Functional Interfaces
/**
* FUNCTIONAL INTERFACES
* ---------------------
* A functional interface is an interface with one abstract method.
*
* Example:
* - Runnable
* - Comparator<T>
* - Predicate<T>
* - BinaryOperator<T>
*
* @FunctionalInterface
* public interface Predicate<T> {
* boolean test(T var1); // abstract
* ...
*
* Functional interfaces are commonly used with lambdas, streams,
* and method references.
*/
package com.minte9.lambdas.functional_interfaces;
import java.util.function.Function;
import java.util.function.Predicate;
public class FunctionalInterfaces {
public static void main(String[] args) {
// -----------------------------------------
// Predicate using a lambda expression
// -----------------------------------------
Predicate<Integer> isEven = x -> x % 2 == 0;
System.out.println(isEven.test(36)); // true
// -------------------------
// Function example
// ----------------------------
Function<Long, Long> increment = x -> x + 1;
System.out.println(increment.apply(20L)); // 21
}
}
Custom Functional Interface
/**
* CUSTOM FUNCTIONAL INTERFACE
* ---------------------------
* You can define your own functional interface when:
* - built-in ones (Predicate, Function, etc) don't fit
* - you want clearer domain-specific meaning
*
* A functional interface may containt:
* - exactly ONE abstract method
* - zero or more default methods
* - zero or more static methods
*
* Only the abstract method counts toward the "functional" rule.
*
* A method reference works when the referenced method
* has the SAME signature as the functional interface method.
*/
package com.minte9.lambdas.functional_interfaces;
public class CustomFunctionalInterface {
public static void main(String[] args) {
MyFuncInterface exclaim = s -> s + "!";
MyFuncInterface question = s -> s + "?";
System.out.println(exclaim.run("Hello World")); // Hello World!
System.out.println(question.run("Java Lambdas")); // Java Lambdas?
System.out.println(exclaim.describe()); // MyFuncInterface: transforms a String
}
}
// Custom functional interface
@FunctionalInterface
abstract interface MyFuncInterface {
// Single abstract method (SAM)
String run(String s);
// Default method (does NOT count as abstract)
default String describe() {
return "MyFuncInterface: transforms a String";
}
}
