Construction patterns
Ways to control instantiation: a singleton guarantees one instance, a static factory names and caches construction, and dependency injection hands collaborators in rather than letting an object build them.
Singleton, done correctly
An enum is the only form that is thread-safe, lazily initialised by the JVM, and immune to both reflection and deserialisation attacks.
public enum Registry {
INSTANCE;
public void register(String key) { /* ... */ }
}The double-checked-locking version needs volatile on the field to be correct,
and is worth writing only when the instance genuinely must be lazy.
Static factory over constructor
A factory method has a name, is not obliged to return a new instance, and can
return a subtype. List.of(), Optional.empty() and Integer.valueOf() all
exploit at least one of these.
Dependency injection
An object that constructs its own collaborators cannot be tested in isolation. Taking them as constructor parameters makes the dependency explicit and lets the caller substitute a fake.