thufir@arrakis:~/sun_tutorial/thufir$ 
thufir@arrakis:~/sun_tutorial/thufir$ cat HelloWorldApp.java 
package thufir;

public class HelloWorldApp {
    public static void main(String[] args) {
        System.out.println("Hello World!"); // Display the string.
    }
}

thufir@arrakis:~/sun_tutorial/thufir$ 
thufir@arrakis:~/sun_tutorial/thufir$ javac HelloWorldApp.java 
thufir@arrakis:~/sun_tutorial/thufir$ 
thufir@arrakis:~/sun_tutorial/thufir$ java HelloWorldApp 
Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorldApp (wrong name: thufir/HelloWorldApp)
        at java.lang.ClassLoader.defineClass1(Native Method)
        at java.lang.ClassLoader.defineClass(ClassLoader.java:620)
        at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:124)
        at java.net.URLClassLoader.defineClass(URLClassLoader.java:260)
        at java.net.URLClassLoader.access$000(URLClassLoader.java:56)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:195)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:276)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:251)
        at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:319)
thufir@arrakis:~/sun_tutorial/thufir$ 
thufir@arrakis:~/sun_tutorial/thufir$ nano HelloWorldApp.java 
thufir@arrakis:~/sun_tutorial/thufir$ 
thufir@arrakis:~/sun_tutorial/thufir$ cat HelloWorldApp.java 
//package thufir;

public class HelloWorldApp {
    public static void main(String[] args) {
        System.out.println("Hello World!"); // Display the string.
    }
}

thufir@arrakis:~/sun_tutorial/thufir$ 
thufir@arrakis:~/sun_tutorial/thufir$ javac HelloWorldApp.java 
thufir@arrakis:~/sun_tutorial/thufir$ 
thufir@arrakis:~/sun_tutorial/thufir$ java -cp . HelloWorldApp 
Hello World!
thufir@arrakis:~/sun_tutorial/thufir$