Strategy Pattern

Swap algorithms at runtime without touching the client.

Intermediate9 min read

Read these first

Builder Pattern

Why this matters

Interface PricingStrategy with price(Order)
CampusOS parking: nearest spot, cheapest spot, handicapped-first
Spring-style: inject strategy via constructor
Strategy: client chooses algorithm; often set once per operation
Patterns are forces and solutions — name the force before the pattern.

Creational

Factorychoose implementation

Builderstep-by-step assembly

Structural

Decoratorstack behaviour

Facadesimple entry point

Behavioral

Strategyswap algorithm

Observernotify subscribers

Statebehaviour by mode

Pick the family first, then the specific pattern.

Overview

Swap algorithms at runtime without touching the client. Study each section below in order — every list is interview-ready reference material.

Pricing strategies

Points to cover

Interface PricingStrategy with price(Order)
Implementations: Regular, StudentDiscount, PremiumMember
Client holds PricingStrategy field — set at runtime
New pricing rule = new class, zero edits to checkout flow

Spot assignment

Points to cover

CampusOS parking: nearest spot, cheapest spot, handicapped-first
Each algorithm is a SpotAssignmentStrategy
ParkingLot delegates to injected strategy

Registration

Points to cover

Spring-style: inject strategy via constructor
Manual: factory picks strategy from config or enum
Test: pass mock strategy into service under test

vs State pattern

Points to cover

Strategy: client chooses algorithm; often set once per operation
State: object changes behaviour as internal mode changes
Vending machine modes → State; payment calculation → Strategy

Java

// CampusOS — apply "Pricing strategies" on paper first, then in code
public final class StrategyPatternExample {
    // 1. Name entities from the problem statement
    // 2. Draw relationships before writing methods
    // 3. Introduce patterns only when a force appears
}

Quick recall

Everything you need if you only revisit this box.

Pricing strategies — review this slice before mock interviews.
Spot assignment — review this slice before mock interviews.
Registration — review this slice before mock interviews.
vs State pattern — review this slice before mock interviews.

Test yourself

Answer these before moving on — recall is what makes it stick.