Adapter Pattern Structure and Participants

The Adapter Pattern has two common implementations: Class Adapter (uses inheritance) and Object Adapter (uses composition). Object Adapter is more commonly used because it follows the composition over inheritance principle.
Object Adapter Structure (Recommended)«interface»
Target
+ request(): void
▲ implements
Adapter
- adaptee: Adaptee
+ request(): void
◆ contains
Adaptee
+ specificRequest(): void
Participants and ResponsibilitiesTarget Interface: Defines the domain-specific interface that the Client uses. This is what the client code expects to work with.
Adapter: Implements the Target interface and holds a reference to the Adaptee. It translates calls from the Target interface to the Adaptee's interface. The translation logic resides here.
Adaptee: The existing class with an incompatible interface that needs adapting. It contains useful functionality but cannot be used directly by the client.
Client: Collaborates with objects conforming to the Target interface. It remains unaware of the Adapter's existence or the Adaptee's incompatible interface.
Call Flow1. Client calls adapter.request()
2. Adapter translates the call
3. Adapter calls adaptee.specificRequest()
4. Adaptee performs the work
5. Adapter returns result to Client (if needed)
Interview Tip: Always clarify whether to use Class Adapter (multiple inheritance, if language supports) or Object Adapter (composition). Object Adapter is preferred in most languages and follows SOLID principles better.

💡 Key Takeaways

✓Object Adapter uses composition (◆) to hold Adaptee reference

✓Class Adapter uses inheritance (▲) from both Target and Adaptee

✓Adapter implements Target interface and delegates to Adaptee

✓Client depends only on Target interface, not concrete Adapter

✓Translation logic is encapsulated within the Adapter class

📌 Examples

1Java's Arrays.asList() adapts arrays to List interface

2InputStreamReader adapts InputStream to Reader interface

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