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.

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