Java10的新特性

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Java语言特性系列

本文主要讲述一下Java10的新特性

特性列表

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。

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