Java 9 到 16 新特性總結(jié)大全！

JAVA 9（2017年9月）

接口里可以添加私有接口

public interface TestInterface {
    default void wrapMethod(){
        innerMethod();
    }
    private void innerMethod(){
        System.out.println("");
    }
}

匿名內(nèi)部類也支持鉆石（diamond）運(yùn)算符

List<Integer> numbers = new ArrayList<>();

List<Integer> numbers = new ArrayList<>() {
    ...
}

增強(qiáng)的 try-with-resources

try (BufferedReader bufferReader = new BufferedReader(...)) {
    return bufferReader.readLine();
}

try (BufferedReader bufferReader0 = new BufferedReader(...);
    BufferedReader bufferReader1 = new BufferedReader(...)) {
    return bufferReader0.readLine();
}

BufferedReader bufferReader0 = new BufferedReader(...);
BufferedReader bufferReader1 = new BufferedReader(...);
try (bufferReader0; bufferReader1) {
    System.out.println(br1.readLine() + br2.readLine());
}

JAVA 10（2018年3月）

局部變量的自動(dòng)類型推斷（var）

var message = "Hello, Java 10";

JAVA 11（2018年9月）

Lambda 中的自動(dòng)類型推斷（var）

List<String> languages = Arrays.asList("Java", "Groovy");
String language = sampleList.stream()
  .map((@Nonnull var x) -> x.toUpperCase())
  .collect(Collectors.joining(", "));

assertThat(language).isEqualTo("Java, Groovy");

javac + java 命令一把梭

$ java HelloWorld.java
Hello Java 11!

Java Flight Recorder 登陸 OpenJDK

JAVA 12（2019年3月）

更簡潔的 switch 語法

DayOfWeek dayOfWeek = LocalDate.now().getDayOfWeek();
String typeOfDay = "";
switch (dayOfWeek) {
    case MONDAY:
    case TUESDAY:
    case WEDNESDAY:
    case THURSDAY:
    case FRIDAY:
        typeOfDay = "Working Day";
        break;
    case SATURDAY:
    case SUNDAY:
        typeOfDay = "Day Off";
}

typeOfDay = switch (dayOfWeek) {
    case MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY -> "Working Day";
    case SATURDAY, SUNDAY -> "Day Off";
};

instanceof + 類型強(qiáng)轉(zhuǎn)一步到位

Object obj = "Hello Java 12!";
if (obj instanceof String) {
    String s = (String) obj;
    int length = s.length();
}

Object obj = "Hello Java 12!";
if (obj instanceof String str) {
    int length = str.length();
}

JAVA 13（2019年9月）

switch 語法再增強(qiáng)

typeOfDay = switch (dayOfWeek) {
    case MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY -> {
        // do sth...
    	yield "Working Day";
    }
    case SATURDAY, SUNDAY -> "Day Off";
};

文本塊（Text Block）的支持

String json = "{\"id\":\"1697301681936888\",\"nickname\":\"空無\",\"homepage\":\"https://juejin.cn/user/1697301681936888\"}";

String json = """
				{
				    "id":"1697301681936888",
				    "nickname":"空無",
				    "homepage":"https://juejin.cn/user/1697301681936888"
				}
				""";

JAVA 14（2020年3月）

新增的 record 類型，干掉復(fù)雜的 POJO 類

public record UserDTO(String id,String nickname,String homepage) { };

public static void main( String[] args ){
	UserDTO user = new UserDTO("1697301681936888","空無","https://juejin.cn/user/1697301681936888");
    System.out.println(user.id);
    System.out.println(user.nickname);
    System.out.println(user.id);
}

IDEA 也早已支持了這個(gè)功能，創(chuàng)建類的時(shí)候直接就可以選：「不過這個(gè)只是一個(gè)語法糖，編譯后還是一個(gè) Class，和普通的 Class 區(qū)別不大」

更直觀的 NullPointerException 提示

Map<String,Map<String,Boolean>> wrapMap = new HashMap<>();
wrapMap.put("innerMap",new HashMap<>());

boolean effected = wrapMap.get("innerMap").get("effected");

// StackTrace:
Exception in thread "main" java.lang.NullPointerException
	at org.example.App.main(App.java:50)

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "java.lang.Boolean.booleanValue()" because the return value of "java.util.Map.get(Object)" is null
	at org.example.App.main(App.java:50)

安全的堆外內(nèi)存讀寫接口，別再玩 Unsafe 的騷操作了

// 分配 200B 堆外內(nèi)存
MemorySegment memorySegment = MemorySegment.allocateNative(200);

// 用 ByteBuffer 分配，然后包裝為 MemorySegment
MemorySegment memorySegment = MemorySegment.ofByteBuffer(ByteBuffer.allocateDirect(200));

// MMAP 當(dāng)然也可以
MemorySegment memorySegment = MemorySegment.mapFromPath(
  Path.of("/tmp/memory.txt"), 200, FileChannel.MapMode.READ_WRITE);

// 獲取堆外內(nèi)存地址
MemoryAddress address = MemorySegment.allocateNative(100).baseAddress();

// 組合拳，堆外分配，堆外賦值
long value = 10;
MemoryAddress memoryAddress = MemorySegment.allocateNative(8).baseAddress();
// 獲取句柄
VarHandle varHandle = MemoryHandles.varHandle(long.class, ByteOrder.nativeOrder());
varHandle.set(memoryAddress, value);

// 釋放就這么簡單，想想 DirectByteBuffer 的釋放……多奇怪
memorySegment.close();

新增的 jpackage 打包工具，直接打包二進(jìn)制程序，再也不用裝 JRE 了

JAVA 15（2020年9月）

ZGC 和 Shenandoah 兩款垃圾回收器正式登陸

封閉（Sealed ）類

public sealed interface Service permits Car, Truck {

    int getMaxServiceIntervalInMonths();

    default int getMaxDistanceBetweenServicesInKilometers() {
        return 100000;
    }

}

JAVA 16（2021年3月）

總結(jié)

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