Error "ClassNotFoundException" in IntelliJ IDEA

Question

I made this simple program:

package main.java;  public class start {     public static void main(String[] args) {         System.out.println("Hello World!");     } } 

And got this error. I have NO idea what is happening,

"C:\Program Files\Java\jdk1.7.0_21\bin\java" -Didea.launcher.port=7532 "-Didea.launcher.bin.path=C:\Program Files (x86)\JetBrains\IntelliJ IDEA Community Edition 12.1.4\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.7.0_21\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.7.0_21\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.7.0_21\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.7.0_21\jre\lib\jce.jar;C:\Program Files\Java\jdk1.7.0_21\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.7.0_21\jre\lib\jfxrt.jar;C:\Program Files\Java\jdk1.7.0_21\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.7.0_21\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.7.0_21\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.7.0_21\jre\lib\resources.jar;C:\Program Files\Java\jdk1.7.0_21\jre\lib\rt.jar;C:\Program Files\Java\jdk1.7.0_21\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.7.0_21\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.7.0_21\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.7.0_21\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.7.0_21\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.7.0_21\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.7.0_21\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.7.0_21\jre\lib\ext\zipfs.jar;C:\Users\Tim\IdeaProjects\Rust\out\production\Rust;C:\Program Files (x86)\JetBrains\IntelliJ IDEA Community Edition 12.1.4\lib\idea_rt.jar" com.intellij.rt.execution.application.AppMain java.start Exception in thread "main" java.lang.ClassNotFoundException: java.start     at java.net.URLClassLoader$1.run(URLClassLoader.java:366)     at java.net.URLClassLoader$1.run(URLClassLoader.java:355)     at java.security.AccessController.doPrivileged(Native Method)     at java.net.URLClassLoader.findClass(URLClassLoader.java:354)     at java.lang.ClassLoader.loadClass(ClassLoader.java:423)     at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)     at java.lang.ClassLoader.loadClass(ClassLoader.java:356)     at java.lang.Class.forName0(Native Method)     at java.lang.Class.forName(Class.java:188)     at com.intellij.rt.execution.application.AppMain.main(AppMain.java:113)  Process finished with exit code 1 

Anyone know what is wrong?

Answer 1

Probably your project structure was src/main/java/start.java but when you added it to IntelliJ you have set src as a source folder, so IntelliJ put main.java as a package.

In Project Setting (Ctrl+Shift+Alt+S) → Modules → Sources tab set src/main/java as Source Folder. Then in your simple program change package to whatever you like (e.g. my.test). After that if IntelliJ reports any error in line with package quick fix with Alt+Enter should help

Answer 2

I had this problem and couldn't solve it with any of these solutions. However I think the problem was that I was using 'open' to create a project from a directory with some java files in. Instead I had to 'create new project' with the same directory as root.

Make sure you delete any '.idea' directories and '.iml' files when 'create new project' since it will otherwise complain about a project already being in the given directory.