Quick Answer
Functional interfaces have exactly one abstract method (can have default and static methods). Annotate with @FunctionalInterface (optional but recommended). Built-in functional interfaces in java.util.function: Predicate (test, returns boolean), Function (apply, transforms T to R), Consumer (accept, takes T returns void), Supplier (get, returns T). Lambdas can be assigned to functional interfaces.
Answer
Functional interfaces have one abstract method (SAM). Examples include Runnable, Callable, Comparator. Marked with @FunctionalInterface and used with lambda expressions.
S
SugharaIQ Editorial Team
Verified Answer
This answer has been peer-reviewed by industry experts holding senior engineering roles to ensure technical accuracy and relevance for modern interview standards.
