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?

Answer 1

If your main method is in the class under a package, you should run it over the hierarchical directory.

Assume there is a source code file (Main.java):

package com.test;

public class Main {

    public static void main(String[] args) {
        System.out.println("salam 2nya\n");
    }
}

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

Answer 2

1. Use the final modifier to enforce good initialization.
2. Avoid returning null in methods, for example returning empty collections when applicable.
3. Use annotations @NotNull and @Nullable
4. Fail fast and use asserts to avoid propagation of null objects through the whole application when they shouldn't be null.
5. Use equals with a known object first: if("knownObject".equals(unknownObject)
6. Prefer valueOf() over toString().
7. Use null safe StringUtils methods StringUtils.isEmpty(null).