Java8新特性小记[参考菜鸟教程]

编程入门行业动态更新时间:2024-10-08 22:45:53
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Java8新特性小记[参考菜鸟教程]

部分新特性

lambda - 减少代码冗余，但可读性会略有降低
操作符
1. map - 对于每一个元素都按照特定函数进行变换
2. flatMap - 可进行元素的一对多/多对多的变换
全部测试案例
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;
import java.nio.charset.StandardCharsets;
import java.time.LocalDateTime;
import java.time.ZoneId;
import java.time.ZonedDateTime;
import java.util.*;
import java.util.function.BiFunction;
import java.util.function.IntBinaryOperator;
import java.util.function.Predicate;
import java.util.function.Supplier;
import java.util.stream.Collectors;public class Java8Test {public static void main(String[] args) {lambda();varScope();methodQuote();methodInterface();methodDefault();stream();optional();nashorn();date();base64();}/*** 1. lambda* 闭包*/private static void lambda() {IntBinaryOperator add = (int a, int b) -> a + b;IntBinaryOperator sub = (a, b) -> a - b;IntBinaryOperator multi = (int a, int b) -> a * b;IntBinaryOperator div = (a, b) -> a / b;BiFunction<String, String, Object> greet = (String s1, String s2) -> s1 + "! " + s2;System.out.println("4 + 2 = " + add.applyAsInt(4, 2));System.out.println("4 - 2 = " + sub.applyAsInt(4, 2));System.out.println("4 * 2 = " + multi.applyAsInt(4, 2));System.out.println("4 / 2 = " + div.applyAsInt(4, 2));System.out.println("greet to catface --> " + greet.apply("hi", "catface"));new Thread(() -> System.out.println("thread running...")).start();}/** 1.2. 变量作用域*/interface Callback {void complete(String msg);}private static void varScope() {String date = new Date().toString();Callback callback = msg -> System.out.println("you have msg: " + msg + " -- " + date);callbackplete("another day has gone, you are not alone...");}/*** 2. 方法引用* 使用::引用方法以减少代码冗余*/static class Person {public static Person create(Supplier<Person> supplier) {return supplier.get();}public static void showCommonMsg(Person person) {System.out.println("person's common msg...");}public void showPrivateMsg(Person person) {System.out.println("person's private msg...");}public void eat() {System.out.println("is eating...");}}/*** 3. 函数式接口* 方便被转换为lambda*/private static void methodQuote() {// 1. 构造器引用[Class<T>::new]Person[] persons = {Person.create(Person::new), Person.create(Person::new)};List<Person> peoples = Arrays.asList(persons);// 2. 静态方法引用[Class::static_method]peoples.forEach(Person::showCommonMsg);// 3. 特定对象的方法引用[instance::methodQuote]peoples.forEach(Person.create(Person::new)::showPrivateMsg);//4. 特定类的任意对象的方法引用[Class::methodQuote]peoples.forEach(Person::eat);// 方法引用实例String[] names = {"alistar", "akali", "ashe", "amumu", "ahri", "anivia", "annie"};Arrays.asList(names).forEach(System.out::println);}private static void methodInterface() {List<Integer> integers = Arrays.asList(-1, 0, 1, 3, 5, 7, 9, 11);System.out.print("集合中所有数据：");eval(integers, i -> true);System.out.print("集合中偶数：");eval(integers, i -> i % 2 == 0);System.out.print("集合中所有>4的数：");eval(integers, i -> i > 4);}private static void eval(List<Integer> list, Predicate<Integer> predicate) {for (Integer i : list) {if (predicate.test(i)) {System.out.print(i + " || ");}}System.out.println();}/*** 4. 接口默认方法* default为接口添加实现方法*/interface Animal {// 1. 接口的默认方法可被重写default void run() {System.out.println("animal is running...[interface's default method]");}// 2. 接口的静态方法不可被重写static void eat() {System.out.println("animal is eating...[interface's static method]");}}static class Cat implements Animal {@Overridepublic void run() {Animal.super.run();System.out.println("cat is running...");}}private static void methodDefault() {new Cat().run();Animal.eat();}/*** 5. Stream* 元素流*/private static void stream() {// forEach[迭代] || limit[获取指定数量的流]new Random().ints().limit(7).forEach(System.out::println);// mapList<Integer> integers = Arrays.asList(1, 5, 9, 3, 7, 9, 5, 2);List<Integer> collect1 = integers.stream().map(i -> i * 2).collect(Collectors.toList());System.out.println("map: " + collect1);// flatMapList<String> greets = Arrays.asList("hi", "hello", "welcome");List<String> names = Arrays.asList("alistar", "akali", "ahri");List<Object> collect = greets.stream().flatMap(strings -> names.stream().map(s -> strings + " - " + s)).collect(Collectors.toList());System.out.println("flatMap: " + collect);// filter || distinct[去重]List<Integer> collect2 = integers.stream().filter(i -> i > 4).distinct().collect(Collectors.toList());System.out.println("filter + distinct: " + collect2);// sorted排序List<Integer> collect3 = integers.stream().sorted(ComparatorparingInt(o -> -o % 2)).collect(Collectors.toList());System.out.println("sorted with Comparator: " + collect3);// parallel[并行运行]List<String> strings = Arrays.asList("qq", "", "ww", "ee", "rr");List<String> collect4 = strings.parallelStream().filter(str -> !str.isEmpty()).collect(Collectors.toList());System.out.println("parallel: " + collect4);// reduceintegers.stream().reduce((i1, i2) -> i1 + i2).ifPresent(System.out::println);integers.stream().reduce((i1, i2) -> i1 > i2 ? i1 : i2).ifPresent(System.out::println);integers.stream().reduce(Integer::min).ifPresent(System.out::println);// multi reducetestMultiReduce();// 统计IntSummaryStatistics intSummaryStatistics = integers.stream().mapToInt(x -> x).summaryStatistics();System.out.println("sum: " + intSummaryStatistics.getSum());System.out.println("max: " + intSummaryStatistics.getMax());System.out.println("min: " + intSummaryStatistics.getMin());System.out.println("average: " + intSummaryStatistics.getAverage());System.out.println("count: " + intSummaryStatistics.getCount());}private static void testMultiReduce() {ArrayList<List<String>> strings = new ArrayList<>();strings.add(Arrays.asList("1", "2", "3", "4"));strings.add(Arrays.asList("2", "3", "4", "5"));strings.add(Arrays.asList("3", "4", "5", "6"));// 非并行流Integer reduce1 = strings.stream().flatMap(Collection::stream).reduce(0,(str, i) -> str + Integer.valueOf(i), (i1, i2) -> {// 非并行流，不会执行第三个参数System.out.println("i1、i2: " + i1 + "、" + i2);// 这里的返回值并没有影响返回结果return null;});System.out.println("reduce1: " + reduce1);// 并行流Integer reduce2 = strings.parallelStream().flatMap(Collection::stream).reduce(0,(str, i) -> str + Integer.valueOf(i), (i1, i2) -> {// u，t分别为并行流每个子任务的结果System.out.println("i1、i2: " + i1 + "、" + i2);return i1 + i2;});System.out.println("reduce2: " + reduce2);}/*** 6. Optional* 可为null的容器对象*/private static void optional() {Integer i1 = null;Integer i2 = 7;Optional<Integer> o1 = Optional.ofNullable(i1);Optional<Integer> o2 = Optional.ofNullable(i2);System.out.println(sum(o1, o2));}private static Integer sum(Optional<Integer> a, Optional<Integer> b) {// Optional.isPresentSystem.out.println("第一个参数值存在: " + a.isPresent());System.out.println("第二个参数值存在: " + b.isPresent());// Optional.orElse: 不为空则值为a否则值为other默认值int i1 = a.orElse(0);//Optional.get(): 必须不为null否则报错int i2 = b.orElse(0);return i1 + i2;}/*** 7. Nashorn JS* 使用ScriptEngineManager在Java中执行JS*/private static void nashorn() {ScriptEngineManager scriptEngineManager = new ScriptEngineManager();ScriptEngine nashorn = scriptEngineManager.getEngineByName("nashorn");String name = "catface";Integer result = null;try {nashorn.eval("print('" + name + "')");result = (Integer) nashorn.eval("10 + 2");} catch (ScriptException e) {System.out.println("执行脚本错误 : " + e.getMessage());}System.out.println(result != null ? result.toString() : null);}/*** 8. 日期时间API*/private static void date() {LocalDateTime now = LocalDateTime.now();System.out.println(now + " || " + now.toLocalDate() + " || " + now.toLocalTime());System.out.println(now.getYear() + " || " + now.getMonthValue() + " || " + now.getDayOfMonth() + " || " + now.getDayOfYear() + " || " + now.getDayOfWeek() + " || " + now.getHour() + " || " + now.getMinute() + " || " + now.getSecond() + " || " + now.getNano());// 时区ZonedDateTime date = ZonedDateTime.parse("2015-12-03T10:15:30+05:30[Asia/Shanghai]");System.out.println(date);ZoneId zoneId = ZoneId.of("Europe/Paris");System.out.println("zoneId: " + zoneId + " || 当前时区：" + zoneId.systemDefault());}/*** 9. base64* 基本：输出被映射到一组字符A-Za-z0-9+/，编码不添加任何行标，输出的解码仅支持A-Za-z0-9+/* URL：输出映射到一组字符A-Za-z0-9+_，输出是URL和文件* MIME：输出隐射到MIME友好格式。输出每行不超过76字符，并且使用'\r'并跟随'\n'作为分割。编码输出最后没有行分割*/private static void base64() {// 基本编码String encode = Base64.getEncoder().encodeToString("catface".getBytes(StandardCharsets.UTF_8));System.out.println("base64编码[基本] : " + encode);// 解码System.out.println("原始字符串 : " + new String(Base64.getDecoder().decode(encode), StandardCharsets.UTF_8));// URL编码encode = Base64.getUrlEncoder().encodeToString("catfaceWYH".getBytes(StandardCharsets.UTF_8));System.out.println("base64编码[URL] : " + encode);// MIME编码StringBuilder stringBuilder = new StringBuilder();for (int i = 0; i < 10; ++i) {stringBuilder.append(UUID.randomUUID().toString());}byte[] mimeBytes = stringBuilder.toString().getBytes(StandardCharsets.UTF_8);System.out.println("base64编码[MIME] : " + Base64.getMimeEncoder().encodeToString(mimeBytes));}
}