Low-Level Design

Single Responsibility: One Reason to Change, One Owner

SRP isn't 'do one thing' — it's 'answer to one actor'. Watch a Marketing copy tweak break Compliance's audit log, then split one NotificationService by who owns each change.

Asked at Amazon, Google, Microsoft +3

Jul 27, 2026·7 min read

Open/Closed: Add a Channel Without Touching the Core

Every new notification channel meant another edit to the same switch — and another regression. Open/Closed makes new behavior a drop-in, not a rewrite.

Asked at Amazon, Google, Microsoft +3

Jul 27, 2026·7 min read

Liskov Substitution: If Clients Must Check the Type, You Broke It

An SmsChannel that can't do what a Channel promised isn't a safe stand-in for its base — so callers start type-checking. Liskov, explained simply: a subtype must work anywhere its parent does.

Asked at Amazon, Google, Microsoft +3

Jul 27, 2026·8 min read

Interface Segregation: Don't Make Callers Depend on What They Don't Use

One fat Notifier interface forces every channel to stub methods it can't support and every caller to depend on methods it never calls. Interface Segregation splits it into roles.

Asked at Amazon, Google, Microsoft +3

Jul 27, 2026·7 min read

Dependency Inversion: Point the Arrow at an Abstraction

A service that news-up its own email sender can't be tested or swapped. Dependency Inversion flips the arrow so high-level policy and low-level detail both depend on an abstraction.

Asked at Amazon, Google, Microsoft +3

Jul 27, 2026·8 min read

Creational Design Patterns6-part series

Creational Patterns: Who Decides What Gets Built

The five creational patterns aren't five clever tricks — they're five answers to one question: how does an object come into existence without the code that uses it hard-wiring the code that makes it? A map, and one interactive way to pick the right one.

Asked at Amazon, Google, Microsoft +3

Jul 28, 2026·5 min read

Factory Method: Let the Subclass Decide What to Build

A switch(type) buried inside your shared workflow is a design smell. Factory Method moves the 'new' to a subclass so the logic that uses an object never learns which one it got.

Asked at Amazon, Google, Microsoft +2

Jul 28, 2026·17 min read

Abstract Factory: Ship a Whole Family, Never a Mismatched Pair

One factory hands you a Button, a Checkbox, and a Menu that already agree with each other — so a macOS button can never end up next to a Windows checkbox.

Asked at Amazon, Google, Microsoft +2

Jul 28, 2026·16 min read

Builder: Assemble Complex Objects Without the Telescoping Constructor

Ten positional arguments and a fistful of nulls is not construction — it's a guessing game. The Builder pattern turns object assembly into readable, validate-once, immutable steps.

Asked at Amazon, Google, Microsoft +2

Jul 28, 2026·17 min read

Prototype: Clone a Ready-Made Object Instead of Rebuilding It

Constructing an object can be expensive and the client shouldn't need its class — so copy a preconfigured instance. The whole game is shallow vs deep copy, and getting it wrong aliases your original.

Asked at Amazon, Google, Microsoft +2

Jul 28, 2026·17 min read

Singleton: One Instance, and the Global It Smuggles In

The GoF pattern everyone knows and half of us regret. Watch a naive getInstance() spawn two instances under load — then see why modern apps get one instance from a DI container, not a global.

Asked at Amazon, Google, Microsoft +2

Jul 28, 2026·20 min read

Structural Design Patterns8-part series

Structural Patterns: Composing Objects Into Bigger Things

The seven structural patterns aren't seven clever tricks — they're seven answers to one question: how do you compose classes and objects into larger structures without the composition itself going rigid? A map, and how to tell the confusing ones apart.

Asked at Amazon, Google, Microsoft +3

Aug 8, 2026·8 min read

Adapter Pattern: Make Two Interfaces That Weren't Built to Talk, Talk

Your code expects one interface; the useful class only speaks another — and you can't edit either. Adapter is the thin impedance-matching layer that lets them work together without a rewrite.

Asked at Amazon, Stripe, Uber +2

Aug 8, 2026·16 min read

Decorator Pattern: Stack Behavior at Runtime Without a Subclass Bomb

You need buffering, then compression, then encryption — in any combination. Subclassing that gives you BufferedEncryptedCompressedInputStream. The Decorator pattern lets you wrap those responsibilities on one object at a time instead.

Asked at Amazon, Google, Microsoft +2

Aug 8, 2026·14 min read

Facade Pattern: One Door Into a Room Full of Machinery

When 'place an order' means orchestrating inventory, payment, shipping, and notifications — every caller re-learns the whole subsystem. A Facade gives them one door, and leaves the machinery open for anyone who still needs it.

Asked at Amazon, Google, Microsoft +3

Aug 8, 2026·14 min read

Proxy Pattern: The Stand-In Your Code Can't Tell Apart

The @Transactional annotation on your service method does nothing on its own — a proxy quietly wrapping the bean is what makes it work. Understand that stand-in and half of Spring's magic stops being magic.

Asked at Amazon, Google, Microsoft +3

Aug 8, 2026·14 min read

Composite Pattern: One size() Call for a File or Ten Thousand

When your data is a tree — files and folders, elements and containers, parts and assemblies — the Composite pattern lets client code treat a single leaf and a whole subtree with the exact same call. The recursion hides inside the tree, not in your if-statements.

Asked at Amazon, Google, Microsoft +2

Aug 8, 2026·15 min read

Hardlow level design design patterns oop

Bridge Pattern: When Two Things Vary, Stop Multiplying Them

Shape × Renderer, Remote × Device, Notification × Transport — the moment two dimensions vary independently, inheritance breeds a class explosion. Bridge splits the hierarchy so each axis grows on its own.

Asked at Amazon, Google, Microsoft +3

Aug 8, 2026·14 min read

Flyweight: One 'e' for a Million Letters

When you must hold millions of near-identical objects, one-object-per-instance quietly blows the heap. Flyweight splits shared shape from per-use position — and hands out one copy for all of them.

Asked at Google, Microsoft, Adobe +3

Aug 8, 2026·14 min read

Strategy Pattern: Swap Behavior Without Touching the Caller

Why a checkout that grows an if-branch every quarter is a design smell — and how the Strategy pattern turns runtime behavior into something you plug in, not rewrite.

Asked at Amazon, Google, Microsoft +2

Jul 27, 2026·14 min read

Behavioral Design Patterns11-part series

Behavioral Patterns: How Objects Talk and Decide

The eleven behavioral patterns aren't eleven tricks — they're eleven answers to one question: when responsibility and communication are tangled inside a class, which one do you pull out, and where does it go? A map, not a re-teach.

Asked at Amazon, Google, Microsoft +3

Aug 8, 2026·8 min read

Observer Pattern: Broadcast State Without Knowing Who's Listening

When one object changes and an unknown, shifting crowd must react, wiring the subject to each dependent is a maintenance trap. Observer inverts it — dependents subscribe, the subject just broadcasts.

Asked at Amazon, Google, Microsoft +3

Aug 8, 2026·15 min read

Command Pattern: Make a Request Something You Can Hold in Your Hand

A method call happens once and is gone. Turn that request into an object with execute() and undo(), and suddenly you can queue it, log it, retry it, and reverse it — the backbone of undo/redo, job queues, and transactional replay.

Asked at Amazon, Google, Microsoft +3

Aug 8, 2026·15 min read

State Pattern: Make Illegal Transitions Impossible

An order that can be paid, shipped, delivered, or cancelled grows a switch(status) block in every method — until one of them forgets an arm and a shipped order gets refunded. The State pattern moves both behavior and the legal next-steps into the state itself.

Asked at Amazon, Google, Microsoft +2

Aug 8, 2026·16 min read

Template Method: Freeze the Recipe, Swap the Ingredients

When CSV, JSON, and XML importers all do the same four steps in the same order but differ in two of them, three near-identical pipelines is the smell. Template Method puts the order in one place and lets subclasses fill only the steps that vary.

Asked at Amazon, Google, Microsoft +2

Aug 8, 2026·15 min read

Iterator Pattern: Walk a Collection Without Knowing Its Shape

Every for-each loop you've ever written is the Iterator pattern in disguise. Here's how one hasNext()/next() surface lets you traverse an array, a tree, or an unbounded paged API the exact same way — and why mutating mid-loop blows up.

Asked at Amazon, Google, Microsoft +2

Aug 8, 2026·16 min read

Mediator Pattern: Turn a Mesh of Dependencies Into a Star

When every object holds a reference to every other, change becomes impossible — an O(N²) tangle where one edit ripples everywhere. The Mediator pulls the wiring into one place so components can stay gloriously ignorant of each other.

Asked at Google, Microsoft, Meta +3

Aug 8, 2026·15 min read

Memento Pattern: Undo Without Cracking the Object Open

Every undo, savepoint, and game save has the same problem — capture an object's private state so you can restore it later, without letting the thing holding the snapshot read or corrupt what's inside.

Asked at Microsoft, Google, JetBrains +3

Aug 8, 2026·14 min read