Create a .java file.

touch foo.java

Add the following markup to the .java file. Since the name of the file is foo.java in this example the name of the class is also "foo".

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

Compile the app. This will create a new file named foo.class.

javac foo.java

Execute foo and "Hello World" should be displayed.

~]# java foo
Hello World