从 java 8到 java 11变化一览

本文列举了从Java8到11各个变化的API：

变量
从Java 10开始，开发人员可以选择让编译器使用var来推断类型:

var x=1.0/2.0

var list = new ArrayList<String>();  

var stream = list.stream();


// Type inference can be (Java 8)
Function helloFunction = s -> "Hello " + s;
 
// Inference of generics (Diamond Operator, since Java 7)
List strings = new ArrayList<>();
strings.add(helloFunction.apply("World"));
 
// Inference of generics (Diamond Operator) with anonymous inner classes (Java 9 -> JEP 213)
Consumer printer = new Consumer<>() {
    @Override
    public void accept(String string) {
        System.out.println(string);
    }
};
strings.forEach(printer::accept);

Java11已经提高了lambda中的var能力：

IntFunction<Integer> doubleIt1 = (int x) -> x * 2; // OK Java 10 and 11
IntFunction<Integer> doubleIt2 = (var x) -> x * 2; // OK Java 11

更复杂案例：

/ /Inference of parameters in Lambda expressions
Consumer<String> printer = (var s) -> System.out.println(s); // statt s -> System.out.println(s);
 
// But no mixture of "var" and declarated types possible
// BiConsumer<String, String> printer = (var s1, String s2) -> System.out.println(s1 + " " + s2);
 
// Useful for type annotations
BiConsumer<String, String> printer = (@Nonnull var s1, @Nullable var s2) -> System.out.println(s1 + (s2 == null ? "" : " " + s2));

// given
Optional<String> value = Optional.of("properValue");
AtomicInteger successCounter = new AtomicInteger(0);
 
AtomicInteger onEmptyOptionalCounter = new AtomicInteger(0);
 
// when
value.ifPresentOrElse(
   v -> successCounter.incrementAndGet(), 
   onEmptyOptionalCounter::incrementAndGet);

String新方法
java 8加入新方法join：

Set<String> set1 = Set.of("a","b", "c");
List<String> list1 = List.of("a","b", "c");
  
System.out.println( String.join("a", "b", "c") );
System.out.println( String.join(".", set1) );
System.out.println( String.join(".", list1) );

Java9 有了一个返回Stream的新方法
Java11 添加了更多String的新方法:

String.repeat(int)
String.lines()
String.strip()
String.stripLeading()
String.stripTrailing()
String.isBlank()

接口
java8:

常量Constant variables
抽象方法Abstract methods
默认方法Default methods
静态方法Static methods

Java9:

Constant variables
Abstract methods
Default methods
Static methods
私有方法Private methods
私有静态方法Private Static methods

Java11:
看一个接口的完整代码：

//generic interface with one type parameter T
interface SomeInterface<T> { 
 int SOME_CONSTANT = 35; // variable declaration
 int abstractMethod(int x, int y);   // method declaration
 T abstractMethodUsingGenericType(T[] array, int i); // method using type parameter
 default int defaultMethod(int x, int y) {
     // implementation of method
  return 0;
 }
 static void main(String[] args) {
     // any static method, including main can be included in interface
 }
 
 
 private void privateMethod(String[] args) {
     // any private  method can be included in interface
 }
 
 private static void staticMethod(String[] args) {
     // any private static method can be included in interface
 }
 
  // nested class definition
 class NestedClass { 
     // members of a class
 }
 // nested interface definition
 interface NestedInterface { 
     // member of an interface
 }
// nested enum definition
 enum NestedEnum {   
     OBJECT1,
     OBJECT2,
     ;
     // methods, variables and constructors
 }
  // nested annotation definition
 @interface NestedAnnotation {   
     String attrib1();
 }
}

Nullable管理
java8引入了新的Optional。

// return an optional empty object
Optional<String> optional = Optional.empty();

// return an optional with value object
String str = "value";
Optional<String> optional = Optional.of(str);

// return an optional with or without value 
Optional<String> optional = Optional.ofNullable(getString());
// how to solve String version = computer?.getSoundcard()?.getUSB()?.getVersion() ?: "UNKNOWN";
// with map
String version = computer.flatMap(Computer::getSoundcard)
                            .flatMap(Soundcard::getUSB)
                            .map(USB::getVersion)
                            .orElse("UNKNOWN");
//list

List<String> list = getList();
List<String> listOpt = list != null ? list : new ArrayList<>();

// is equivalent to
List<String> listOpt = getList().orElseGet(() -> new ArrayList<>());

java9中可以返回另外一个Optional ，使用增强方法ifPresentOrElse :

String defaultString = "default";
Optional<String> value = Optional.empty();
Optional<String> defaultValue = Optional.of(defaultString);

// when
Optional<String> result = value.or(() -> defaultValue);


// given
Optional<String> value = Optional.of("properValue");
AtomicInteger successCounter = new AtomicInteger(0);
 
AtomicInteger onEmptyOptionalCounter = new AtomicInteger(0);
 
// when
value.ifPresentOrElse(
   v -> successCounter.incrementAndGet(), 
   onEmptyOptionalCounter::incrementAndGet);

也增加了stream方法。
Java10中有新的方法叫：orElseThrow
Java11中代码如下：
Optional.of(string).isEmpty()

Stream
Java8 引入了新的Stream：

// starting from list
List<String> myList =
    Arrays.asList("a1", "a2", "b1", "c2", "c1");

myList
    .stream()
    .filter(s -> s.startsWith("c"))
    .map(String::toUpperCase)
    .sorted()
    .forEach(System.out::println);

// native stream 
Stream.of("a1", "a2", "a3")
    .findFirst()
    .ifPresent(System.out::println);

可以和number和map一起运行：

// working with number
IntStream.range(1, 4)
    .forEach(System.out::println);

// working with map
Arrays.stream(new int[] {1, 2, 3})
    .map(n -> 2 * n + 1)
    .average()
    .ifPresent(System.out::println);

流提供者可以多次重用流Stream，从而避免了集合中臭名昭著的错误：

java.lang.IllegalStateException: stream has already been operated upon or closed

Supplier<Stream<String>> streamSupplier =
    () -> Stream.of("d2", "a2", "b1", "b3", "c")
            .filter(s -> s.startsWith("a"));

streamSupplier.get().anyMatch(s -> true);   // ok
streamSupplier.get().noneMatch(s -> true);  // ok

收集器可以转换到几种List:

List<Person> filtered =
    persons
        .stream()
        .filter(p -> p.name.startsWith("P"))
        .collect(Collectors.toList());

使用flatmap管理nullability

Outer outer = new Outer();
if (outer != null && outer.nested != null && outer.nested.inner != null) {
    System.out.println(outer.nested.inner.foo);
}

// similar to类似上面代码的新写法
Optional.of(new Outer())
    .flatMap(o -> Optional.ofNullable(o.nested))
    .flatMap(n -> Optional.ofNullable(n.inner))
    .flatMap(i -> Optional.ofNullable(i.foo))
    .ifPresent(System.out::println);

Java9中引入了iterate 和 takeWhile/dropWhile方法：

Stream.iterate(0, i -> i < 10, i -> i + 1) .forEach(System.out::println);
System.out.println("stream take while");


Stream<String> stream1 = Stream.iterate("", s -> s + "s")
      .takeWhile(s -> s.length() < 10);
  
stream1.forEach(System.out::println);

Java11引入了Not谓词，以前代码：

lines.stream()          
         .filter(s -> !s.isBlank())

现在可以是：

lines.stream()          
         .filter(Predicate.not(String::isBlank))

响应式Stream
Java9 引入了java.util.concurrent.Flow的四个接口：

Flow.Processor
Flow.Publisher
Flow.Subscriber
Flow.Subscription

还有不可变集合：

// empty immutable collections
 List<String> emptyImmutableList = List.of();
 Set<String> emptyImmutableSet = Set.of();
 Map emptyImmutableMap = Map.of();

// immutable collections
List<String> immutableList = List.of("one", "two");
Set<String> immutableSet = Set.of("value1", "value2");
Map<String, String> immutableMap = Map.of("key1", "Value1", "key2", "Value2", "key3", "Value3");

Java9引入了MultiResolutionImage和提高了Process API

Java10提高了通过List.copyOf(), Set.copyOf(), Map.copyOf()创建不可变集合能力，Collectors类有了新方法toUnmodifiableList, toUnmodifiableSet, toUnmodifiableMap.

新语法

Java 9 try-with-resource现在支持内部可自定义：

try (new MyAutoCloseable() { }.finalWrapper.finalCloseable) {
     // do some stuff with finalCloseable
  } catch (Exception ex) { }

Java 9还支持带有菱形运算符的匿名类：

List<String> list = new ArrayList<>(){ };

Java 9支持支持HTTP / 2和websockets的新HTTP客户端API。
Java 11改善了HTTP客户端API支撑

int numLetters = switch (day) {
    case MONDAY, FRIDAY, SUNDAY -> 6;
    case TUESDAY                -> 7;
    case THURSDAY, SATURDAY     -> 8;
    case WEDNESDAY              -> 9;
};

Lambdas
Java8中已经引入了Lambda语法：s -> do(s)
为了支持Lambda，引入了5个接口：Consumer, Function, Supplier, Predicate, Operator
在Iterable & Stream & Optional上增加了几个方法：

forEach
filter
map
flatMap
collect
...

Java 11提高了Lambda的var推断能力

Deprecated
Java 9 short list (https://docs.oracle.com/javase/9/docs/api/deprecated-list.html)

java.activation
java.corba
java.se.ee
java.transaction
java.xml.bind
java.xml.ws
java.xml.ws.annotation
jdk.policytool

Java 11 short list
Java EE packages removed

javax.activation (java.activation)
javax.activity, javax.rmi, javax.rmi.CORBA, org.omg.* (java.corba)
javax.transaction (java.transaction)
javax.xml.bind.* (java.xml.bind)
javax.jws, javax.jws.soap, javax.xml.soap, javax.xml.ws.*(java.xml.ws)
javax.annotation (java.xml.ws.annotation)

日志
Java9提供了日志级别和作用域：

java -Xlog:all=debug:file=application.log -version

Annotations
Java 8

@FunctionalInterface : to specify an interface "for lambda" purpose

Java 9
has improved the following annotations

@SafeVarargs : it improves now private methods
@Deprecated : it provide the scope of deprecation (JSCOR-GUID-BB859EA8-E6F7-4239-9A52-4F1BDE27BB09>https://docs.oracle.com/javase/9/core/enhanced-deprecation1.htmJSCOR-GUID-BB859EA8-E6F7-4239-9A52-4F1BDE27BB09)