What is the Adapter pattern?

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creational

Adapter

Convert the interface of a class into another interface that clients expect.

You have code that expects one interface, and a class that offers a different one. Both work fine. They just do not fit.

This happens constantly with third-party libraries and old systems. You cannot edit the library, and rewriting your own code to match it would spread that library's shape through your whole project.

Adapter is a small class that sits between them. It implements the interface your code expects, and translates each call into whatever the other class actually needs.

What the interviewer is probing.

1. "What does Adapter do?" *Probing:* the purpose. *Stalls:* "Converts." *Moves up:* makes an existing interface usable through the one your code expects, without changing either side.

2. "How does it differ from Facade?" *Probing:* the distinction. *Stalls:* "Nothing." *Moves up:* Adapter converts one interface to another specific one; Facade simplifies a whole subsystem behind a new interface.

3. "What is the object versus class adapter?" *Probing:* the implementation detail. *Stalls:* "Nothing." *Moves up:* object adapter composes the adaptee and is the usual choice; class adapter inherits, which needs multiple inheritance and is more brittle.

4. "Where does it appear in real codebases?" *Probing:* the practical angle. *Stalls:* "Rarely." *Moves up:* wrapping a third-party SDK behind your own interface, which is also what makes replacing that vendor possible.

A power plug adapter — your device expects round pins; the wall has flat pins. The adapter bridges the gap.

Key Concepts

The adapter implements the interface your code already uses, so your code needs no changes at all.

Inside, it holds a reference to the class it is adapting. Each method converts the arguments, calls the real method, and converts the result back.

All the awkwardness lives in this one small class. If you later replace the library, you rewrite the adapter and nothing else.

java

public class LegacyAnalytics {
    public void trackEvent(String cat, String action, int val) { }
}
public interface Analytics {
    void logEvent(String name, Map<String, Object> props);
}
public class AnalyticsAdapter implements Analytics {
    private final LegacyAnalytics legacy;
    public AnalyticsAdapter(LegacyAnalytics l) { this.legacy = l; }
    public void logEvent(String name, Map<String, Object> props) {
        String cat = (String) props.getOrDefault("category", "general");
        int val = (int) props.getOrDefault("value", 0);
        legacy.trackEvent(cat, name, val);
    }
}