The four levels of visibility in Java

More specifically:

• Public access means that any object can directly access the variable or method.
• Protected access means that subclass objects, as well as members of classes in the same package, can access the thing being modified
• Package access means that classes within the same package can directly access that which is modified.
• Finally, the Private access modifier denies access to all objects not in the same class; practically speaking, almost always, even those should use accessor and mutator methods.

Still more (scandalously basic) trivia!

• "Package" is the default level of accessibility.
  (Thanks to amien the nice catch - I had stated that the default class was protected. Y'all know you get 'em confused, too.)
• A Java source file may have no more than one public class, after which the source will be named.
• Inner classes/nested classes/anonymous classes/etc. do not use access modifiers, because it would be silly.
• Members of an interface are public by default.
• Access to protected fields and methods in a superclass is accomplished only by using a fully qualified name ThisClass.method where ThisClass an instance of a type which is a subclass of the class containing the desired method or field.

Also, there's no reason you can't have a private constructor if you really, really want one. Don't let kill-joys tell you there is. Here, have a look at this:

// For your pleasure alone, I have used hardlinks in lieu
// of syntax highlighting. Enjoy.

public class LookyHere extends FancySecretSuperClass {

   private LookyHere (String str) {
      super(str);    //this string does something important
   }
   public LookyHere (Integer i) {
      this(i.toString());
   }
   public LookyHere (Object o) {
      this(o.toString());
   }
   public LookyHere (char[] chrs) {
      this(new String(chrs));
   }
   // etc.

}

It makes sense in the same way using accessors to fiddle with your member variables does. Granted, it's kind of a trivial example. (I had intended to pass in the different possible parameters for a Connection object, but I'm lousy at that database stuff and it would never have compiled. Hell, even this may not compile. It's just an illustration of my argument.)