How do I run Java class files

Question

I ve compiled a HelloWorld program  and I m using the command prompt to run it   The  class file is named HelloWorld2 class  The file is located in C  Users Matt workspace HelloWorld2 bin Here s what I m getting when I go to command prompt  and type  Java HelloWorld2     C  Users Matt gt Java HelloWorld2 Exception in thread  main  java lang NoClassDefFoundError  HelloWorld2 Caused by  java lang ClassNotFoundException  HelloWorld2         at java net URLClassLoader 1 run Unknown Source          at java security AccessController doPrivileged Native Method          at java net URLClassLoader findClass Unknown Source          at java lang ClassLoader loadClass Unknown Source          at sun misc Launcher AppClassLoader loadClass Unknown Source          at java lang ClassLoader loadClass Unknown Source  Could not find the main class  HelloWorld2   Program will exit    I was expecting to see a HelloWorld printed out   What am I doing wrong   I have the JDK installed

User · Answer

You have to put java in lower case and you have to add  class   java HelloWorld2 class

User · Answer

To run Java class file from the command line  the syntax is   java -classpath  path to jars  lt packageName gt   lt MainClassName gt    where packageName  usually starts with either com or org  is the folder name where your class file is present   For example if your main class name is App and Java package name of your app is com foo app  then your class file needs to be in com foo app folder  separate folder for each dot   so you run your app as     java com foo app App   Note    is indicating shell prompt  ignore it when typing  If your class doesn t have any package name defined  simply run as  java App   If you ve any other jar dependencies  make sure you specified your classpath parameter either with -cp -classpath or using CLASSPATH variable which points to the folder with your jar war ear zip class files  So on Linux you can prefix the command with  CLASSPATH  path to jars  on Windows you need to add the folder into system variable  If not set  the user class path consists of the current directory         Practical example  Given we ve created sample project using Maven as     mvn archetype generate -DgroupId com foo app -DartifactId my-app -DarchetypeArtifactId maven-archetype-quickstart -DinteractiveMode false    and we ve compiled our project by mvn compile in our my-app  dir  it ll generate our class file is in target classes com foo app App class   To run it  we can either specify class path via -cp or going to it directly  check examples below     find   -name    class    target classes com foo app App class   CLASSPATH target classes  java com foo app App Hello World    java -cp target classes com foo app App Hello World    java -classpath    path to other-jars target classes com foo app App Hello World    cd target classes  amp  amp  java com foo app App Hello World    To double check your class and package name  you can use Java class file disassembler tool  e g      javap target classes com foo app App class Compiled from  App java  public class com foo app App     public com foo app App      public static void main java lang String         Note  javap won t work if the compiled file has been obfuscated

User · Answer

You need to set the classpath to find your compiled class   java -cp C  Users Matt workspace HelloWorld2 bin HelloWorld2

User · Answer

Go to the path where you saved the java file you want to compile  Replace path by typing cmd and press enter  Command Prompt Directory will pop up containing the path file like C  blah blah foldercontainJava Enter javac javafile java Press Enter  It will automatically generate java class file

User · Answer

This can mean a lot of things  but the most common one is that the class contained in the file doesn t have the same name as the file itself  So  check if your class is also called HelloWorld2

[java] How do I run Java .class files?

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