非常简单且实用的函数式编程(国庆day1)

函数式：用Lambda表达式实现函数的方式

1 什么时候使用函数式编程？

1.1 集合的操作

1.1.1 转换

list set map之间转换

1.1.2 聚合操作

count：获取数据数量 max：获取按照规则约定的最大值 min：获取按照规则约定的最小值 avg：获取平均值

1.1.3 循环处理

foreach：对数据进行循环处理

1.1.4 转换为String

joining：字符串数据指定分隔符

1.1.5 分组

groupingBy：按照特定功能处理分组

1.1.6 获取数组

toArray：数据转换为Object数组

1.2 Stream的操作

1.2.1 过滤

filter：按照规则筛选数据 distinct：过滤掉相同的数据

1.2.2 转换

map：接收一个T，返回一个R flatMap：处理Stream嵌套问题

1.2.3 其他操作

skip：忽略前n条数据 limit：限制只需要前n条数据 sorted：将数据排序

1.3 Optional的操作

1.3.1 检测是否为空

isPresent：存在返回true isEmpty：为空返回true

1.3.2 条件运算

ifPresent：存在则执行

1.3.3 默认值

orElse：当Optional为空时的的默认值

1.3.4 获取值

get：不为空时，获取数据

1.3.5 过滤

filter：对数据进行过滤

1.4 函数接口

1.4.1 Predicate

有输入且输出为布尔值

1.4.2 Function

有输入且有输出

1.4.3 Supplier

有输出没有输入

1.4.4 Consumer

有输入且没有输出

1.4.5 Operator

输入和输出同一类型

1.4.6 Comparator

输入两个值且输出为布尔值

1.5 方法引用

1.5.1 构造函数引用

::new

1.5.2 静态方法的引用

Integer:: toString

1.5.3 实例方法引用

person:: getName

1.5.4 引用任意对象的方法

forEach(person:: getName)

1.6 lambda表达式使用

()输入参数->()返回值

2 集合操作示例

public class _03finalStream { public static void main(String[] args) { List<Person> people = Arrays.asList(new Person("zhangsan", "男"), new Person("lisi", "男"), new Person("xiaohong", "女")); /** * 聚合操作 */ //数量 people.stream().count(); //最大值 people.stream().max(Comparator.comparing(Person::getWeight)); /** * 循环 */ people.stream().forEach(System.out::println); /** * 匹配 */ //全匹配 people.stream().allMatch(Person::isMale); //任意匹配 people.stream().anyMatch(Person::isMale); //不匹配 people.stream().noneMatch(Person::isMale); /** * 查找 */ //获取任意数据 Person person = people.stream().findAny().get(); //获取第一条数据 Person person1 = people.stream().findFirst().get(); /** * 获取数组 */ Object[] peopleArray = people.stream().toArray(); /** * 累计聚合操作 */ Integer r1 = Stream.of(1, 2, 3).reduce((a, b) -> a + b).get(); /** * collect */ Stream<String> nameStream = Stream.of("zhangsan", "lisi"); //转为集合 List<String> nameList = nameStream.collect(Collectors.toList()); // Set<String> nameSet = nameStream.collect(Collectors.toSet()); //转为String // String nameString = nameStream.collect(Collectors.joining(",")); //聚合操作 //数量 // Long nameCount = nameStream.collect(Collectors.counting()); //最大值 /// nameStream.collect(Collectors.maxBy(Comparator.comparing(String::length))).get(); //平均值 // Double nameLength = nameStream.collect(Collectors.averagingLong(String::length)); //排序，最大值，最小值，平均值，数量,分组 List<Integer> numList = Arrays.asList(3, 2, 1, 4, 8, 6, 7); Stream<Integer> numStream = numList.stream(); numStream.sorted(Comparator.comparing(Integer::intValue)).forEach(System.out::println); IntSummaryStatistics numSummary = numStream.collect(Collectors.summarizingInt(Integer::intValue)); System.out.println(numSummary.getMax() + " " + numSummary.getMin() + " " + numSummary.getAverage() + " " + numSummary.getCount()); Map<Integer, List<Integer>> map = numStream.collect(Collectors.groupingBy(Integer::intValue)); /** * 生成map */ Map<Integer, Integer> map1 = numStream.collect(Collectors.toMap(Function.identity(), Integer::intValue)); } }

3 Stream操作示例

public class _02IntermediateStream { public static void main(String[] args) { /** * filter */ List<Person> people = Arrays.asList(new Person("zhangsan", "男"), new Person("lisi", "男"), new Person("xiaohong", "女")); //1 函数接口 people.stream().filter(person -> person.getGender().equals("男")) .forEach(person -> System.out.println(person)); //2 方法引用 people.stream().filter(Person::isMale).forEach(System.out::println); /** * distinct */ people.stream().distinct() .forEach(System.out::println); people.stream().distinct() .filter(person -> person.getGender().equals("男")) .forEach(System.out::println); /** * map */ Stream<Integer> nameLength = people.stream() .map(Person::getName) .map(String::length); /** * flatMap:解决stream嵌套问题 */ List<Person> people1 = Arrays.asList(new Person("zhangsan", "man", 100)); List<Person> people2 = Arrays.asList(new Person("lisi", "woman", 200)); List<List<Person>> peopleList = Arrays.asList(people1, people2); /** * 使用map需要处理嵌套的Stream */ peopleList.stream() .map(Collection::stream) .forEach(personStream -> personStream.forEach(System.out::println)); /** * 将一个流转为两个流，然后再合成为一个流 */ peopleList.stream() .flatMap(Collection::stream) .forEach(System.out::println); /** * 其他操作 */ /** * 忽略前N条 */ people1.stream().skip(1).forEach(System.out::println); /** * 只取前N条 */ people1.stream().limit(1).forEach(System.out::println); /** * 将数据排序 */ people1.stream().sorted(Comparator.comparing(Person::getWeight)).forEach(System.out::println); } }

4 Optioanl操作示例

public class _01getOptional { public static void main(String[] args) { Optional<Object> emptyOptional = Optional.empty(); Optional<String> stringOptional = Optional.of("zhangsan"); String str1 = "zhangsan"; String str2 = null; Optional<String> stringOptional1 = Optional.ofNullable(str1); Optional<String> nullOptional2 = Optional.ofNullable(str2); //检查是否存在 System.out.println(stringOptional.isPresent()); System.out.println(nullOptional2.isPresent()); System.out.println(nullOptional2.isEmpty()); //条件运算 stringOptional1.ifPresent(System.out::println); //默认值 nullOptional2.orElse("lisi"); //获得值 if (nullOptional2.isPresent()) { nullOptional2.get(); } //数据过滤 nullOptional2.filter(s -> s.equals("zhangsan")).isPresent(); //转换处理 String str2 = nullOptional2.map(s -> "map" + s).get(); } }

5 函数式接口

5.1 Predicate

public class _01Predicate { public static void main(String[] args) { /** * i>0 */ IntPredicate greaterThan0 = i -> i > 0; System.out.println(greaterThan0.test(10)); /** * negate: i<=0 */ IntPredicate negate = greaterThan0.negate(); System.out.println(negate.test(10)); /** * and: i>0 and i<100 */ IntPredicate lessThan100 = i -> i <= 100; IntPredicate and = greaterThan0.and(lessThan100); System.out.println(and.test(10)); /** * or: i>0 or i>0 or i<100 */ IntPredicate or = greaterThan0.or(lessThan100); System.out.println(or.test(-100)); /** * BiPredicate:两个参数 */ BiPredicate<String,Integer> isLenThanGivenLen = (str, len) -> str.length() > len; System.out.println(isLenThanGivenLen.test("123",3)); } }

5.2 Function

public class _02Function { public static void main(String[] args) { /** * apply */ Function<String, Integer> lengthFunction = str -> str.length(); System.out.println(lengthFunction.apply("123")); Function<Integer, Integer> plusFunction = x -> x + x; Function<Integer,Integer> multiFunction = x -> x * x; /** * andThen */ Function<Integer, Integer> andThen = plusFunction.andThen(multiFunction); System.out.println(andThen.apply(2)); /** * compose */ Function<Integer, Integer> compose = plusFunction.compose(multiFunction); System.out.println(compose.apply(2)); /** * 参数固化 */ IntFunction<Integer> intPlusFunction = x -> x + x; System.out.println(intPlusFunction.apply(2)); /** * 返回值固化 */ ToIntFunction<Integer> toIntPlusFunction = x -> x + x; System.out.println(toIntPlusFunction.applyAsInt(2)); /** * 参数和返回值都固化 */ IntToLongFunction intToLongPlusFunction = x -> x + x; System.out.println(intToLongPlusFunction.applyAsLong(2)); /** * 两个参数 */ BiFunction<String,String,Integer> biFunction = (str1, str2) -> str1.length() + str2.length(); System.out.println(biFunction.apply("123","456")); } }

5.3 Consumer

public class _03Consumer { public static void main(String[] args) { Consumer<String> lengthConsumer = str -> System.out.println(str.length()); lengthConsumer.accept("123"); } }

5.4 Supplier

public class _04Supplier { public static void main(String[] args) { Supplier<Long> supplier = () -> System.currentTimeMillis(); System.out.println(supplier.get()); } }

5.5 Operator

public class _05Operator { public static void main(String[] args) { IntUnaryOperator operatorFunction = x -> x + x; System.out.println(operatorFunction.applyAsInt(2)); } }

5.6 Comparator

public class _06Comparator { public static void main(String[] args) { Comparator<Integer> compareFunction = (num1, num2) -> num1.compareTo(num2); System.out.println(compareFunction.compare(1,2)); } }

6 方法引用

6.1 Constructor

public class _01Constructor { public static void main(String[] args) { //1 无参数构造，只生产不消费 Supplier<Person> emptyConstructor = Person::new; Person person = emptyConstructor.get(); person.setName("zhangsan"); System.out.println(person); //2 一个参数构造，既生产又消费 Function<String, Person> nameConstructor = Person::new; Person person1 = nameConstructor.apply("zhangsan"); System.out.println(person1); //3 两个参数构造，既生产又消费 BiFunction<String, String, Person> nameAndGenderConstructor = Person::new; Person person2 = nameAndGenderConstructor.apply("zhangsan", "man"); System.out.println(person2); } }

6.2 Static

public class _02Static { public static void main(String[] args) { /** * toString方法是Integer类的静态方法 */ IntFunction<String> intToStringFunction = Integer::toString; System.out.println(intToStringFunction.apply(123)); } }

6.3 Instance

public class _03Instance { public static void main(String[] args) { Person person = new Person("zhangsan", "man", 100); Consumer<String> walkConsumer = person::walk; walkConsumer.accept("上海路"); } }

6.4 AnyInstance

public class _04AnyInstance { public static void main(String[] args) { List<Person> people = Arrays.asList(new Person("zhagnsan", "man", 123) , new Person("lisi", "man", 456)); people.forEach(Person::sayName); } }