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feat: 增加 Java 21 新特性示例代码
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‎core-java-modules/core-java-20/src/main/java/JEP431Test.java‎

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/**
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* JDK 21 之前，顺序集合中操作体验不一致
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* @author https://www.wdbyte.com
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* @date 2023/10/12ø
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*/
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public class JEP431Test {
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public static void main(String[] args) {
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System.out.println();
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// sortedSet linkedHashMap 逆序输出很难操作
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}
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}

‎core-java-modules/core-java-21/README.md‎

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## Java 21 新特性示例

‎core-java-modules/core-java-21/pom.xml‎

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<?xml version="1.0" encoding="UTF-8"?>
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<project xmlns="http://maven.apache.org/POM/4.0.0"
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xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
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xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
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<modelVersion>4.0.0</modelVersion>
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<parent>
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<groupId>com.wdbyte.core-java-modules</groupId>
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<artifactId>core-java-modules</artifactId>
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<version>1.0.0-SNAPSHOT</version>
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</parent>
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<groupId>com.wdbyte.jdk21</groupId>
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<artifactId>core-java-21</artifactId>
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<properties>
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<maven.compiler.source>21</maven.compiler.source>
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<maven.compiler.target>21</maven.compiler.target>
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<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
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</properties>
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</project>

‎core-java-modules/core-java-21/src/main/java/Jep430StringTemplate.java‎

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import java.time.LocalDate;
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import java.util.FormatProcessor;
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import java.util.Locale;
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/**
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* 字符串模版
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*
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* @author https://www.wdbyte.com
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* @date 2023/10/13
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*/
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public class Jep430StringTemplate {
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public static void main(String[] args) {
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int x = 20;
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int y = 3;
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String s = x + " + " + y + " = " + (x + y);
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System.out.println(s);
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String sb = new StringBuilder().append(x).append(" + ").append(y).append(" = ").append(x + y).toString();
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System.out.println(sb);
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String sFormat = String.format("%d + %d = %d", x, y, x + y);
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System.out.println(sFormat);
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System.out.println("--------------------");
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// JDK 21 使用字符串模版 STR 进行插值
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String sTemplate = StringTemplate.STR."\{x} + \{y} = \{x+y}";
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System.out.println(sTemplate);
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// 字符串模版也可以先定义模版，再处理插值
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StringTemplate st = StringTemplate.RAW."\{x} + \{y} = \{x+y}";
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String sTemplate2 = StringTemplate.STR.process(st);
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System.out.println(sTemplate2);
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System.out.println("--------------------");
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LocalDate now = LocalDate.now();
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String nowStr = StringTemplate.STR."现在是 \{now.getYear()} 年 \{now.getMonthValue()} 月 \{now.getDayOfMonth()} 号";
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System.out.println(nowStr);
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// 字符串模版读取数组，字符串模版也可以嵌套
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String[] infoArr = { "Hello", "Java 21", "https://www.wdbyte.com" };
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String sArray = StringTemplate.STR."\{infoArr[0]}, \{STR."\{infoArr[1]}, \{infoArr[2]}"}";
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System.out.println(sArray);
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// 字符串模版也可以结合多行文本
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String name = "https://www.wdbyte.com";
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String address = "程序猿阿朗";
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String json = StringTemplate.STR."""
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{
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"name": "\{name}",
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"address": "\{address}"
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}
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""";
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System.out.println(json);
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System.out.println("--------------------");
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}
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}
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record Rectangle(String name, double width, double height) {
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double area() {
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return width * height;
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}
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}

‎core-java-modules/core-java-21/src/main/java/Jep431SequencedCollection.java‎

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import java.util.ArrayDeque;
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import java.util.ArrayList;
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import java.util.Deque;
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import java.util.LinkedHashMap;
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import java.util.LinkedHashSet;
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import java.util.List;
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import java.util.SequencedCollection;
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import java.util.TreeSet;
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import java.util.function.Consumer;
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/**
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* @author https://www.wdbyte.com
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* @date 2023/10/12
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*/
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public class Jep431SequencedCollection {
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public static void main(String[] args) {
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// JDK 21 之后，为所有元素插入有序集合提供了一致的操作 API
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List<Integer> listTemp = List.of(1, 2, 3, 4, 5);
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ArrayList<Integer> list = new ArrayList(listTemp);
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Deque<Integer> deque = new ArrayDeque<>(listTemp);
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LinkedHashSet<Integer> linkedHashSet = new LinkedHashSet<>(listTemp);
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TreeSet<Integer> sortedSet = new TreeSet<>(listTemp);
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LinkedHashMap<Integer, Integer> linkedHashMap = new LinkedHashMap<>();
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for (int i = 1; i <= 5; i++) {
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linkedHashMap.put(i, i);
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}
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// 输出第一个元素
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System.out.println(list.getFirst());
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System.out.println(deque.getFirst());
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System.out.println(linkedHashSet.getFirst());
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System.out.println(sortedSet.getFirst());
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System.out.println(linkedHashMap.firstEntry());
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System.out.println("-----------------------");
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// 输出最后一个元素
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System.out.println(list.getLast());
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System.out.println(list.getLast());
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System.out.println(deque.getLast());
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System.out.println(sortedSet.getLast());
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System.out.println(linkedHashMap.lastEntry());
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System.out.println("-----------------------");
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// 逆序遍历
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Consumer<SequencedCollection> printFn = s -> {
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// reversed 逆序元素
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s.reversed().forEach(System.out::print);
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System.out.println();
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};
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printFn.accept(list);
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printFn.accept(deque);
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printFn.accept(linkedHashSet);
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printFn.accept(sortedSet);
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// 有序 map 接口是 SequencedMap,上面的 consume类型不适用
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linkedHashMap.reversed().forEach((k, v) -> {
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System.out.print(k);
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});
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}
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}

‎core-java-modules/core-java-21/src/main/java/Jep440Record.java‎

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/**
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* @author https://www.wdbyte.com
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* @date 2023/10/13
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*/
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public class Jep440Record {
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public static void main(String[] args) {
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Object obj = "Hello www.wdbyte.com";
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if (obj instanceof String s) {
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System.out.println(s);
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}
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Dog dog = new Dog("Husky", 1);
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if (dog instanceof Dog(String name, int age)) {
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String res = StringTemplate.STR."name：\{name} age：\{age}";
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System.out.println(res);
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}
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Object myDog = new MyDog(dog, Color.BLACK);
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if (myDog instanceof MyDog(Dog(String name,int age),Color color)){
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String res = StringTemplate.STR."name：\{name} age：\{age} color:\{color}";
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System.out.println(res);
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}
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}
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}
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record Dog(String name, int age) {}
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enum Color{WHITE,GREY,BLACK};
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record MyDog(Dog dog,Color color){};
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‎core-java-modules/core-java-21/src/main/java/Jep441SwitchPatten.java‎

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/**
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* @author https://www.wdbyte.com
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* @date 2023/10/13
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*/
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public class Jep441SwitchPatten {
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public static void main(String[] args) {
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String r1 = formatterPatternSwitch(Integer.valueOf(1));
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String r2 = formatterPatternSwitch(new String("www.wdbyte.com"));
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String r3 = formatterPatternSwitch(Double.valueOf(3.14D));
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System.out.println(r1);
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System.out.println(r2);
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System.out.println(r3);
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}
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static String formatterPatternSwitch(Object obj) {
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return switch (obj) {
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case Integer i -> String.format("int %d", i);
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case Long l -> String.format("long %d", l);
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case Double d -> String.format("double %f", d);
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case String s -> String.format("String %s", s);
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default -> obj.toString();
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};
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}
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static void testFooBarNew(String s) {
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switch (s) {
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case null -> System.out.println("Oops");
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case "Foo", "Bar" -> System.out.println("Great");
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default -> System.out.println("Ok");
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}
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}
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}

‎core-java-modules/core-java-21/src/main/java/Jep443Unname.java‎

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import java.util.List;
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/**
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* @author https://www.wdbyte.com
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* @date 2023/10/15
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*/
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public class Jep443Unname {
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public static void main(String[] args) {
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String _ = "123213";
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Integer _ = 123;
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List<Integer> list = List.of(1, 2, 3, 4, 5, 6, 7, 8);
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int count = 0;
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for (Integer _ : list) {
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count++;
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}
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System.out.println(count);
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}
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}

‎core-java-modules/core-java-21/src/main/java/Jep444VirtualThread.java‎

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import java.time.Duration;
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import java.util.concurrent.Executors;
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import java.util.stream.IntStream;
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/**
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* 虚拟线程
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*
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* @author https://www.wdbyte.com
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* @date 2023/10/10
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*/
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public class Jep444VirtualThread {
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public static void main(String[] args) throws InterruptedException {
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// 创建并提交执行虚拟线程
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long start = System.currentTimeMillis();
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try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
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IntStream.range(0, 10_000).forEach(i -> {
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executor.submit(() -> {
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Thread.sleep(Duration.ofSeconds(1));
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return i;
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});
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});
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}
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System.out.println("time:" + (System.currentTimeMillis() - start) + "ms");
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// 创建一个虚拟线程指定虚拟线程名称
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Thread thread1 = Thread.ofVirtual().name("v-thread").unstarted(() -> {
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String threadName = Thread.currentThread().getName();
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System.out.println(String.format("[%s] Hello Virtual Thread", threadName));
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});
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thread1.start();
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System.out.println(thread1.isVirtual());
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//创建一个线程，启动为虚拟线程
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Thread thread2 = new Thread(() -> {
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String threadName = Thread.currentThread().getName();
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System.out.println(String.format("[%s] Hello Virtual Thread 2", threadName));
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});
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Thread.startVirtualThread(thread2);
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// 判断一个线程是否是虚拟线程
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System.out.println(thread1.isVirtual());
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System.out.println(thread2.isVirtual());
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Thread.sleep(1000);
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}
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}

‎core-java-modules/core-java-21/src/main/java/Jep445HelloJava.java‎

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void main(){
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System.out.println("Hello, Java 21!");
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}
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