函数式编程和lambda表达式--学习笔记

1. 认识lambda

   package com.yu.lambda;
   
   public class TestLambda {
       public static void main(String[] args) throws Exception {
   
           new Thread(() -> System.out.println("OK")).start();
   
           IBookNumber book = (i) -> i * 2;
           System.out.println(book.isbn(897654));
           
           System.out.println(book.name("a"));
       }
   }
   
   @FunctionalInterface
   interface IBookNumber {
       int isbn(int i);
       default String name(String name) {
           return "java " + name;
       }
   }

   OK
   1795308
   java a

2. 函数接口

   package com.yu.lambda;
   
   import java.text.DecimalFormat;
   import java.util.function.Function;
   
   public class TestFunction {
       public static void main(String[] args) throws Exception {
           A a = new A(1234567890);
           Function mf = 
                   i -> new DecimalFormat("#,###").format(i);
           a.pM(mf.andThen(s -> " RMB: ¥ " + s));
   
           //只关注输入输出, 不关注细节
       }
   }
   
   class A {
       private int m;
       public A(int m) {
           this.m = m;
       }
       public void pM(Function mf) {
           System.out.println("--test--" + mf.apply(m));
       }
   }

   --test-- RMB: ¥ 1,234,567,890

3. 方法引用

   package com.yu.lambda;
   
   import java.util.function.*;
   
   public class TestMethodRefrence {
       public static void main(String[] args) throws Exception {
           Consumer c = s -> System.out.println(s);
           c.accept("p");
   
           Consumer m = System.out::println;
           m.accept("ps");
   
           Cat cat = new Cat();
           Consumer q = Cat::sleep;
           q.accept(cat);
   
           //分析  输入, 输出
           Function w = cat::eat;
           System.out.println("food surplus " + w.apply(1));
   
           IntUnaryOperator e = cat::eat;
           System.out.println("food surplus " + e.applyAsInt(2));
   
           //jdk 默认把当前实例传入到非静态方法
           //public int eat(Cat cat, int number) {
           BiFunction t = Cat::eat;
           System.out.println("food surplus " + t.apply(cat, 3));
   
           Supplier s = Cat::new;
           System.out.println("create new cat : " + s.get());
   
           //输入 String, 输出 Cat
           Function d = Cat::new;
           System.out.println("create new cat : " + d.apply("mm"));
       }
   }
   
   class Cat {
       private String name;
       private int food = 10;
       public Cat() {
           this.name = "xiaobai";
       }
       public Cat(String name) {
           this.name = name;
       }
       public static void sleep(Cat cat) {
           System.out.println("cat sleep......");
       }
       public int eat(int number) {
           System.out.println("cat eat food " + number);
           this.food -= number;
           return this.food;
       }
       public String toString() {
           return this.name;
       }
   }

   p
   ps
   cat sleep......
   cat eat food 1
   food surplus 9
   cat eat food 2
   food surplus 7
   cat eat food 3
   food surplus 4
   create new cat : xiaobai
   create new cat : mm

4. 类型推断

   package com.yu.lambda;
   
   public class TestTypeInference {
       public static void main(String[] args) throws Exception {
           IMath im = (x, y) -> x + y;
           System.out.println("x + y = " + im.add(1, 2));
   
           IMath[] ims = { (x, y) -> x + y };
   
           Object obj = (IMath)(x, y) -> x + y;
   
           IMath m = getLambda();
   
           new TestTypeInference().testParam((x, y) -> x + y);
       }
   
       public static IMath getLambda() {
           return (x, y) -> x + y;
       }
   
       public void testParam(IMath math) {
           System.out.println("x + y = " + math.add(3, 4));
       } 
   }
   
   @FunctionalInterface
   interface IMath {
       int add(int x, int y);
   }

   x + y = 3
   x + y = 7

5. 变量引用
   编译器说 Lambda 表达式中的变量必须是 final 的，我偏不信