After getting used to the spoon-feeding IDE’s, writing some one-off classes might lead to this Exception.

Exception in thread “main” java.lang.NoClassDefFoundError: HelloWorld
Caused by: java.lang.ClassNotFoundException: HelloWorld
at java.net.URLClassLoader$1.run(URLClassLoader.java:200)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
at java.lang.ClassLoader.loadClass(ClassLoader.java:252)
at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:320)
Could not find the main class: HelloWorld. Program will exit.

Very common problem. Your classpath needs a .: followed by the classpath, to include the class(es) in your current directory.

java -cp .:./lib/mysql.jar HelloWorld

Here’s a method that shows how to get all the static fields in a Java class and return them.

	public static List printFields(Class c) throws ClassNotFoundException, IllegalAccessException {
    		List fields = new ArrayList();
        java.lang.reflect.Field[] f = c.getDeclaredFields();
        for (java.lang.reflect.Field field : f) {
            int modifier = field.getModifiers();
            if (java.lang.reflect.Modifier.isStatic(modifier)){
          		String val = (String) field.get( null );
          		System.out.print(field.getName() +  ": " + val.toString() + ", ");
          		fields.add(val);
            }
        }
        return fields;
    }

So you want to populate a new column in MySQL with fake dates?

Here’s how to add a new datetime column in SQL

alter table city_new add column mytime datetime;

Here’s how to populate it with fake timestamps data

update city_new set mytime = select from_unixtime(
unix_timestamp(’2009-01-01 01:00:00′)+floor(rand()*31536000)
);

Here’s a more generic formula

select from_unixtime(
unix_timestamp( ‘start timestamp’)
+floor(rand()* (max interval in seconds) )
);