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It explains Java 9 concept areas, platform independence, bytecode and the JVM/JRE/JDK distinctions, and the ClassLoader execution order. 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Reference variables store a reference (memory address) to an object created in memory.","https://schema.org",{"og:url":78,"og:type":120,"og:title":59,"og:site_name":91,"og:description":61},"article",{"robots":122,"canonical":78},"index,follow",{"doc_id":124,"site_id":56},573479,1792051011,{"code":4,"msg":5,"data":127},{"doc_id":124,"user_id":128,"nickname":88,"user_avatar":129,"doc_module":9,"category_id":8,"category_name":11,"doc_title":59,"doc_description":61,"doc_content":130,"file_id":131,"file_url":132,"file_type":133,"file_size":134,"view_count":73,"is_deleted":4,"is_public":9,"is_downloadable":9,"audit_status":9,"page_count":135,"language":136,"language_code":57,"site_id":56,"html_lang":57,"table_of_contents":137,"faqs":138,"seo_title":139,"seo_description":61,"update_tm":140,"read_time":141},3985747872567,"https://ap-avatar.wpscdn.com/davatar_276721f389ce27ea32af1340a28f341c","Java Tutorial For Beginners-A Cheat Sheet  \n\n| Review Java 9 Concepts at Jet Speed.\u003Cbr>Complete Java Course\u003Cbr>\u003Cbr>\u003Cbr>|\n| --- |\n| |\n| Introduction |\n| Background |\n\nPopularity of Java  \n Platform Independent or Portable  \n Object Oriented Language  \n Security  \n Rich API  \n Great IDE's  \n Omnipresent  \n Web Applications (Java EE (JSP, Servlets), Spring, Struts..)  \n Mobile Apps(Android)  \n Microservices (Spring Boot)  \nPlatform Independence  \n Build once, run anywhere  \n Java bytecode is the instruction set of the Java virtual machine  \ngraph TD  \nA[Java Code] -->|Compiled| B(Bytecode) B--> C{Run}  \nC-->|bytecode| D[Windows JVM] D--> K[Windows Instructions] C-->|bytecode| E[Unix JVM] E--> L[Unix Instructions] C-->|bytecode| F[Linux JVM] F--> M[Linux Instructions]  \nC-->|bytecode| G[Any other platform JVM] G--> N[Linux Instructions]  \nJDK vs JVM VS JRE  \n JVM (Java Virtual Machine)  \n runs the Java bytecode.  \n JRE  \n JVM + Libraries + Other Components (to run applets and other java applications)  JDK  \n JRE + Compilers + Debuggers  \nClassLoader  \n Find and Loads Java Classes! Three Types  \n System Class Loader-Loads all application classes from CLASSPATH  \n Extension Class Loader-Loads all classes from extension directory  \n Bootstrap Class Loader-Loads all the Java core files Order of execution of ClassLoaders  \n JVM needs to find a class, it starts with System Class Loader.  \n If it is not found, it checks with Extension Class Loader.  \n If it not found, it goes to the Bootstrap Class Loader.  \n If a class is still not found, a ClassNotFoundException is thrown.  \nFirst Java Program  \npublic class HelloWorld {  \npublic static void main (String[] args) { System .out .println ( \"Hello World\" );  \n}  \n}  \nNotes  \n Every line of code we write in Java is part of something called Class. We will talk about Class later.  \n First line defines a public class called HelloWorld. All the code in a class is between { and } .  \n When a program runs, Java should know which line of code has to be run first. public static void main(String[] args) is the first method that is run when a program is executed.  \n Java, like any other programming language, is particular about syntax!!  \nUsing Java and JavaC  \nThere are two steps involved in running a Java Program  \n Compilation  \n Execution  \nCompilation  \nWe use javac to compile java code.  \n Open CommandPrompt/Terminal and cd to the folder where [HelloWorld.java](HelloWorld.java) file is present  \n execute the command below  \njavac [HelloWorld.java](HelloWorld.java)  \n You should see two fi[les HelloWorld.java](les HelloWorld.java) and HelloWorld.class in the folder.  \n HelloWorld.class contains the java bytecode  \nExecution  \n Now we can run the program using JVM  \n execute the command below  \njava HelloWorld  \n You should see the output \"Hello World\" printed in the console.  \nClass and Object  \n What is a class?  \n Definining an instance of a class-an object  \n Invoking a method on the object  \nVariables  \n Value of a variable changes during the course of a program execution.  \nint number;  \nnumber = 5 ;  \nSystem .out .println (number);//5 number = number + 2 ;  \nSystem .out .println (number);//7 number = number + 2 ;  \nSystem .out .println (number);//9  \nDeclaring and Initializing Variables  \n Declaration is give a variable a name and type  \nTYPE variableName;  \nTips  \n Two or more variables of single type can be declared together.  \n Variable can be local or global. The local variables can be referenced (ie, are valid) only within the scope of their method (or function) .  \n All six numeric types in Java are signed.  \nPrimitive Variables  \nVariables that store value.  \nJava defines few types like int (numbers), float(floating point numbers), char (characters) . Variables of these types store the value of the variable directly. These are not objects. These are called primitive variables.  \nAn example is shown below: Primitive Variables contains bits representing the value of the variable.  \nin","cbCailT3oVxDVnDy","https://ap.wps.com/l/cbCailT3oVxDVnDy","pdf",1375497,175,"English","# Introduction\n## Background\n# Popularity of Java\n## Platform independence\n## Bytecode and JVM/JRE/JDK\n# ClassLoader\n## Class loading order\n# First Java Program\n## Compilation and execution\n# Class and Object\n## Variables\n# Primitive and Reference Variables\n## Identifiers and keywords","[{\"question\":\"What are the roles of JVM, JRE, and JDK in running Java programs?\",\"answer\":\"JVM runs Java bytecode. JRE includes JVM plus libraries and other components for running applications. JDK adds compilers and debuggers on top of JRE.\"},{\"question\":\"How does the ClassLoader find and load classes?\",\"answer\":\"JVM first starts with the System Class Loader to look in the CLASSPATH, then checks the Extension Class Loader, and finally the Bootstrap Class Loader. If a class is still not found, ClassNotFoundException is thrown.\"},{\"question\":\"What is the difference between primitive variables and reference variables?\",\"answer\":\"Primitive variables store values directly, such as int, char, boolean, float, and others. Reference variables store a reference (memory address) to an object created in memory.\"}]","Java Tutorial For Beginners-A Cheat Sheet | PDF",1791940659,61]