Adapter pattern

Introduction

The Adapter pattern lets a client use an existing class through an interface it already understands. It is a structural boundary that translates calls, data, and sometimes exceptions between your contract and an incompatible SDK or legacy component.

TL;DR / mental model: Keep your application talking to its own target interface, then put a thin adapter at the boundary that translates to the adaptee's API.

Problem and Context

Your checkout service charges credit cards through a PaymentProcessor interface. Every service, every test, every retry wrapper depends on that interface. Then the business signs a deal with PayPal. PayPal's SDK has completely different method names, different parameter shapes, and returns different result types.

You are stuck between two interfaces that do not match. The third-party SDK cannot be modified, and changing your interface would touch every caller. The adapter pattern gives you a third option: write a thin wrapper that translates one interface into the other.

When It Helps

Use Adapter when your code owns a stable target interface but an existing library or legacy implementation has a different API. It is especially useful when changing either side would spread vendor details through many callers.

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Participants and Structure

PaymentProcessor is the target interface your code depends on. StripeGateway and PayPalClient are the adaptees, third-party classes with incompatible APIs. StripeAdapter and PayPalAdapter are the adapters, implementing the target and delegating to the adaptee. Your checkout service never knows which gateway is behind the adapter.

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Idiomatic Example and Implementation Notes

The adapter is deliberately thin. Its only job is translation: map parameter types, convert result types, and delegate. No business logic lives here.

Use an object adapter (composition) as the default. It wraps the adaptee as a field, so it can adapt final classes, swap adaptees at runtime, and accept test doubles. A class adapter uses inheritance and, where supported, multiple inheritance; Java does not support multiple class inheritance, and inheritance couples the adapter to one adaptee type.