[psch@mnpc javatest]$ cat Foo.java
public class Foo {
    public static void bar() {
        System.out.println("yes, bar!");
    }
}
[psch@mnpc javatest]$ cat Baz.java
public class Baz {
    public static void main(String... args) {
        Foo.bar();
    }
}
[psch@mnpc javatest]$ javac Foo.java Baz.java
[psch@mnpc javatest]$ jar cf Foo.jar Foo.class && rm Foo.class
[psch@mnpc javatest]$ CLASSPATH=. java Baz
Exception in thread "main" java.lang.NoClassDefFoundError: Foo
	at Baz.main(Baz.java:3)
Caused by: java.lang.ClassNotFoundException: Foo
	at java.net.URLClassLoader$1.run(URLClassLoader.java:372)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:361)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(URLClassLoader.java:360)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
	... 1 more
[psch@mnpc javatest]$ CLASSPATH=.:Foo.jar java Baz
yes, bar!
[psch@mnpc javatest]$