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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