Compile java file in linux [closed]

Question

I have added environment variables in /etc/profile and the compiled file has no package name. Why java Hello can not work and appear java.lang.ClassNotFoundException: HelloWorld. I would like to know where is the problem, thanks.

Here is the code.

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

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Linux execute java command Error: Main class not found or loaded
java环境变量配置可参考
Java环境变量配置（CentOS）

以上回答转自博客园提问 谢谢 ycyzharry