@FunctionalInterface注解的函数式接口

@FunctionalInterface
interface IService {
    void say(String message);
}

public class FunctionalInterfaceTest {
    public static void main(String[] args) {
        IService serviceObj = message -> System.out.println("Hello, " + message);
        serviceObj.say("Sam");
    }
}

输出结果：

Hello, Sam

Java8前已有的函数式接口

• java.lang.Runnable
• java.util.concurrent.Callable
• java.security.PrivilegedAction
• java.util.Comparator
• java.io.FileFilter
• java.nio.file.PathMatcher
• java.lang.reflect.InvocationHandler
• java.beans.PropertyChangeListener
• java.awt.event.ActionListener
• javax.swing.event.ChangeListener

Java8新增的java.util.function

内含43个函数式接口，如下：

函数式接口用法

Consumer

import java.util.function.BiConsumer;
import java.util.function.Consumer;

/**
 * Consumer传入一个对象，BiConsumer传入两个对象，实质都是对传入的T实体进行操作处理
 */
public class ConsumerDemo {

    public static void main(String[] args) {
        Consumer<String> consumer = System.out::println;
        consumer.accept("hello");
        BiConsumer<Integer, Integer> biConsumer = (a, b) -> System.out.println(a + "+" + b + "=" + (a + b));
        biConsumer.accept(3, 4);
    }

}

Function

import java.util.function.Function;

/**
 * Function实际上是对类型T实体进行相应的操作并返回类型为R的实体
 */
public class FunctionDemo {

    private static class OrderItem {
        double unitPrice;
        int count;

        public OrderItem(double unitPrice, int count) {
            this.unitPrice = unitPrice;
            this.count = count;
        }

        public double getPrice() {
            return unitPrice * count;
        }
    }

    public static void main(String[] args) {
        Function<OrderItem, Double> function = OrderItem::getPrice;
        OrderItem orderItem = new OrderItem(12.0, 5);
        System.out.printf("%.2f", function.apply(orderItem));
    }

}

Predicate

/**
 * Predicate确定实体T是否满足约束，返回boolean
 */
public class PredicateDemo {

    public static void main(String[] args) {
        Predicate<String> predicate = s -> s.length() < 20;
        System.out.println(predicate.test("123456"));
        System.out.println(predicate.test("1234567890123456789012"));
    }

}

Supplier

import java.util.function.Supplier;

/**
 * Supplier实际是创建了T实体并返回
 */
public class SupplierDemo {

    private static class Student {
        int id;
        String name;

        Student() {
            this.id = 0;
            this.name = "";
        }

        Student(int id, String name) {
            this.id = id;
            this.name = name;
        }

        @Override
        public String toString() {
            return "Student{" + "id=" + id + ", name='" + name + '\'' + '}';
        }
    }

    public static void main(String[] args) {
        Supplier<Student> supplier1 = Student::new;
        System.out.println(supplier1.get());
        Supplier<Student> supplier2 = () -> new Student(123, "Sam");
        System.out.println(supplier2.get());
    }

}