pattern · 12 MIN READ

Adapter

Translate an existing API into the contract your client already understands. An adapter preserves the client’s vocabulary while isolating vendor-specific units and return shapes.

Reading completion is independent of quizzes and challenges.

What you’ll understand

Translate an existing API into the contract your client already understands. An adapter preserves the client’s vocabulary while isolating vendor-specific units and return shapes.
ShippingAdapter wraps LegacyShipping, converts kilograms to grams, and rounds hours up to whole delivery days.
Evaluate the tradeoff: Translation adds a boundary to maintain. Keep business pricing policy out of the adapter and document rounding rules. A facade simplifies a subsystem; an adapter changes its contract.

Useful first: Classes and Objects

The idea and when to recognize it

Translate an existing API into the contract your client already understands. An adapter preserves the client’s vocabulary while isolating vendor-specific units and return shapes.

Requirement: A checkout asks for a shipping quote using kilograms and expects integer cents plus delivery days. The vendor accepts grams and reports transit hours.
Change to anticipate: Integrate the legacy SDK without modifying ShippingClient, including fractional kilogram weights and transit times that are not whole days.
Design goal: ShippingAdapter wraps LegacyShipping, converts kilograms to grams, and rounds hours up to whole delivery days.

Start with the concrete problem

A checkout asks for a shipping quote using kilograms and expects integer cents plus delivery days. The vendor accepts grams and reports transit hours.

Integrate the legacy SDK without modifying ShippingClient, including fractional kilogram weights and transit times that are not whole days.

Direct implementation	Revised design
Relabeling fields without converting units silently changes the shipping price or delivery promise.	ShippingAdapter wraps LegacyShipping, converts kilograms to grams, and rounds hours up to whole delivery days.
The baseline case can hide the problem. Compare the extension and boundary fixtures before accepting this design.	Translation adds a boundary to maintain. Keep business pricing policy out of the adapter and document rounding rules. A facade simplifies a subsystem; an adapter changes its contract.

Responsibilities and contracts

ShippingAdapter wraps LegacyShipping, converts kilograms to grams, and rounds hours up to whole delivery days.

ShippingClient: Depends only on the normalized quote contract. Public operations: quote(order)
ShippingAdapter: Translates the shipping contract. Public operations: quote(order)
LegacyShipping: Owns the vendor API. Public operations: estimate(grams, hours)

Read the design

Select a participant to inspect its responsibility and public operations. Follow the labeled relationships; an arrow indicates direction, while the legend distinguishes ownership, substitution, and temporary use.

class diagramSelect a participant below

ShippingClient

Depends only on the normalized quote contract

quote(order)

Relationships and notation

ShippingClient → ShippingAdapter: quote contract. Dependency → uses temporarily
ShippingAdapter → LegacyShipping: wraps. Association · knows another object

Preparing the design walkthrough…

Follow the interaction

Choose a supplied workload and compare the direct and revised implementations. Step through the same operation in both languages. Object values are recorded after each highlighted statement; previous steps remain unchanged.

GUIDED SIMULATIONExample language

Scenario Implementation

Convert 1 kg to 1,000 g and 24 hours to 1 day.

A finite, authored walkthrough of this fixture. Source code is verified in tests; it is not executed by this page.

class diagramSelect a participant below

ShippingClient

Depends only on the normalized quote contract

quote(order)

Relationships and notation

ShippingClient → ShippingAdapter: quote contract. Dependency → uses temporarily
ShippingAdapter → LegacyShipping: wraps. Association · knows another object

Object state after the highlighted operation

{}

function example(input) {
  class LegacyShipping { estimate(grams, hours) { return { fee: grams * 2, hours }; } }
  class ShippingAdapter {
    constructor(sdk) { this.sdk = sdk; }
    quote(order) { const raw = this.sdk.estimate(Math.round(order.kg * 1000), order.hours); return { costCents: raw.fee, deliveryDays: Math.ceil(raw.hours / 24) }; }
  }
  class ShippingClient { constructor(provider) { this.provider = provider; } quote(order) { return this.provider.quote(order); } }
  const client = new ShippingClient(new ShippingAdapter(new LegacyShipping()));
  const quote = client.quote(input);
  return quote;
}

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Textual interaction history

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Implementation and expected output

Read the complete example alongside its input and expected output. The implementation selector changes the displayed program; the language selector preserves your current scenario and playback position.

Example language

Revised design · Call example(input) with the selected fixture.

Complete JavaScript implementation

function example(input) {
  class LegacyShipping { estimate(grams, hours) { return { fee: grams * 2, hours }; } }
  class ShippingAdapter {
    constructor(sdk) { this.sdk = sdk; }
    quote(order) { const raw = this.sdk.estimate(Math.round(order.kg * 1000), order.hours); return { costCents: raw.fee, deliveryDays: Math.ceil(raw.hours / 24) }; }
  }
  class ShippingClient { constructor(provider) { this.provider = provider; } quote(order) { return this.provider.quote(order); } }
  const client = new ShippingClient(new ShippingAdapter(new LegacyShipping()));
  const quote = client.quote(input);
  return quote;
}

Input

{
  "kg": 1,
  "hours": 24
}

Expected contract result

{
  "costCents": 2000,
  "deliveryDays": 1
}

Preparing the design walkthrough…

Try the idea in another setting

A sensor adapter can convert a device’s millivolt readings into the temperature contract expected by a monitoring dashboard.

Name the new client contract before adapting the example.
Identify which state belongs to each object and who controls its lifetime.
Write one normal case and one boundary case before extending the design.

Tradeoffs and interview takeaways

Translation adds a boundary to maintain. Keep business pricing policy out of the adapter and document rounding rules. A facade simplifies a subsystem; an adapter changes its contract.

Common mistake: Relabeling fields without converting units silently changes the shipping price or delivery promise.

Explain the choice: ShippingAdapter wraps LegacyShipping, converts kilograms to grams, and rounds hours up to whole delivery days.
Challenge your choice: Translation adds a boundary to maintain. Keep business pricing policy out of the adapter and document rounding rules. A facade simplifies a subsystem; an adapter changes its contract.
Describe a requirement that would make the simpler implementation sufficient.

Design challenge and knowledge check

Preparing the design walkthrough…

Navigation

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Adapter

What you’ll understand

The idea and when to recognize it

Start with the concrete problem

Responsibilities and contracts

Read the design

Follow the interaction

Implementation and expected output

Try the idea in another setting

Tradeoffs and interview takeaways

Design challenge and knowledge check

Meet the contract across every scenario

Five decisions to remember

Make it your own

Meet the contract across every scenario

Five decisions to remember