When should you use the Adapter pattern in a project?

The Adapter pattern should be used when you need to connect incompatible interfaces: that is, to make classes or modules that were not originally designed to work together do so.

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#1. Integrating Legacy Code with a New System

When you are modernizing a system but cannot change the legacy components. The adapter "translates" the old interface into the new one, preserving functionality without rewriting the logic.

Example: The new system uses PaymentService.process(), while the old module uses LegacyPay.runPayment(). The adapter implements PaymentService, internally calling LegacyPay.runPayment().

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#2. Using Third-Party Libraries and APIs

If an external SDK, library, or API has a different interface, an adapter lets you integrate it without changing either the library or your code.

Example: You use an external email service whose methods are named differently - the adapter fits it to your internal EmailSender interface.

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#3. Aligning Interfaces of Different Classes

When you have several similar classes with different interfaces that need to be used in a uniform way. The adapter creates a single interaction contract.

Example: The classes XmlDataReader and JsonDataReader are brought to a common DataReader interface through adapters.

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#4. Working with Legacy Systems

Enterprise projects often keep old modules that are expensive or impossible to change. The adapter lets you temporarily "cover" the old code with a new interface, preserving backward compatibility.

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#5. Gradual Transition Between Architectures

During migration from a monolith to microservices, an API change, or a move to new data formats (XML to JSON). The adapter acts as a temporary bridge until the systems are unified.

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#6. Conclusion

Use Adapter when:

• classes cannot be changed but need to be combined;
• interfaces differ, but the logic must be shared;
• you need to preserve legacy code or external dependencies without breaking the architecture.

Summary: Adapter is applied when you need to ensure compatibility without modifying the original code, acting as a "translator" between two worlds.