Java11新特性 - 新加一些实用的API

不能对集合进行添加、删除、替换、排序等操作,否则会报java.lang.UnsupportedOperationException错误。
示例代码:

List<String> list = List.of("aa", "bb", "cc"); list.add("dd");

报错信息:

Exception in thread "main" java.lang.UnsupportedOperationException at java.base/java.util.ImmutableCollections.uoe(ImmutableCollections.java:71) at java.base/java.util.ImmutableCollections$AbstractImmutableCollection.add(ImmutableCollections.java:75)

扩展:Arrays.asList()创建的集合也是一个不可变的集合

1|2Question2:of方法与copyOf方法之间的区别


示例代码:

var list1 = List.of("aa", "bb", "cc"); var copy1 = List.copyOf(list1); System.out.println(list1 == copy1); //运行结果: true var list2 = new ArrayList<String>(); var copy2 = List.copyOf(list2); System.out.println(list2 == copy2); //运行结果: false

注意:var也是Java11新推出的特性局部变量类型推断,这里面不再赘述。

第一段代码和第二段代码差不多,为什么返回结果不一样呢?我们来看一下他们的源码:

static <E> List<E> of(E e1, E e2, E e3) { return new ImmutableCollections.ListN<>(e1, e2, e3); } static final class ListN<E> extends AbstractImmutableList<E> implements Serializable { static final List<?> EMPTY_LIST = new ListN<>(); @Stable private final E[] elements; @SafeVarargs ListN(E... input) { // copy and check manually to avoid TOCTOU @SuppressWarnings("unchecked") E[] tmp = (E[])new Object[input.length]; // implicit nullcheck of input for (int i = 0; i < input.length; i++) { tmp[i] = Objects.requireNonNull(input[i]); } elements = tmp; } @Override public boolean isEmpty() { return size() == 0; } @Override public int size() { return elements.length; } @Override public E get(int index) { return elements[index]; } private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException { throw new InvalidObjectException("not serial proxy"); } private Object writeReplace() { return new CollSer(CollSer.IMM_LIST, elements); } } static <E> List<E> copyOf(Collection<? extends E> coll) { return ImmutableCollections.listCopy(coll); } static <E> List<E> listCopy(Collection<? extends E> coll) { if (coll instanceof AbstractImmutableList && coll.getClass() != SubList.class) { return (List<E>)coll; } else { return (List<E>)List.of(coll.toArray()); } }

从源码可以看出,copyOf 方法会先判断来源集合是不是 AbstractImmutableList 类型的,如果是,就直接返回,如果不是,则调用 of 创建一个新的集合。第二段代码因为用的 new 创建的集合,不属于不可变 AbstractImmutableList 类的子类,所以 copyOf 方法又创建了一个新的实例,所以为false。

扩展:使用Set.of()方法创建Set对象时,不可以包含重复数据

2|0Stream加强


Stream是Java8的新特性,我之前的博客对其进行了详细的介绍:Java8新特性 - Stream API。Java9开始对Stream新增了4个新方法。

2|1增加单个参数构造方法,可为null


在Java8,新建一个值为null的Stream,会报错java.lang.NullPointerException错误。
示例代码:

Stream stream = Stream.of(null);

错误:

Exception in thread "main" java.lang.NullPointerException at java.util.Arrays.stream(Arrays.java:5004) at java.util.stream.Stream.of(Stream.java:1000)

错误分析:

查看Stream.of()源码

@SafeVarargs @SuppressWarnings("varargs") // Creating a stream from an array is safe public static<T> Stream<T> of(T... values) { return Arrays.stream(values); } public static <T> Stream<T> stream(T[] array) { return stream(array, 0, array.length); }

可以看见传入null会被解析为时一个数组对象,会进一步访问它的长度信息,导致了NullPointerException异常。

在Java11中,新增了ofNullable方法,可以传入null。因为无法推断类型,所以返回的值为Object,示例代码如下:

Stream stream = Stream.ofNullable(null); stream.forEach(System.out::println);

从源码中可以看出,当传入的是一个null时,返回的是一个空的对象。

public static<T> Stream<T> ofNullable(T t) { return t == null ? Stream.empty() : StreamSupport.stream(new Streams.StreamBuilderImpl<>(t), false); }

2|2增加 takeWhile 和 dropWhile 方法


takeWhile

首先来看一段示例代码:

Stream<Integer> stream = Stream.of(3, 6, 9, 12, 15); stream.takeWhile(n -> n % 2 != 0) .forEach(System.out::println);

第一次看这段代码,可能第一印象就是找出流中的奇数打印出来,但是实际输出为:

3

所以takeWhile方法在计算到n % 2 == 0的时候就终止了,takeWhile方法的作用是:从流中一直获取判定器为真的元素,一旦遇到元素为假,就终止处理!

dropWhile

与上面takeWhile一样的代码:

Stream<Integer> stream = Stream.of(3, 6, 9, 12, 15); stream.dropWhile(n -> n % 2 != 0) .forEach(System.out::println);

返回的结果为:

6
9
12
15

结果正好与takeWhile相反,dropWhile方法的作用为:只要判定器为真,就一直丢弃元素,直到为假,取为假后的所有元素!

2|3iterate重载


流的迭代,主要用于创建流,在指定初值的情况下,经过处理,然后将处理过的值作为初值,不断迭代。所以iterate创建的是一个无限流。
示例代码:

Stream.iterate(1, t -> 2 * t + 1) .limit(10) .forEach(System.out::println);

无限流有一个问题,在数值溢出之后,会一直重复-1的值。Java11进行了优化,以让你提供一个 Predicate (判断条件)来指定什么时候结束迭代,进行有限的迭代,示例代码如下:

Stream.iterate(1, t -> t< 1000, t -> 2 * t + 1) .forEach(System.out::println);

3|0增加了一系列的字符串处理方法


3|1判断字符串是否为空白


示例代码:

String s1 = " \t \r\n"; System.out.println(s1.isBlank()); // true

3|2去除字符串首尾空白


示例代码:

String s2 = " \t 123\r\n".strip(); System.out.println(s2); // 123

trim()大部分情况下效果等同于strip(),但是trim()只能去除码值小于等于32的空白字符,不能去除中文情况下的空白字符,strip()可以去除所有语言中的空白

3|3去除尾/首部空格


示例代码:

String s2 = " \t 123\r\n"; System.out.println(s2.strip().length()); // 3 // 去除首部的空白 System.out.println(s2.stripLeading().length()); // 5 // 去除尾部的空白 System.out.println(s2.stripTrailing().length()); // 10

3|4复制字符串


String s3 = "做一个好人"; System.out.println(s3.repeat(3)); // 做一个好人做一个好人做一个好人

3|5行数统计


String s2 = " \t 123\r\n123"; System.out.println(s2.lines().count()); //2

s2.lines()获取的是一个流,将字符串根据换行符切割转换为Stream

4|0Optional加强


Optional是Java中引进的容器类,主要用于避免空指针异常,我之前的博客对其进行了详细的介绍:Java8新特性 - Optional容器类。Java11中Opthonal 也增加了几个非常酷的方法。

4|1ofNullable方法


参照前面Stream增强的介绍,使用Optional.of(T value);传入的参数是null时,会抛出空指针异常,所以Java11中新增了ofNullable,可以兼容空指针,但是实际传入null后要小心,不能用get接收,最好利用orElse方法接收,示例代码如下。

Optional<String> optional = Optional.ofNullable(null); // 如果内部引用为空,则返回参数中引用,否则返回内部引用 System.out.println(optional.orElse("做一个好人")); // 做一个好人

4|2orElseThrow方法


也可以使用orElseThrow()方法接收,直接在orElseThrow()时抛出异常。

Optional<String> optional = Optional.ofNullable(null); System.out.println(optional.orElseThrow());

错误如下:
Exception in thread "main" java.util.NoSuchElementException: No value present
at java.base/java.util.Optional.orElseThrow(Optional.java:382)

源码如下:

public T orElseThrow() { if (value == null) { throw new NoSuchElementException("No value present"); } return value; }

4|3or方法


也可以使用or()方法接收,当一个空 Optional 时给它一个替代的Optional,示例代码如下:

Optional<String> optional = Optional.ofNullable(null); System.out.println(optional.or(() -> Optional.of("做一个好人!")) .get()); // 做一个好人!

5|0InputStream加强


Java11中新增了transferTo方法,把输入流中的所有数据直接自动地复制到输出流中,这是在处理原始数据流时非常实用的一种用法,不用写循环,也不需要缓冲区,示例代码如下:

public void test() throws Exception { var classLoader = this.getClass().getClassLoader(); var inputStream = classLoader.getResourceAsStream("file"); try (var outputStream = new FileOutputStream("file2")) { inputStream.transferTo(outputStream); } inputStream.close(); }  

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