There can be only one

Before Java release 1.5 you had to do the following to create a Singleton:

public class ImASingleton {

 private static final ImASingleton INSTANCE = new ImASingleton();

 private ImASingleton() {
 }

 public static ImASingleton getInstance() {
  return INSTANCE;
 }
}

And to be able to serialize this singleton, you had to do 3 things:
1. Implement the Serializable interface.
2. Mark the instance fields transient if you didn't want them to be serialized.
3. Define a readResolve() method to return the single instance. This is a hook method called in the deserialization process. If you don't provide this method, the default implementation will always create a new instance in stead of your unique instance!

public class ImASingleton implements Serializable {

 private static final ImASingleton INSTANCE = new ImASingleton();
 private final transient Object nonSerializableField;

 private ImASingleton() {
 }

 public static ImASingleton getInstance() {
          return INSTANCE;
 }

 private Object readResolve() {
   return INSTANCE; 
 }
}

As of Java 1.5 the preferred way to create a singleton is simply by using an enum. This get's you the Serialization for free.

public enum MySingleton() {
 INSTANCE;
}