Java语言特性系列

序

本文主要讲述一下Java10的新特性

特性列表

286: Local-Variable Type Inference(重磅)

相关解读: java10系列(二)Local-Variable Type Inference

相关解读: Java10来了，来看看它一同发布的全新JIT编译器

319: Root Certificates

相关解读: OpenJDK 10 Now Includes Root CA Certificates

322: Time-Based Release Versioning

相关解读: java10系列(一)Time-Based Release Versioning

细项解读

上面列出的是大方面的特性，除此之外还有一些api的更新及废弃，主要见What's New in JDK 10 - New Features and Enhancements，这里举几个例子。

Optional.orElseThrow()

/Library/Java/JavaVirtualMachines/jdk-10.jdk/Contents/Home/lib/src.zip!/java.base/java/util/Optional.java

源码

    /**
     * If a value is present, returns the value, otherwise throws
     * {@code NoSuchElementException}.
     *
     * @return the non-{@code null} value described by this {@code Optional}
     * @throws NoSuchElementException if no value is present
     * @since 10
     */
    public T orElseThrow() {
        if (value == null) {
            throw new NoSuchElementException("No value present");
        }
        return value;
    }

实例

    @Test
    public void testOrElseThrow(){
        var data = List.of("a","b","c");
        Optional<String> optional = data.stream()
                .filter(s -> s.startsWith("z"))
                .findAny();
        String res = optional.orElseThrow();
        System.out.println(res);
    }

新增了orElseThrow与get相对应

输出

java.util.NoSuchElementException: No value present

	at java.base/java.util.Optional.orElseThrow(Optional.java:371)
	at com.example.FeatureTest.testOrElseThrow(FeatureTest.java:19)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:564)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
	at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
	at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
	at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
	at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)

APIs for Creating Unmodifiable Collections

java9新增的of工厂方法的接口参数是一个个元素，java10新增List.copyOf, Set.copyOf,及Map.copyOf用来从已有集合创建ImmutableCollections

List.copyOf源码

    /**
     * Returns an <a href="#unmodifiable">unmodifiable List</a> containing the elements of
     * the given Collection, in its iteration order. The given Collection must not be null,
     * and it must not contain any null elements. If the given Collection is subsequently
     * modified, the returned List will not reflect such modifications.
     *
     * @implNote
     * If the given Collection is an <a href="#unmodifiable">unmodifiable List</a>,
     * calling copyOf will generally not create a copy.
     *
     * @param <E> the {@code List}'s element type
     * @param coll a {@code Collection} from which elements are drawn, must be non-null
     * @return a {@code List} containing the elements of the given {@code Collection}
     * @throws NullPointerException if coll is null, or if it contains any nulls
     * @since 10
     */
    @SuppressWarnings("unchecked")
    static <E> List<E> copyOf(Collection<? extends E> coll) {
        if (coll instanceof ImmutableCollections.AbstractImmutableList) {
            return (List<E>)coll;
        } else {
            return (List<E>)List.of(coll.toArray());
        }
    }

实例

    @Test(expected = UnsupportedOperationException.class)
    public void testCollectionCopyOf(){
        List<String> list = IntStream.rangeClosed(1,10)
                .mapToObj(i -> "num"+i)
                .collect(Collectors.toList());
        List<String> newList = List.copyOf(list);
        newList.add("not allowed");
    }

Collectors新增了toUnmodifiableList, toUnmodifiableSet,以及 toUnmodifiableMap方法

    @Test(expected = UnsupportedOperationException.class)
    public void testCollectionCopyOf(){
        List<String> list = IntStream.rangeClosed(1,10)
                .mapToObj(i -> "num"+i)
                .collect(Collectors.toUnmodifiableList());
        list.add("not allowed");
    }

小结

java10最主要的新特性，在语法层面就属于Local-Variable Type Inference，而在jvm方面307: Parallel Full GC for G1以及317: Experimental Java-Based JIT Compiler都比较重磅，值得深入研究。

doc

JDK 10 Features
Introducing Java SE 10(官方解读)
What's New in JDK 10 - New Features and Enhancements(官方细项解读)
JDK-8140281 : (opt) add no-arg orElseThrow() as preferred alternative to get()
JDK-8177290 : add copy factory methods for unmodifiable List, Set, Map