v4

• Singleton<E> now implements Supplier<E>
• Lazy singletons are now thread-safe. Eager versions already were.

Full Changelog: v3...v4

singleton v3

Singleton v3

• Reject null for Supplier arguments. (With NullPointerException)
• Continue to allow null for actual Singleton instances.

singleton v2

Singleton v2

• Adds an implementation for Singleton.eager(Supplier)
• Polishes documentation
• Singleton.Eager and Singleton.Lazy are no longer public. (This wasn't intended in v1)

singleton v1

Singleton v1

• Adds a simple Singleton implementation.