .class file?

compiled java bytecode that contains machine instructions for the JVM to run the program

What is a .jar file?

a java archive that bundles class files and other resources including a manifest

When does a JAR become runnable?

when the Manifest specifies a Main-Class, which tells Java which class to execute

How do you run a JAR?

java -jar Program.jar

How do you compile a Java source?

javac

How do you run a class?

java ClassName

How do you get the number of arguments?

args.length

JDK

Java Development Kit

What does the JDK do?

it is the development toolkit that is used for developing/compiling Java programs

JRE

Java Runtime Environment

What does a JRE do?

this environment need to run .class Java files which includes runtime libraries and files

JVM

Java Virtual Machine

What does a JVM do?

executes the actual Java bytecode and interfaces with the underlying operating system as an implementation of a JRE

JIT

Just-In-Time compiler

What does the JIT do?

it is part of the JVM that translates specific JVM instructions to CPU instructions

Why is Java portable?

you compile Java into JVM bytecode rather than directly into instructions for one specific operating system/CPU and then the JVM handles the platform-specific execution

Java’s main idea

“Write once, run anywhere”

A main-class must have a…

public static void main(String[ ] ) method