How to execute a java class from the command line

Question

I have a compiled java class   Echo class  public class Echo       public static void main  String arg                 System out println arg             I cd to the directory and enter  java Echo  hello   I get this error   C  Documents and Settings joe My Documents projects Misc bin gt java Echo  hello  Exception in thread  main  java lang NoClassDefFoundError  Echo Caused by  java lang ClassNotFoundException  Echo         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          at java lang ClassLoader loadClassInternal Unknown Source  Could not find the main class  Echo   Program will exit    What is the simplest way to get my java code in a form that I can run from the command line as apposed to having to use Eclipse IDE

User · Answer

You have no valid main method    The signature should be  public static void main String   args    Hence  in your case the code should look like this   public class Echo       public static void main  String   arg                 System out println arg 0              Edit  Please note that Oscar is also right in that you are missing   in your classpath  you would run into the problem I solve after you have dealt with that error

User · Answer

My situation was a little complicated   I had to do three steps since I was using a  dll in the resources directory  for JNI code   My files were  S  Accessibility tools src main resources dlls HelloWorld dll S  Accessibility tools src test java com accessibility HelloWorld class   My code contained the following line  System load HelloWorld class getResource   dlls HelloWorld dll   getPath       First  I had to move to the classpath directory  cd  D  S  Accessibility tools src test java    Next  I had to change the classpath to point to the current directory so that my class would be loaded and I had to change the classpath to point to he resources directory so my dll would be loaded   set classpath  classpath             src main resources     Then  I had to run java using the classname   java com accessibility HelloWorld

User · Answer

Try   java -cp   Echo  hello      Assuming that you compiled with   javac Echo java    Then there is a chance that the  current  directory is not in your classpath   where java looks for  class definitions     If that s the case and listing the contents of your dir displays   Echo java Echo class   Then any of this may work   java -cp   Echo  hello    or   SET CLASSPATH  CLASSPATH      java Echo  hello    And later as Fredrik points out you ll get another error message like       Exception in thread  main  java lang NoSuchMethodError  main   When that happens  go and read his answer

User · Answer

You need to specify the classpath  This should do it   java -cp   Echo  hello    This tells java to use    the current directory  as its classpath  i e  the place where it looks for classes  Note than when you use packages  the classpath has to contain the root directory  not the package subdirectories  e g  if your class is my package Echo and the  class file is bin my package Echo class  the correct classpath directory is bin

User · Answer

First  have you compiled the class using the command line javac compiler   Second  it seems that your main method has an incorrect signature - it should be taking in an array of String objects  rather than just one   public static void main String   args     Once you ve changed your code to take in an array of String objects  then you need to make sure that you re printing an element of the array  rather than array itself   System out println args 0     If you want to print the whole list of command line arguments  you d need to use a loop  e g   for int i   0  i  lt  args length  i         System out print args i      System out println

User · Answer

If you have in your java source   package mypackage    and your class is hello java with   public class hello     and in that hello java you have    public static void main String   args      Then  after compilation   changeDir  cd  to the directory where your hello class is   Then   java -cp   mypackage hello   Mind the current directory and the package name before the class name  It works for my on linux mint and i hope on the other  os s also  Thanks Stack overflow for a wealth of info

User · Answer

With Java 11 you won t have to go through this rigmarole anymore   Instead  you can do this    gt  java MyApp java   You don t have to compile beforehand  as it s all done in one step   You can get the Java 11 JDK here  JDK 11 GA Release

[java] How to execute a java .class from the command line

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