Design Patterns

Centralize object creation behind one method so callers depend on the created type's contract, not its construction logic.

7 min 1 code practice

Provides an interface for creating a family of related objects without specifying their concrete classes — e.g. a UI kit that produces LightButton + LightDialog together.

6 min 1 code practice

Construct complex objects step-by-step with readable, fluent calls — especially when there are many optional or immutable fields.

7 min 1 code practice

Clone an existing instance instead of re-running expensive construction. Rare in interviews — know the intent, not the ceremony.

7 min 1 code practice

One instance per JVM, with a globally reachable accessor. Interviews love it because naive implementations break under concurrency.

7 min 1 code practice

Make an incompatible interface look like the one your client expects — a translator between contracts.

6 min 1 code practice

Wrap an object to add behavior at runtime, stackable without touching the original class.

6 min 1 code practice

One simplified entry point that hides a messy subsystem of many classes.

7 min 1 code practice

A stand-in that controls access to the real object: lazy init, access control, caching, remote calls.

7 min 1 code practice

Treat individual objects and groups of objects uniformly — a tree where leaves and nodes share an interface.

6 min 1 code practice

Decouple an abstraction from its implementation so both can vary independently — avoids the 2D class explosion.

7 min 1 code practice

Share the intrinsic (immutable, common) part of many fine-grained objects and pass the extrinsic (per-use) part in, so a million objects cost the memory of a few hundred.

7 min 1 code practice

Encapsulate a family of algorithms behind one interface and swap them at runtime — the single most used pattern in LLD interviews.

8 min 1 code practice

One subject notifies many listeners when its state changes — the publish/subscribe idea in-process.

7 min 1 code practice

Wrap a request (and its receiver) into an object — enables undo/redo, queuing, logging, and macros.

7 min 1 code practice

An object changes behavior when its state changes — state logic moves from if/else soup into per-state classes. The vending machine pattern.

8 min 1 code practice

Define the skeleton of an algorithm in a base class, let subclasses fill in specific steps — inversion of control inside one class hierarchy.

6 min 1 code practice

Pass a request along a chain of handlers; each handler decides to process it or pass it on. Middleware, filters, validators.

6 min 1 code practice

Provide sequential access over a collection without exposing its internals. Mostly baked into Java today — nested iterators (tree walks, pagination) still show up.

6 min 1 code practice

Centralize interactions between many objects so they talk to one mediator instead of each other — star topology.

7 min 1 code practice

Save a snapshot of an object's state so you can restore it later, without exposing its private fields. Think 'save game' in a video game.

7 min 1 code practice

Add new operations to a group of classes without changing those classes. The operation 'visits' each object and does the right thing for its type.

7 min 1 code practice