design patterns

28 posts

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

Mediumlow level design design patterns oop

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

Mediumlow level design design patterns oop

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

Mediumlow level design design patterns oop

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

Mediumlow level design design patterns oop

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

Easylow level design design patterns oop

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

Mediumlow level design design patterns oop

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

Easylow level design design patterns oop

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

Mediumlow level design design patterns oop

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

Easylow level design design patterns oop

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

Mediumlow level design design patterns oop

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

Mediumlow level design design patterns oop

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

Mediumlow level design design patterns oop

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

Easylow level design design patterns oop

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

Mediumlow level design design patterns oop

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

Mediumlow level design design patterns oop

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

Mediumlow level design design patterns oop

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

Mediumlow level design design patterns oop

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

Easylow level design design patterns oop

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

Easylow level design design patterns oop

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

Mediumlow level design design patterns oop

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

Mediumlow level design design patterns oop

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

Mediumlow level design design patterns oop

Chain of Responsibility: Let the Request Find Its Own Handler

Why the sender of a request should never hard-code who processes it — and how linking handlers into a chain turns a brittle escalation switch into the reorderable pipeline behind every middleware stack.

Asked at Amazon, Google, Microsoft +3

Aug 8, 2026·15 min read

Hardlow level design design patterns oop

Visitor Pattern: Add Operations Without Editing the Classes

When your class hierarchy is stable but you keep bolting new operations onto every node — typecheck, pretty-print, evaluate, export — the Visitor pattern moves each operation into its own class so the nodes never change again.

Asked at Google, Microsoft, Amazon +3

Aug 8, 2026·15 min read

Hardlow level design design patterns oop

Interpreter Pattern: When Your Config Grows a Grammar

Feature-flag rules, search filters, and pricing conditions all quietly become little languages. The Interpreter pattern turns that grammar into a tree of classes that evaluate themselves — and tells you exactly when to stop and reach for a real parser instead.

Asked at Google, Amazon, Microsoft +2

Aug 8, 2026·15 min read

Easylow level design oop design patterns

Designing Tic-Tac-Toe: A Low-Level Design Walkthrough

A complete object-oriented design for Tic-Tac-Toe — a playable board, the full class model (players, board, pluggable rules), an O(1) win check, a codeable interface you can implement, and the patterns that scale it to N×N, K-in-a-row, and bots.

Asked at Google, Amazon, Microsoft +3

Jul 24, 2026·20 min read

Mediumlow level design oop design patterns

Designing a Parking Lot: A Low-Level Design Walkthrough

A complete object-oriented design for a multi-level parking lot — the class model, pluggable spot-allocation and pricing strategies, an O(1) availability lookup, a live simulator you can drive cars into, and a codeable interface you can implement against.

Asked at Google, Amazon, Microsoft +3

Jul 27, 2026·30 min read