本文主要介绍Java 8的 Lambda 表达式的简单使用

基本语法

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(parameters) -> expression
(parameters) -> {statements;}

Apple

相关代码托管在java8_demo

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@Data
@AllArgsConstructor
@NoArgsConstructor
public class Apple {
    public static enum COLOR { GREEN, RED }
    public static final int HEAVY_WEIGHT = 200;
    private COLOR color;
    private Integer weight;
}

实例

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(String s) -> s.length()
// 等价于
int f(String s){ return s.length(); }

(Apple a) -> a.getWeight() > Apple.HEAVY_WEIGHT
// 等价于
boolean f(Apple a){ return a.getWeight() > Apple.HEAVY_WEIGHT; }

(int x, int y) -> {
    System.out.println("Result:");
    Syetem.out.println(x+y;
}
// 等价于
void f(int x, int y){
    System.out.println("Result:");
    Syetem.out.println(x+y;
}

() -> 42
// 等价于
int f(){ return 42; }

(Apple a1 , Apple a2) -> a1.getWeight() > a2.getWeight
// 等价于
boolean f(Apple a1 , Apple a2){ return a1.getWeight() > a2.getWeight;}

函数式接口 + 函数描述符

函数式接口

  1. 定义：只定义了一个抽象方法（排除 default 方法）的接口
  2. 用途：Lambda 表达式允许直接以内联的形式，为函数式接口的抽象方法提供实现，并把整个表达式作为函数式接口的实例

函数描述符

  1. 定义：函数式接口的抽象方法的签名
  2. 函数描述符 = 函数式接口的抽象方法的签名 = Lambda表达式的签名

Lambda 出现的地方

  1. 赋值给一个变量（类型为函数式接口）
  2. 传递给一个接受函数式接口为参数的方法

自定义函数式接口

函数式接口

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@FunctionalInterface
public interface Change { // 只有一个抽象方法
    Integer action(Integer param);
}

函数描述符：Integer -> Integer

单元测试

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private int localChange(int param, Change change) {
   return change.action(param);
}

@Test
public void multipyTest() {
   // Lambda 能够赋值给一个函数式接口的变量（前提是函数式接口的抽象方法的签名 = Lambda 表达式的签名）
   Change multiply = param -> param * 3;
   assertEquals(15, localChange(5, multiply));
   // Lambda 能够传递给一个接受函数式接口为参数的方法（前提是函数式接口的抽象方法的签名 = Lambda 表达式的签名）
   assertEquals(4, localChange(3, param -> param + 1));
}

常用内置函数式接口

函数式接口 函数描述符
Predicate<T> T -> boolean
Consumer<T> T -> void
Supplier<T> () -> T
Function<T,R> T -> R
BiFunction<T,U,R> (T,U) -> R
BiPredicate<T,U> (T,U) -> boolean
BinaryOperator<T> extends BiFunction<T,T,T> (T,T) -> T
BiConsumer<T,U> (T,U) -> void
UnaryOperator<T> extends Function<T,T> T -> T

Predicate

定义

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@FunctionalInterface
public interface Predicate<T> {
    boolean test(T t);
}

函数描述符：T -> boolean

实例

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private <T> List<T> predicate(List<T> list, Predicate<T> predicate) {
   List<T> result = new ArrayList<>();
   for (T t : list) {
       if (predicate.test(t)) {
           result.add(t);
       }
   }
   return result;
}

@Test
public void predicateTest() {
   assertEquals(2, predicate(Arrays.asList("zhongmingmao", "", null),
           s -> null == s || s.isEmpty()).size());
}

Consumer

定义

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@FunctionalInterface
public interface Consumer<T> {
    void accept(T t);
}

函数描述符：T -> void

实例

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private <T> void consume(List<T> list, Consumer<T> consumer) {
   for (T t : list) {
       consumer.accept(t);
   }
}

@Test
public void consumerTest() {
   consume(Arrays.asList(1, 2, 3), integer -> System.out.println(integer));
}

Supplier

定义

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@FunctionalInterface
public interface Supplier<T> {
    T get();
}

函数描述符：() -> T

实例

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private <T> T supply(Supplier<T> supplier) {
   return supplier.get();
}

@Test
public void supplierTest() {
   assertEquals(LocalDate.now(), supply(() -> LocalDate.now()));
}

Function<T,R>

定义

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@FunctionalInterface
public interface Function<T, R> {
    R apply(T t);
}

函数描述符：T -> R

实例

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private <T, R> R function(T t, Function<T, R> func) {
   return func.apply(t);
}

@Test
public void functionTest() {
   assertEquals(Integer.valueOf(12), function(Integer.valueOf(4), integer -> integer * 3));
}

BiFunction<T, U, R>

定义

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@FunctionalInterface
public interface BiFunction<T, U, R> {
    R apply(T t, U u);
}

函数描述符：(T,U) -> R

实例

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private <T, U, R> R biFunction(T t, U u, BiFunction<T, U, R> biFunc) {
   return biFunc.apply(t, u);
}

@Test
public void biFunctionTest() {
   assertEquals("40", biFunction(4, "0", (integer, s) -> integer + s));
}

BiPredicate<T, U>

定义

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@FunctionalInterface
public interface BiPredicate<T, U> {
    boolean test(T t, U u);
}

函数描述符：(T,U) -> boolean

实例

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private <T, U> boolean biPredicate(T t, U u, BiPredicate<T, U> biPred) {
   return biPred.test(t, u);
}

@Test
public void biPredicateTest() {
   assertTrue(biPredicate(4, "zhognmingmao", (integer, s) -> null != s && s.length() > integer));
}

BinaryOperator

定义

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@FunctionalInterface
public interface BinaryOperator<T> extends BiFunction<T,T,T> {
}

函数描述符：(T,T) -> T

实例

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private <T> T binaryOperator(T t, BinaryOperator<T> bOp) {
   return bOp.apply(t, t);
}

@Test
public void binaryOperatorTest() {
   assertEquals(Integer.valueOf(16), binaryOperator(4, (integer, integer2) -> integer * integer2));
}

BiConsumer<T, U>

定义

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@FunctionalInterface
public interface BiConsumer<T, U> {
    void accept(T t, U u);
}

函数描述符：(T,U) -> void

实例

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private <T, U> void biConsumer(T t, U u, BiConsumer<T, U> bCon) {
   bCon.accept(t, u);
}

@Test
public void biConsumerTest() {
   biConsumer(4, "zhongmingmao", (integer, s) -> System.out.println(integer + s.length()));
}

UnaryOperator

定义

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@FunctionalInterface
public interface UnaryOperator<T> extends Function<T, T> {
}

函数描述符：T -> T

实例

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private <T> T unaryOperator(T t, UnaryOperator<T> uOp) {
   return uOp.apply(t);
}

@Test
public void unaryOperatorTest() {
   assertEquals("zhongmingmao" , unaryOperator("zhongming" , s -> s + "mao"));
}

IntPredicate

  1. IntPredicate是Predicate<Integer>的**原始类型特化**，节省了自动装箱和自动拆箱的开销
  2. 类似函数式接口的还有 LongPredicate、IntFunction<R> 等

定义

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@FunctionalInterface
public interface IntPredicate {
    boolean test(int value);
}

实例

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@Test
public void intPredicateTest() {
   IntPredicate intPredicate = value -> value % 2 == 0;
   Predicate<Integer> integerPredicate = integer -> integer % 2 == 0;

   int max = 1 << 30;
   long p1 = System.currentTimeMillis();
   for (int i = 0; i < max; i++) {
       intPredicate.test(i);
   }
   long p2 = System.currentTimeMillis();
   for (int i = 0; i < max; i++) {
       integerPredicate.test(i);
   }
   long p3 = System.currentTimeMillis();
   System.out.println((p3 - p2) / (p2 - p1 + 0.0)); // 本地结果在100~220之间
   assertTrue((p3 - p2) > (p2 - p1));
}

类型检查

引用上面的例子进行分析

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// 函数定义
private <T> List<T> predicate(List<T> list, Predicate<T> predicate) {...}
// 函数调用
predicate(Arrays.asList("zhongmingmao", "", null), s -> null == s || s.isEmpty()).size());
  1. 函数定义中形参为Predicate<T> predicate
  2. Predicate<T>中的抽象函数的签名为boolean test(T t)，因此Predicate<T>的函数描述符为T -> boolean
  3. 函数调用中 Lambda 的签名为String -> boolean与Predicate<T>的函数描述符匹配，类型检查通过

方法引用

方法引用是Lambda的一种快捷写法（**语法糖**）

静态方法

  1. Lambda：(args) -> ClassName.staticMethod(args)
  2. 方法引用：ClassName::staticMethod
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Function<String, Integer> str2Integer = s -> Integer.parseInt(s);
str2Integer = Integer::parseInt;

任意类型的实例方法

  1. Lambda：(arg0,rest) -> arg0.instanceMethod(rest)（arg0是 ClassName 类型）
  2. 方法引用：ClassName::instanceMethod
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BiPredicate<List<String>, String> contains = (strings, s) -> strings.contains(s);
contains = List::contains;

现有对象的实例方法

  1. Lambda：(args) -> expr.instanceMethod(args)
  2. 方法引用：expr::instanceMethod
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List<String> list = Arrays.asList("a", "b", "A", "B");
Predicate<String> contain = s -> list.contains(s);
contain = list::contains;

构造函数

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Supplier<Apple> c1 = () -> new Apple();
c1 = Apple::new;// 默认构造函数
Apple apple = c1.get();

BiFunction<Apple.COLOR, Integer, Apple> c2 = (color, integer) -> new Apple(color, integer);
c2 = Apple::new;// 2个参数构造函数
apple = c2.apply(Apple.COLOR.GREEN, Integer.valueOf(200));

复合Lambda

Comparator复合

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@FunctionalInterface
public interface Comparator<T> {
    int compare(T o1, T o2);
}

// <T, U extends Comparable<? super U>> Comparator<T> comparing(
// Function<? super T, ? extends U> keyExtractor)
Comparator<Apple> comparator = Comparator.comparing(Apple::getWeight); // Apple -> Integer
Comparator<Apple> reversedComparetor = comparator.reversed();
Comparator<Apple> linkedComparator = Comparator.comparing(Apple::getWeight).reversed()
                                              .thenComparing(Apple::getColor).reversed();

Predicate复合

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Predicate<Apple> greenApple = apple -> Apple.COLOR.GREEN.equals(apple.getColor());
Predicate<Apple> notGreenApple = greenApple.negate();
Predicate<Apple> heavyApple = apple -> apple.getWeight() > Apple.HEAVY_WEIGHT;
Predicate<Apple> greenAndHeavyApple = greenApple.and(heavyApple);
Predicate<Apple> redApple = apple -> Apple.COLOR.RED.equals(apple.getColor());
Predicate<Apple> greenAndHeavyOrRedApple = greenAndHeavyApple.or(redApple);
// 优先级从左至右，（red or green） and heavy
Predicate<Apple> redOrGreenAndHeavyApple = redApple.or(greenApple).and(heavyApple);

Function复合

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Function<Integer, Integer> f = integer -> integer + 1;
Function<Integer, Integer> g = integer -> integer * 2;
Function<Integer, Integer> h = f.andThen(g); // g(f(x)) = (x+1)*2
Function<Integer, Integer> i = g.andThen(f); // f(g(x)) = (x*2)+1