Default & Static Methods in Interfaces

Java 8 reshaped interfaces by letting them carry real method bodies, not just signatures. Default methods add behaviour that implementing classes inherit for free — which is how core interfaces like Collection grew without breaking old code — while also raising the diamond problem and its three resolution rules. Static methods, by contrast, give an interface its own helper and factory methods (like List.of()), called on the interface name and never inherited. Together, these two features explain how modern Java interfaces actually work. This pair of guides covers both, with hands-on examples.