Java Could not find or load main class

A popular error message for new Java users.

Error: Could not find or load main class ClassName.class
Caused by: java.lang.ClassNotFoundException: ClassName.class

1. No Package

1.1 Reviews a simple Java Hello World, no package.

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello World");

# Compile Java source code
C:\projects> javac HelloWorld.java
C:\projects> dir
15/04/2019  01:20 PM               425 HelloWorld.class
15/04/2019  01:42 PM               138 HelloWorld.java
# Run Java
C:\projects>java HelloWorld.class
Error: Could not find or load main class HelloWorld.class
Caused by: java.lang.ClassNotFoundException: HelloWorld.class

To fix it, run java ClassName, without .class extension.

C:\projects>java HelloWorld
Hello World

2. With Package

2.1 A simple Hello World and package it as com.mkyong

package com.mkyong;
public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello World");

# Compile Java source code
C:\projects> javac HelloWorld.java
C:\projects> dir
15/04/2019  01:20 PM               425 HelloWorld.class
15/04/2019  01:42 PM               138 HelloWorld.java
C:\projects>java HelloWorld
Error: Could not find or load main class HelloWorld
Caused by: java.lang.NoClassDefFoundError: com/mkyong/HelloWorld (wrong name: HelloWorld)

2.2 If package is defined, the Java classes .class must place in the correct folder location.

For example :

package com.mkyong;

The above generated class file must place in C:\projects\com\mkyong\HelloWorld.class

2.3 To fix it, we can use -d to specify where to place generated class files.

# Compile Java source code
C:\projects> javac -d . HelloWorld.java
C:\projects> dir
15/04/2019  02:01 PM    <DIR>          com
15/04/2019  01:51 PM               161 HelloWorld.java
C:\projects\com\mkyong\HelloWorld.class
C:\projects\HelloWorld.java

2.4 Now, we can run like this java {package}.HelloWorld

C:\projects>java HelloWorld
Error: Could not find or load main class HelloWorld
Caused by: java.lang.ClassNotFoundException: HelloWorld
C:\projects>java com.mkyong.HelloWorld
Hello World