Java could not find or load main class

Question

Problem:

What is the meaning of Java could not find or load main class,  and why does it happen?

I’ll appreciate your sharing.

Answer 1

This error can occur for a couple of reasons. Let me walk through every possible solution step by step.

1. Calling  .class file from java command:

public class HelloWorld {

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

Let's assume I have the basic program above. If I will compile it using the command, HelloWorld.class will be created. 

javac HelloWorld.java

Now I want to run the .class file. I will get the error message if I run

java HelloWorld.class

Instead of

java HelloWorld

 

2. Incorrect Casing: We all know that Java is a case sensitive language. We need to keep in mind whenever we declare a variable, function or even class name.

So, we’ll get the error message if we run

 

java helloworld

instead of

java HelloWorld

 

3. Class in a package

Have a close look at the program below.

package com.naymulhasan;
/**
 * Java program to demonstrate 
 * Error :Could not find or load main class
 * 
 * @author Naymul Hasan
 */
public class HelloWorld {

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

Here I have HelloWorld class inside com.naymulhasan package. In this case, if I try to call

java HelloWorld

it will give the same error message. Because I had to call the class file with the package name. Let me try this one

java com.naymulhasan.HelloWorld

Oops! I am still getting the error message because the CLASSPATH environment variable is not set. Let’s try with the CLASSPATH command -cp

java -cp . com.naymulhasan.HelloWorld

BOOM. You are all set now.