Creational: Singleton, Factory, Builder, Prototype

Singleton means one instance for the whole app, and in Java the safe, lazy, simple ways are the Bill Pugh holder idiom and the enum singleton.

Factory Method picks one product, Abstract Factory picks a matching family, Builder assembles one complex product step by step, and Prototype copies an existing product.

Prototype copies an existing object to make a new one; the shallow-clone trap is when the copy still shares the original's inner mutable objects, and a copy constructor avoids the mess of Cloneable.

The Builder assembles a complex object field by field through chained calls and locks it with build(), replacing an unreadable constructor with many parameters.

Prototype creates a new object by copying a ready-made template, saving the cost of building from scratch and preserving shared defaults.

The Factory hides the new keyword behind one place, so callers ask for a product by type and get back an interface, and adding a new type is just adding a class.

Factory produces one product; Abstract Factory produces a whole matching family of products through a factory of factories.

A lazy thread-safe Singleton builds the single instance only on first use, safely under concurrency, and the cleanest ways are the Bill Pugh holder idiom and the enum.

Stop clone from creating a second Singleton by refusing to implement Cloneable, overriding clone to throw, or returning the one instance, and prefer the enum which cannot be cloned at all.

The Builder pattern constructs a complex object step by step through a fluent interface, producing an immutable, consistent result.

The Factory Method centralises object creation so callers ask for a type and receive the right concrete instance without naming the concrete class.

Singleton guarantees exactly one shared instance, created safely and accessed globally through a static method.