What is a Functional Interface in Java?
Definition A functional interface in Java must have exactly one abstract method and may have other default or static methods. Example @FunctionalInterface public interface Calculator { int add ( int a, int b ); default void log ( String message ) { System.out.println ( "Log: " + message ); } static void showResult ( int result ) { System.out.println ( "Result: " + result ); } } // Using Lambda Calculator calculator = ( a, b ) -> a + b ; int result = calculator.add ( 5, 3 ); Calculator.showResult ( result ); A default method in an interface offers a built-in implementation that can be replaced by any class using it. This feature is exclusive to interfaces.