Could not find main class HelloWorld

Question

I installed Java 1 7 0 in the following folder C  Program Files Java    My operating system is Windows XP Version 2002  with Service pack 3   The environment variables which I set are    CLASSPATH   C  Program Files Java jdk1 7 0 jre lib rt jar  Path   C  Program Files Java jdk1 7 0 bin  JAVA HOME   C  Program Files Java    I have presented here the class names which are in my system    Next I wrote a program  HelloWorld java   import java io     class HelloWorld       public static void main String   args                 System out println  Hello World               When I am compiling using javac HelloWorld java it is compiling fine   But after I issue java HelloWorld I am encountering the below error   Error  Could not find main class HelloWorld Exception in thread  main  java lang NoClassDefFoundError  HelloWorld         at sun launcher LauncherHelper checkAndLoadMain LauncherHelper java 198   Caused by  java lang ClassNotFoundException  HelloWorld         at java net URLClassLoader 1 run URLClassLoader java 299          at java net URLClassLoader 1 run URLClassLoader java 288          at java security AccessController doPrivileged Native Method          at java net URLClassLoader findClass URLClassLoader java 287          at java lang ClassLoader loadClass ClassLoader java 422          at sun misc Launcher AppClassLoader loadClass Launcher java 325          at java lang ClassLoader loadClass ClassLoader java 355          at sun launcher LauncherHelper checkAndLoadMain LauncherHelper java 195    After a bit of searching around  I found that may be something wrong in the environment variable  I tried to play with that but no luck   I even RESTARTED the machine and then again I tried to run but with same fate

User · Answer

JAVA HOME is not necessary if you start java and javac from the command line  But JAVA HOME should point to the real jdk directory  C  Program Files Java jdk1 7 0 in your case   I d never use the CLASSPATH environment variable outside of build scripts  especially not global defined  The -cp flag is better  But in your case  as you do not need additional libraries  rt jardoesn t count   you won t need a classpath declaration  A missing -cp is equivalent to a -cp   and that s what you need here   The HelloWorld class needs to be declared as public  This actually may be the cause for your problems   I was pretty sure  that a source file needs one public class    or was it one public class at most

User · Answer

Java is not finding where your compiled class file  HelloWorld class  is  It uses the directories and JAR-files in the CLASSPATH environment variable for searching if no -cp or -classpath option is given when running java exe     You don t need the rt jar in the CLASSPATH  these was only needed for older versions of Java  You can leave it undefined and the current working directory will be used  or just add    a single point   separated by      to the CLASSPATH variable to indicate the current directory     CLASSPATH    C      some jar    Alternatively you can use the -cp or -classpath option   java -cp   HelloWorld   And  as Andreas wrote  JAVA HOME is not needed by Java  just for some third-party tools like ant  but should point to the correct location

User · Answer

Tell it where to look for you class  it s in      which is the current directory   java -classpath   HelloWorld   No need to set JAVA HOME or CLASSPATH in this case

User · Answer

It looks that you had done all setup properly but there might be one area where it might be causing problem  Check the value of your  CLASSPATH  variable and make sure at the end you kept      Note     is for end separator   is for including existing path at the end

User · Answer

Just remove your  classpath  from you environment variable  Then try running   java HelloWorld    This should work fine

User · Answer

You are not setting a classpath that includes your compiled class  java can t find any classes if you don t tell it where to look   java -cp  compiler outpur dir  HelloWorld   Incidentally you do not need to set CLASSPATH the way you have done

User · Answer

put    at classpath value in beginning  it will start working   it happens because it searches the class file in classpath which is mentioned in path variable

User · Answer

You either want to add     to your CLASSPATH to specify the current directory  or add it manually at run time the way unbeli suggested

User · Answer

I have also faced same problem       Actually this problem is raised due to the fact that your program  class files are not saved in that directory  Remove your CLASSPATH from your environment variable  you do no need to set classpath for simple Java programs  and reopen cmd prompt  then compile and execute   If you observe carefully your  class file will save in the same location   I am not an expert  I am also basic programer if there is any mistake in my sentences please ignore it  -

User · Answer

I had the same problem  Perhaps  the problem is that you have compiled and executed the class with different Java versions   Make sure the version of the compiler is the same as the command  java    javac -version  java -version   In Linux  use   sudo update-alternatives --config java  to change the version of Java

[java] Could not find main class HelloWorld

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