Java10的新特性

時間 2019-11-06

標籤 java10 java 特性欄目 Java 简体版

原文原文鏈接

Java語言特性系列

序

本文主要講述一下Java10的新特性html

特性列表

286: Local-Variable Type Inference(重磅)

相關解讀: java10系列(二)Local-Variable Type Inference

相關解讀: Java10來了，來看看它一同發佈的全新JIT編譯器

319: Root Certificates

相關解讀: OpenJDK 10 Now Includes Root CA Certificates

322: Time-Based Release Versioning

相關解讀: java10系列(一)Time-Based Release Versioning

細項解讀

上面列出的是大方面的特性，除此以外還有一些api的更新及廢棄，主要見What's New in JDK 10 - New Features and Enhancements，這裏舉幾個例子。java

Optional.orElseThrow()

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

源碼

/**
     * 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;
    }

實例

@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與get相對應

輸出api

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工廠方法的接口參數是一個個元素，java10新增List.copyOf, Set.copyOf,及Map.copyOf用來從已有集合建立ImmutableCollections

List.copyOf源碼

/**
     * 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());
        }
    }

實例

@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");
    }