What are Extension Methods (C#) or Default Methods (Java)? When should you use them?

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.