Java 1.5 introduced the concept of Enum types. They are type-safe constants, which implements equals(), hashCode() and cannot be extended. Each constant can have attributes and override an abstract method created on each Enum class.
Although Singletons are not encouraged, the best way to create it is using Enum types. Here is an example:
public enum Singleton {
INSTANCE;
public void sayHello() {
System.out.println("Hello World!");
}
}
And then you call it this way:
Singleton.INSTANCE.sayHello();
Using Enums to create Singletons brings the serialization mechanism already bundled in the Enum type. This technique is described on the Effective Java Second Edition book by Joshua Block.