What is an interface in Java?

An interface is a reference type in Java that defines a contract — a set of abstract methods that implementing classes must provide. Interfaces support multiple inheritance: a class can implement multiple interfaces. Before Java 8, interface methods were all abstract and public. Since Java 8, interfaces can have default methods (concrete methods with a default implementation) and static methods. Since Java 9, they can have private methods. All fields in an interface are implicitly public static final. Use interfaces to define capabilities (e.g., Comparable, Serializable, Runnable) that unrelated classes can implement.