When would you choose an interface or an abstract class in Java?

Compares Java interfaces and abstract classes through contract, shared state, implementation reuse, and evolution trade-offs.

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TL;DR

I use an interface when I want to define a capability or contract that otherwise unrelated classes can implement. A class can implement multiple interfaces. I use an abstract class when implementations share a meaningful base identity, common state, protected behavior, or construction rules. A class can extend only one class. Interfaces can have default and static methods, but they do not own per-instance state, so they are not a complete substitute for a shared base implementation.

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A design choice based on type relationships and ownership rather than a memorized feature table.

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Core answer
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Overview
Both can define polymorphic behavior, but they express different relationships and provide different implementation tools.
I use an interface when I want to define a capability or contract that otherwise unrelated classes can implement. A class can implement multiple interfaces. I use an abstract class when implementations share a meaningful base identity, common state, protected behavior, or construction rules. A class can extend only one class. Interfaces can have default and static methods, but they do not own per-instance state, so they are not a complete substitute for a shared base implementation.
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A PaymentGateway is usually an interface because different providers need the same application-facing contract without sharing identity or state. An abstract FileImporter might be appropriate if all importers share lifecycle steps, configuration, and protected parsing support. Default interface methods can evolve a contract or share small behavior, but substantial stateful implementation often becomes awkward there. Composition is another option when implementations only need to reuse a helper rather than belong to one hierarchy.

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Choosing only by counting methods or fields.
Claiming interfaces contain no implemented methods.
Using an abstract class only to share unrelated utility code.
Adding an interface for every concrete class without a real contract.

When would you choose an interface or an abstract class in Java?

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