Adapter

Allow objects with incompatible interfaces to collaborate.

Real-World Analogy

A travel adapter lets you plug a US plug (flat pins) into a European socket (round holes). The adapter converts one interface to another.

The Pattern

flowchart LR
    Client --> Target[Target Interface]
    Target --> Adapter[Adapter]
    Adapter --> Adaptee[Adaptee<br/>Incompatible Interface]

    style Client fill:#3b82f6,color:#fff
    style Target fill:#7c3aed,color:#fff
    style Adapter fill:#f59e0b,color:#fff
    style Adaptee fill:#ef4444,color:#fff

❌ Bad Example — Directly Using Incompatible Code

// Old system — returns XML
class OldWeatherAPI {
  getXML() {
    return '<weather><temp>25</temp></weather>';
  }
}

// New system — expects JSON
class WeatherApp {
  display(data) {
    console.log(`Temperature: ${data.temp}°C`);
  }
}

// ❌ Can't directly use OldWeatherAPI with WeatherApp
const oldAPI = new OldWeatherAPI();
const app = new WeatherApp();
// app.display(oldAPI.getXML()); // Won't work — expects JSON

✅ Fixed Example

// Target interface
class WeatherApp {
  display(data) {
    console.log(`Temperature: ${data.temp}°C`);
  }
}

// Adaptee (incompatible)
class OldWeatherAPI {
  getXML() {
    return '<weather><temp>25</temp></weather>';
  }
}

// Adapter
class WeatherAdapter {
  constructor(oldAPI) {
    this.oldAPI = oldAPI;
  }

  getJSON() {
    const xml = this.oldAPI.getXML();
    // Simple XML to JSON conversion
    const temp = xml.match(/<temp>(\d+)<\/temp>/)[1];
    return { temp: parseInt(temp) };
  }
}

// Usage
const oldAPI = new OldWeatherAPI();
const adapter = new WeatherAdapter(oldAPI);
const app = new WeatherApp();

app.display(adapter.getJSON()); // "Temperature: 25°C" ✅

When to Use

You have an existing class with an incompatible interface
You want to use a third-party library without changing your code
You need to wrap legacy code behind a modern interface

When NOT to Use

You can modify the source code — just change the interface directly
The interfaces are already compatible

In Simple Words

Wraps an incompatible object so it looks like what the client expects
Like a power adapter between two different plug types
Lets old and new code work together without changing either