How to run Java class file on Ubuntu

Let say you compile the following Java codes

public class hello {
    public static void main(String[] args) {
    System.out.println("rizauddin.com");
    }
}

using the following command:

javac hello.java

You will get a class file `hello.class`.

However, when you try to run the Java class file using

java hello

like you usually did on Windows, it won't run.

Exception in thread "main" java.lang.NoClassDefFoundError: hello Caused by: java.lang.ClassNotFoundException: hello at java.net.URLClassLoader$1.run(URLClassLoader.java:202) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(URLClassLoader.java:190) at java.lang.ClassLoader.loadClass(ClassLoader.java:307) at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301) at java.lang.ClassLoader.loadClass(ClassLoader.java:248) Could not find the main class: hello. Program will exit.

Actually, to run Java class file on Ubuntu, you need to always specify the classpath, using the `-cp` options.

As an example,

java -cp . hello

The `.` means the current working directory. In other words, you are telling Java to look for `hello class` in the same directory that you invoke the `java` command.

You can also add the current directory to the CLASSPATH, like this:

export CLASSPATH=.:$CLASSPATH

If you don't like to do that each time you want to run a Java class file, you can set the CLASSPATH permanently in /etc/environment or in ~/.profile.