What are Extension Methods (C#) or Default Methods (Java)? When should you use them?
Extension Methods and Default Interface Methods
C# extension methods allow developers to add callable methods to an existing type without modifying or inheriting from that type. They are statically resolved and do not actually modify the target class. Java default interface methods instead provide an implementation as part of an interface and are dynamically dispatched according to interface method rules. I use extension methods for small, discoverable utility behavior and default methods primarily for evolving interfaces or providing genuinely shared contract behavior.
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C# extension methods are useful for focused helper behavior and fluent APIs.
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They cannot access private members of the extended type merely because they are extensions.
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Java default methods help evolve interfaces without forcing every implementation to immediately provide a new method.
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Do not use extension methods to hide important domain behavior that belongs to the type itself.
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Do not put large business workflows into default interface methods.
Follow-up Questions
- Are C# extension methods virtual?
- Can extension methods access private fields?
- Why were Java default methods introduced?
- What happens when multiple interfaces provide conflicting defaults?