mySystemDesign

BEGINNER TO ADVANCED

Encapsulation, Abstraction, Inheritance, Polymorphism. Foundation of every LLD interview.

SRP, OCP, LSP, ISP, DIP. 5 rules for clean, maintainable, extensible OOP code.

Prefer has-a over is-a. More flexible. Avoids brittle deep class hierarchies.

Draw classes, interfaces, and relationships before writing a single line of code.

Interface = contract, supports multiple. Abstract = partial implementation. Know when to use each.

Race conditions, deadlocks, mutex, semaphore, ReentrantLock, ThreadLocal, immutability.

One global instance. Thread-safe via enum or double-checked locking. Logger, Config, DB Pool.

Let subclasses decide which object to create. Decouples instantiation from usage. Open/Closed.

Factory of factories. Create families of related objects. Cross-platform UI toolkit example.

Construct complex objects step by step. Fluent API. Avoids telescoping constructor anti-pattern.

Clone existing objects instead of constructing from scratch. Shallow vs deep copy.

Reuse a fixed set of pre-created objects. Avoids expensive construction. DB connections, threads.

Convert an incompatible interface into the expected one. Wrap legacy or third-party classes.

Add responsibilities to objects dynamically. Wrap in decorator objects. Java I/O streams.

Provide a simple interface to a complex subsystem. Hide internal complexity from clients.

Surrogate for another object. Lazy init, access control, logging, caching proxy.

Treat individual and composed objects uniformly. Tree structures. File system, UI component trees.

Decouple abstraction from implementation. Both can vary independently. Cross-platform rendering.

Share fine-grained objects to save memory. Intrinsic (shared) vs extrinsic (per-use) state.

Subject notifies subscribed observers when state changes. Foundation of event systems.

Define a family of algorithms, encapsulate each, make them interchangeable at runtime.

Allow object to change behaviour when internal state changes. Finite State Machine in OOP.

Encapsulate a request as an object. Enables undo/redo, queuing, logging. Text editor actions.

Define skeleton of algorithm in base class, let subclasses fill in the steps. Hollywood Principle.

Sequentially access elements of a collection without exposing its structure. Java Iterator.

Pass request along a chain of handlers. Each decides to process or pass on. Middleware, logging.

Centralise communication between objects. Reduces many-to-many coupling. Chat room example.

Capture and restore object state without exposing internals. Undo functionality.

Add new operations to object structures without modifying them. Double dispatch.

Define a grammar for a language, provide an interpreter to deal with it. SQL, regex parsing.