What does Could not find or load main class mean

Question

A common problem that new Java developers experience is that their programs fail to run with the error message   Could not find or load main class      What does this mean  what causes it  and how should you fix it

User · Answer

What fixed the problem in my case was   Right click on the project class you want to run  then Run As- Run Configurations  Then you should either fix your existing configuration or add new in the following way   open the Classpath tab  click on the Advanced    button then add bin folder of your project

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If this issue is Eclipse-related   Try adding the project to your class path   See the below image     This method worked for me

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By default  Java uses    the current working directory  as the default CLASSPATH  What this means is that when you type a command at the prompt e g  java MyClass  the command is interpreted as if you had type java -cp   MyClass  Did you see that dot between -cp and MyClass   cp is short for the longer classpath option   This is sufficient for most cases and things seems to work just fine until at some time you try to add a directory to your CLASSPATH  In most cases when programmers need to do this  they just run a command like set CLASSPATH path to some dir  This command creates a new environment variable called CLASSPATH having the value path to some dir or replaces its value with path to some dir if CLASSPATH was already set before   When this is done  you now have a CLASSPATH environment variable and Java no longer uses its default classpath     but the one you ve set  So the next day you open your editor  write some java program  cd to the directory where you saved it  compile it  and try to run it with the command java MyClass  and you are greeted with a nice output  Could not find or load main class      If your commands were working well before and you are now getting this output  then this might be the case for you    What happens is that when you run the command java MyClass  Java searches for the class file named MyClass in the directory or directories that you have set in your CLASSPATH and not your current working directory so it doesn t find your class file there and hence complains   What you need to do is add   to your class path again which can be done with the command set CLASSPATH  CLASSPATH     notice the dot after the semicolon   In plain english this command says  Pick what was initially the value of CLASSPATH   CLASSPATH    add   to it      and assign the result back to CLASSPATH    And voila  you are once again able to use your command java MyClass as usual

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Excluding the following files solved the problem  META-INF   SF META-INF   DSA META-INF   RSA Added the following code in build gradle jar       from           configurations compile collect   it isDirectory     it   zipTree it                        exclude  quot META-INF   SF quot          exclude  quot META-INF   DSA quot          exclude  quot META-INF   RSA quot            manifest           attributes                   Main-Class    mainclass

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This is a specific case  but since I came to this page looking for a solution and didn t find it  I ll add it here   Windows  tested with 7  doesn t accept special characters  like     in class and package names  Linux does  though   I found this out when I built a  jar in NetBeans and tried to run it in command line  It ran in NetBeans but not in command line

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Java Version  11  If you are using Java 11 then you don t need to compile and run your java file  Just run like Java ClassName java Example  class abc       public static void main String   args           System out println  quot hello Jarvis  quot                 Now Run the command java abc java

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Reason  2 - the application s classpath is incorrectly specified Read the three documents linked above   Yes     READ them  It is important that a Java programmer understands at least the basics of how the Java classpath mechanisms works   I want to add this documentation to this very good post from above     JDK Tools and Utilities     General    General Information  file structure  classpath  how classes are found  changes  Enhancements enhancements in JDK 7  Standard JDK Tools and Utilities https   docs oracle com javase 7 docs technotes tools index html https   docs oracle com javase 7 docs technotes tools findingclasses html https   docs oracle com javase 7 docs technotes tools windows classpath html How the Java Launcher Finds Classes Understanding the class path and package names https   docs oracle com javase 7 docs technotes tools solaris javac html ClassLoader in Java The Java ClassLoader is a part of the Java Runtime Environment that dynamically loads Java classes into the Java Virtual Machine  The Java run time system does not need to know about files and file systems because of classloaders  Java classes aren   t loaded into memory all at once  but when required by an application  At this point  the Java ClassLoader is called by the JRE and these ClassLoaders load classes into memory dynamically  https   en wikipedia org wiki Java Classloader https   www geeksforgeeks org classloader-in-java  https   en wikipedia org wiki Java virtual machine

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in Intellij Check your Global libraries and local libraries check in pom xml the libraries version   maybe it is an old library they are so many possibilities mention above that need to be tried too

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Here s another issue that took me a bit of time  The command line class path param doesn t behave as you d expect  I m on MacOS calling the CLI directly  and I m including two jars in the call   For example  both of these were confusing the tool about the name of the main class   This one because the asterisk was causing it to parse the args incorrectly   java -cp path to jars   com mypackage Main  And this one because -- I m not sure why   java -cp    jar  com mypackage Main  This worked   java -cp  path to jars    com mypackage Main  Listing the two jars explicitly also worked   java -cp path to jars jar1 jar path to jars jar2 jar com mypackage Main

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For a database connection I was getting this one  I just added the following in the class path   export CLASSPATH  lt path gt  db2jcc4 jar          Here I appended    in last so that my class also get identified while loading   Now just run   java ConnectionExample  lt args gt    It worked perfectly fine

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In this instance you have      Could not find or load main class  classpath   It s because you are using  -classpath   but the dash is not the same dash used by java on the command prompt  I had this issue copying and pasting from Notepad to cmd

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When the same code works on one PC  but it shows the error in another  the best solution I have ever found is compiling like the following   javac HelloWorld java java -cp   HelloWorld

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I was unable to solve this problem with the solutions stated here  although the answer stated has  no doubt  cleared my concepts   I faced this problem two times and each time I have tried different solutions  in the Eclipse IDE     Firstly  I have come across with multiple main methods in different classes of my project  So  I had deleted the main method from subsequent classes  Secondly  I tried following solution    Right click on my main project directory  Head to source then clean up and stick with the default settings and on Finish  After some background tasks you will be directed to your main project directory   After that I close my project  reopen it  and boom  I finally solved my problem

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If it s a Maven project    Go to the POM file  Remove all the dependencies  Save the POM file  Again import only the necessary dependencies   Save the POM file    The issue should go away

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I got this issue for my demo program created in IntelliJ   There are two key points to solve it    the package name of program the current working dir of the terminal cmd prompt    my demo program   package io rlx tij c2   public class Ex10       public static void main String   args               do something           the path of source code       projectRoot src main java io rlx tij c2 Ex10 java    go to java dir  cd    projectRoot src main java compile to class  javac   io rlx tij c2 Ex10 java run program  java io rlx tij c2 Ex10   if I run program in    projectRoot src main java io rlx tij c2 or I run it without package name  I will get this error  Error  Could not find or load main class

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Class file location          C  test com company  File Name                    Main class  Fully qualified class name   com company Main  Command line command     java  -classpath  C  test  com company Main   Note here that class path does NOT include  com company

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I had same problem and finally found my mistake    I used this command for compiling and it worked correctly   javac -cp   home omidmohebbi AAAATest jars core-1 7 jar  home omidmohebbi AAAATest jars javase-1 7 jar  home omidmohebbi AAAATest jars qrgen-1 2 jar  qrcode java   But this command did not work for me  I could not find or load the main class  qrcode    java -cp   home omidmohebbi AAAATest jars core-1 7 jar  home omidmohebbi AAAATest jars javase-1 7 jar  home omidmohebbi AAAATest jars qrgen-1 2 jar  qrcode   Finally I just added the     character at end of the classpath and the problem was solved   java -cp   home omidmohebbi AAAATest jars core-1 7 jar  home omidmohebbi AAAATest jars javase-1 7 jar  home omidmohebbi AAAATest jars qrgen-1 2 jar   qrcode

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Sometimes what might be causing the issue has nothing to do with the main class  and I had to find this out the hard way  It was a referenced library that I moved  and it gave me the      Could not find or load main class xxx Linux   I just deleted that reference  added it again  and it worked fine again

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On Windows put    at the CLASSPATH value in the beginning   The    dot  means  look in the current directory   This is a permanent solution   Also you can set it  one time  with set CLASSPATH  CLASSPATH     This will last as long as your cmd window is open

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This is how I solved my issue  I noticed if you are including jar files with your compilation  adding the current directory      to the classpath helps  javac -cp  quot abc jar efg jar quot  MyClass java java -cp  quot abc jar efg jar quot  MyClass  VS   javac -cp  quot    abc jar efg jar quot  MyClass java java -cp  quot    abc jar efg jar quot  MyClass

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I spent a decent amount of time trying to solve this problem  I thought that I was somehow setting my classpath incorrectly but the problem was that I typed     java -cp C  java MyClasses C  java MyClasses utilities myapp Cool     instead of      java -cp C  java MyClasses utilities myapp Cool      I thought the meaning of fully qualified meant to include the full path name instead of the full package name

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In my case  I got the error because I had mixed UPPER- and lower-case package names on a Windows 7 system   Changing the package names to all lower case resolved the issue  Note also that in this scenario  I got no error compiling the  java file into a  class file  it just wouldn t run from the same  sub-sub-sub-  directory

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I got this error after doing mvn eclipse eclipse This messed up my  classpath file a little bit   Had to change the lines in  classpath from   lt classpathentry kind  src  path  src main java  including       java   gt   lt classpathentry kind  src  path  src main resources  excluding       java   gt    to    lt classpathentry kind  src  path  src main java  output  target classes    gt   lt classpathentry kind  src  path  src main resources  excluding       output  target classes    gt

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I had a weird one      Error  Could not find or load main class mypackage App   It turned out I had a reference to POM  parent  coded up in my project s pom xml file  my project s pom xml was pointing to a parent pom xml  and the relativePath was off wrong   Below is a partial of my project s pom xml file    lt parent gt       lt groupId gt myGroupId lt  groupId gt       lt artifactId gt pom-parent lt  artifactId gt       lt version gt 0 0 1-SNAPSHOT lt  version gt       lt relativePath gt    badPathHere pom xml lt  relativePath gt   lt  parent gt    Once I resolved the POM relativePath  the error went away   Go figure

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Right click the project    Select  Open module settings  Mark the src folder as  sources  Go to edit configurations  and then select your main class Click OK or the Apply button   This worked for me

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In my case  error appeared because I had supplied the source file name instead of the class name    We need to supply the class name containing the main method to the interpreter

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Seems like when I had this problem  it was unique   Once I removed the package declaration at the top of the file  it worked perfectly   Aside from doing that  there didn t seem to be any way to run a simple HelloWorld java on my machine  regardless of the folder the compilation happened in  the CLASSPATH or PATH  parameters or folder called from

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In Java  when you sometimes run the JVM from the command line using the java executable and are trying to start a program from a class file with public static void main  PSVM   you might run into the below error even though the classpath parameter to the JVM is accurate and the class file is present on the classpath   Error  main class not found or loaded   This happens if the class file with PSVM could not be loaded  One possible reason for that is that the class may be implementing an interface or extending another class that is not on the classpath  Normally if a class is not on the classpath  the error thrown indicates as such  But  if the class in use is extended or implemented  java is unable to load the class itself   Reference  https   www computingnotes net java error-main-class-not-found-or-loaded

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I ran into this error while trying to execute this code written in intellij idea on onlinegdb compiler  For me commenting out this line helped me prevent this error    package com company

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Sometimes  in some online compilers that you might have tried you will get this error if you don t write public class  Classname  but just class  Classname

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Try -Xdiag   Steve C s answer covers the possible cases nicely  but sometimes to determine whether the class could not be found or loaded might not be that easy  Use java -Xdiag  since JDK nbsp 7   This prints out a nice stacktrace which provides a hint to what the message Could not find or load main class message means   For instance  it can point you to other classes used by the main class that could not be found and prevented the main class to be loaded

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If your source code name is HelloWorld java  your compiled code will be HelloWorld class   You will get that error if you call it using   java HelloWorld class   Instead  use this   java HelloWorld

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This happened to me too  In my case  it only happened when a HttpServlet class was present in source code  IntelliJ IDEA didn t give a compile time error  the servlet package got imported just fine  however at run time there was this main class error    I managed to solve it  I went to menu File  rarr  Project Structure        Then to Modules     There was a Provided scope near the servlet module  I changed it to Compile     And it worked

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In the context of IDE development  Eclipse  NetBeans or whatever  you have to configure your project properties to have a main class  so that your IDE knows where the main class is located to be executed when you hit  Play     Right click your project  then Properties Go to the Run category and select your Main Class Hit the Run button

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After searching for 2 days I found this solution and this works  It is pretty weird but it works for me  package javaapplication3  public class JavaApplication3            param args the command line arguments     public static void main String   args           TODO code application logic here     System out println  quot Hello quot         this is my program i want to run that locates at C  Java Projects JavaApplication3 src javaapplication3 Now open cmd on this location and compile program using this command javac JavaApplication3 java  After compiling navigate one directory down i e  C  Java Projects JavaApplication3 src now run following command to execute program java javaapplication3 JavaApplication3

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Answering with respect to an external library -  Compile   javac -cp    lt external lib jar gt    lt program-name gt  java   Execute   java -cp    lt external lib jar gt    lt program-name gt    The above scheme works well in OS nbsp X and Linux systems  Notice the   in the classpath

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One more scenario that got me scratch my head  and I found no reference to it herein  is  package com me Public class Awesome extends AwesomeLibObject                public static void main String   argv             System out println  quot YESS0 quot             Where AwesomeLibObject is a class defined in an external lib  I got the same confusing error message for it  Error  Could not find or load main class com Awesome The resolution is simple  the external lib must be in classpath as well

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Solving  Could not Load main class error   After reading all the answers  I noticed most didn t work for me  So I did some research and here is what I got  Only try this if step 1 doesn t work    Try to install JRE 32 or 64  If it doesn t work  Open go to C  Program Files  x86  Java or C  Program Files Java   i  open the jdk folder and then the bin folder  ii  Copy the path and add it to environment variables  Make sure you separate variables with a semi-colon     For example   C  Yargato bin C  java bin    If you don t  it will cause more errors  iii  Go to the jre folder and open its bin folder  iv  Here search for rt jar file  Mine is      C  Program Files  x86  Java jre1 8 0 73 lib rt jar   Copy and under environment variable and search for the classpath variable   and paste it there   v  Now restart cmd and try running again  The error will disappear  vi  I will post a link to my YouTube video tutorial

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If your classes are in packages then you have to cd to the root directory of your project and run using the fully qualified name of the class  packageName MainClassName    Example   My classes are in here   D  project com cse    The fully qualified name of my main class is   com cse Main   So I cd back to the root project directory   D  project   Then issue the java command   java com cse Main   This answer is for rescuing newbie java programmers from the frustration caused by a common mistake  I recommend you read the accepted answer for more in depth knowledge about the java classpath

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I also faced similar errors while testing a Java MongoDB JDBC connection  I think it s good to summarize my final solution in short so that in the future anybody can directly look into the two commands and are good to proceed further   Assume you are in the directory where your Java file and external dependencies  JAR files  exist   Compile   javac -cp mongo-java-driver-3 4 1 jar JavaMongoDBConnection java    -cp - classpath argument  pass all the dependent JAR files one by one   java - This is the Java class file which has main method  sdsd   Run   java -cp mongo-java-driver-3 4 1 jar  JavaMongoDBConnection    Please do observe the colon  Unix    comma  Windows  after all the dependency JAR files end At the end  observe the main class name without any extension  no  class or  java

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First set the path using this command   set path  paste the set path address    Then you need to load the program  Type  cd  folder name   in the stored drive and compile it  For Example  if my program stored on the D drive  type  D   press enter and type   cd  folder name

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In my case I just change the JRE in Eclipse   Please find the attached screen shot

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If you use Maven to build the JAR file  please make sure to specify the main class in the pom xml file    lt build gt       lt plugins gt           lt plugin gt               lt artifactId gt maven-jar-plugin lt  artifactId gt               lt configuration gt                   lt archive gt                       lt manifest gt                           lt mainClass gt class name us com test abc MyMainClass lt  mainClass gt                       lt  manifest gt                   lt  archive gt               lt  configuration gt           lt  plugin gt       lt  plugins gt   lt  build gt

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When running the java with the -cp option as advertised in Windows PowerShell you may get an error that looks something like   The term  ClassName  is not recognized as the name of a cmdlet  function  script       In order to for PowerShell to accept the command  the arguments of the -cp option must be contained in quotes as in   java -cp  someDependency jar    ClassName   Forming the command this way should allow Java process the classpath arguments correctly

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If you use IntelliJ and get the error while running the main method from the IDE  just make sure your class is located in java package  not in kotlin

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I had such an error in this case   java -cp lib jar com mypackage Main   It works with   for Windows and   for Unix   java -cp lib jar  com mypackage Main

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Scenario  using command prompt CMD in Windows  for compile and run a simple  java  program which have only  Main java  file  with specified  package main   Source file path    some-project-name-folder src main Main java  Destination folder    some-project-name-folder dest  Destination file path  folder   main  and file   Main class  will be produced by  javac      some-project-name-folder dest main Main class  Main java is as follow   package main   public class Main       public static void main String   args            System out println  quot Hello world quot             Compilation       javac  compiler will produce  Main class  in the  dest main  folder      main  folder is created because in the source file in our case   Main java   is    specified  package main    javac -d   dest   src main Main java  Run compiled file  in our case   Main class         -cp  is the same as  -classpath         dest  means destination folder  where resides compiled  Main class        main Main  means  package   main   which contains class  Main   Main class       WARNING  when run  java  class  MUST NOT type extension   class              after  class name              in our case   main Main   lt package gt   lt class-name gt   WITHOUT extension                 class     java -cp   dest main Main     Hello world

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All answers here are directed towards Windows users it seems  For Mac  the classpath separator is    not    As an error setting the classpath using   is not thrown then this can be a difficult to discover if coming from Windows to Mac   Here is corresponding Mac command   java -classpath      lib    com test MyClass   Where in this example the package is com test and a lib folder is also to be included on classpath

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This might help you if your case is specifically like mine   as a beginner I also ran into this problem when I tried to run a Java program   I compiled it like this   javac HelloWorld java   And I tried to run also with the same extension   java Helloworld java   When I removed the  java and rewrote the command like java HelloWorld  the program ran perfectly

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What helped me was specifying the classpath on the command line  for example    Create a new folder  C  temp Create file Temp java in C  temp  with the following class in it   public class Temp       public static void main String args              System out println args 0             Open a command line in folder C  temp  and write the following command to compile the Temp class   javac Temp java  Run the compiled Java class  adding the -classpath option to let JRE know where to find the class   java -classpath C  temp Temp Hello

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According to the error message   Could not find or load main class    there are two categories of problems    Main class could not be found Main class could not be loaded  this case is not fully discussed in the accepted answer    Main class could not be found when there is typo or wrong syntax in the fully qualified class name or it does not exist in the provided classpath   Main class could not be loaded when the class cannot be initiated  typically the main class extends another class and that class does not exist in the provided classpath   For example   public class YourMain extends org apache camel spring Main   If camel-spring is not included  this error will be reported

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The java  lt class-name gt  command syntax First of all  you need to understand the correct way to launch a program using the java  or javaw  command  The normal syntax1 is this      java    lt options gt     lt class-name gt    lt arg gt        where  lt option gt  is a command line option  starting with a  quot - quot  character    lt class-name gt  is a fully qualified Java class name  and  lt arg gt  is an arbitrary command line argument that gets passed to your application   1 - There are some other syntaxes which are described near the end of this answer  The fully qualified name  FQN  for the class is conventionally written as you would in Java source code  e g      packagename packagename2 packagename3 ClassName  However some versions of the java command allow you to use slashes instead of periods  e g      packagename packagename2 packagename3 ClassName  which  confusingly  looks like a file pathname  but isn t one   Note that the term fully qualified name is standard Java terminology     not something I just made up to confuse you  -  Here is an example of what a java command should look like      java -Xmx100m com acme example ListUsers fred joe bert  The above is going to cause the java command to do the following   Search for the compiled version of the com acme example ListUsers class  Load the class  Check that the class has a main method with signature  return type and modifiers given by public static void main String       Note  the method argument s name is NOT part of the signature   Call that method passing it the command line arguments   quot fred quot    quot joe quot    quot bert quot   as a String     Reasons why Java cannot find the class When you get the message  quot Could not find or load main class     quot   that means that the first step has failed   The java command was not able to find the class   And indeed  the  quot     quot  in the message will be the fully qualified class name that java is looking for  So why might it be unable to find the class  Reason  1 - you made a mistake with the classname argument The first likely cause is that you may have provided the wrong class name    Or     the right class name  but in the wrong form     Considering the example above  here are a variety of wrong ways to specify the class name   Example  1 - a simple class name  java ListUser  When the class is declared in a package such as com acme example  then you must use the full classname including the package name in the java command  e g  java com acme example ListUser   Example  2 - a filename or pathname rather than a class name  java ListUser class java com acme example ListUser class   Example  3 - a class name with the casing incorrect  java com acme example listuser   Example  4 - a typo java com acme example mistuser   Example  5 - a source filename  except for Java 11 or later  see below  java ListUser java   Example  6 - you forgot the class name entirely java lots of arguments    Reason  2 - the application s classpath is incorrectly specified The second likely cause is that the class name is correct  but that the java command cannot find the class   To understand this  you need to understand the concept of the  quot classpath quot    This is explained well by the Oracle documentation   The java command documentation Setting the Classpath  The Java Tutorial - PATH and CLASSPATH  So     if you have specified the class name correctly  the next thing to check is that you have specified the classpath correctly   Read the three documents linked above    Yes     READ them   It is important that a Java programmer understands at least the basics of how the Java classpath mechanisms works   Look at command line and   or the CLASSPATH environment variable that is in effect when you run the java command   Check that the directory names and JAR file names are correct  If there are relative pathnames in the classpath  check that they resolve correctly     from the current directory that is in effect when you run the java command  Check that the class  mentioned in the error message  can be located on the effective classpath  Note that the classpath syntax is different for Windows versus Linux and Mac OS   The classpath separator is   on Windows and   on the others   If you use the wrong separator for your platform  you won t get an explicit error message   Instead  you will get a nonexistent file or directory on the path that will be silently ignored    Reason  2a - the wrong directory is on the classpath When you put a directory on the classpath  it notionally corresponds to the root of the qualified name space   Classes are located in the directory structure beneath that root  by mapping the fully qualified name to a pathname   So for example  if  quot  usr local acme classes quot  is on the class path  then when the JVM looks for a class called com acme example Foon  it will look for a  quot  class quot  file with this pathname     usr local acme classes com acme example Foon class  If you had put  quot  usr local acme classes com acme example quot  on the classpath  then the JVM wouldn t be able to find the class  Reason  2b - the subdirectory path doesn t match the FQN If your classes FQN is com acme example Foon  then the JVM is going to look for  quot Foon class quot  in the directory  quot com acme example quot    If your directory structure doesn t match the package naming as per the pattern above  the JVM won t find your class   If you attempt rename a class by moving it  that will fail as well     but the exception stacktrace will be different   It is liable to say something like this  Caused by  java lang NoClassDefFoundError   lt path gt   wrong name   lt name gt    because the FQN in the class file doesn t match what the class loader is expecting to find    To give a concrete example  supposing that   you want to run com acme example Foon class  the full file path is  usr local acme classes com acme example Foon class  your current working directory is  usr local acme classes com acme example    then    wrong  FQN is needed java Foon    wrong  there is no  com acme example  folder in the current working directory java com acme example Foon    wrong  similar to above java -classpath   com acme example Foon    fine  relative classpath set java -classpath          com acme example Foon    fine  absolute classpath set java -classpath  usr local acme classes com acme example Foon  Notes   The -classpath option can be shortened to -cp in most Java releases   Check the respective manual entries for java  javac and so on  Think carefully when choosing between absolute and relative pathnames in classpaths   Remember that a relative pathname may  quot break quot  if the current directory changes   Reason  2c - dependencies missing from the classpath The classpath needs to include all of the other  non-system  classes that your application depends on    The system classes are located automatically  and you rarely need to concern yourself with this    For the main class to load correctly  the JVM needs to find   the class itself  all classes and interfaces in the superclass hierarchy  e g  see Java class is present in classpath but startup fails with Error  Could not find or load main class  all classes and interfaces that are referred to by means of variable or variable declarations  or method call or field access expressions    Note  the JLS and JVM specifications allow some scope for a JVM to load classes  quot lazily quot   and this can affect when a classloader exception is thrown   Reason  3 - the class has been declared in the wrong package It occasionally happens that someone puts a source code file into the the wrong folder in their source code tree  or they leave out the package declaration   If you do this in an IDE  the IDE s compiler will tell you about this immediately   Similarly if you use a decent Java build tool  the tool will run javac in a way that will detect the problem   However  if you build your Java code by hand  you can do it in such a way that the compiler doesn t notice the problem  and the resulting  quot  class quot  file is not in the place that you expect it to be  Still can t find the problem  There lots of things to check  and it is easy to miss something   Try adding the -Xdiag option to the java command line  as the first thing after java    It will output various things about class loading  and this may offer you clues as to what the real problem is  Also  consider possible problems caused by copying and pasting invisible or non-ASCII characters from websites  documents and so on   And consider  quot homoglyphs quot   where two letters or symbols look the same     but aren t  Finally  you can apparently run into this problem if you try to launch from a JAR file with incorrect signatures in  META-INF   SF    Alternative syntaxes for java There are three alternative syntaxes for the launching Java programs using the java command   The syntax used for launching an  quot executable quot  JAR file is as follows  java    lt options gt    -jar  lt jar-file-name gt    lt arg gt        e g  java -Xmx100m -jar  usr local acme-example listuser jar fred  The name of the entry-point class  i e  com acme example ListUser  and the classpath are specified in the MANIFEST of the JAR file   The syntax for launching an application from a module  Java 9 and later  is as follows  java    lt options gt    --module  lt module gt    lt mainclass gt     lt arg gt        The name of the entrypoint class is either defined by the  lt module gt  itself  or is given by the optional  lt mainclass gt    From Java 11 onwards  you can compile and run a single source code file  and run it with the following syntax  java    lt options gt     lt sourcefile gt    lt arg gt        where  lt sourcefile gt  is  typically  a file with the suffix  quot  java quot     For more details  please refer to the official documentation for the java command for the Java release that you are using   IDEs A typical Java IDE has support for running Java applications in the IDE JVM itself or in a child JVM   These are generally immune from this particular exception  because the IDE uses its own mechanisms to construct the runtime classpath  identify the main class and create the java command line  However it is still possible for this exception to occur  if you do things behind the back of the IDE   For example  if you have previously set up an Application Launcher for your Java app in Eclipse  and you then moved the JAR file containing the  quot main quot  class to a different place in the file system without telling Eclipse  Eclipse would unwittingly launch the JVM with an incorrect classpath  In short  if you get this problem in an IDE  check for things like stale IDE state  broken project references or broken launcher configurations  It is also possible for an IDE to simply get confused   IDE s are hugely complicated pieces of software comprising many interacting parts   Many of these parts adopt various caching strategies in order to make the IDE as a whole responsive   These can sometimes go wrong  and one possible symptom is problems when launching applications   If you suspect this could be happening  it is worth trying other things like restarting your IDE  rebuilding the project and so on   Other References  From the Oracle Java Tutorials - Common Problems  and Their Solutions

User · Answer

All right  there are many answers already  but no one mentioned the case where file permissions can be the culprit   When running  a user may not have access to the JAR  file or one of the directories of the path  For example  consider   Jar file in   dir1 dir2 dir3 myjar jar   User1 who owns the JAR file may do     Running as User1 cd  dir1 dir2 dir3  chmod  r myjar jar   But it still doesn t work     Running as User2 java -cp   dir1 dir2 dir3  dir1 dir2 javalibs  MyProgram Error  Could not find or load main class MyProgram   This is because the running user  User2  does not have access to dir1  dir2  or javalibs or dir3  It may drive someone nuts when User1 can see the files  and can access to them  but the error still happens for User2

User · Answer

You really need to do this from the src folder  There you type the following command line    name of the package   Class Name   arguments    Let s say your class is called CommandLine class  and the code looks like this   package com tutorialspoint java                  Created by mda21185 on 15-6-2016               public class CommandLine           public static void main String args                 for int i 0  i lt args length  i                     System out println  args     i           args i                                    Then you should cd to the src folder and the command you need to run would look like this   java com tutorialspoint java CommandLine this is a command line 200 -100   And the output on the command line would be   args 0   this args 1   is args 2   a args 3   command args 4   line args 5   200 args 6   -100

User · Answer

If your classes are in packages  The main class is defined in a package   you should run it over the hierarchical directory  using the full name of the class  packageName MainClassName   Assume there is a source code file  Main java   package com test   public class Main        public static void main String   args            System out println  quot salam 2nya n quot             For running this code  you should place Main Class in the package like directory   com test Main Java  And in the root directory use java com test Main

User · Answer

Use this command   java -cp    PACKAGE  CLASSNAME   Example  If your classname is Hello class created from Hello java then use the below command   java -cp   Hello   If your file Hello java is inside package com demo then use the below command  java -cp   com demo Hello   With JDK 8 many times it happens that the class file is present in the same folder  but the java command expects classpath and for this reason we add -cp   to take the current folder as reference for classpath

[java] What does "Could not find or load main class" mean?

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