无涯教程-Java9 - Stream API增强

Java 9 改进的 Stream API 添加了一些便利的方法，使流处理更容易，并使用收集器编写复杂的查询。

Java 9 为 Stream 新增了几个方法:dropWhile、takeWhile、ofNullable，为 iterate 方法新增了一个重载方法。

takeWhile 语法

default Stream<T> takeWhile(Predicate<? super T> predicate)

takeWhile方法使用一个断言作为参数，返回给定 Stream 的子集直到断言语句第一次返回 false。如果第一个值不满足断言条件，将返回一个空的 Stream

import java.util.stream.Stream;
public class Tester {
public static void main(String[] args) {
Stream.of("a","b","c","","e","f").takeWhile(s->!s.isEmpty())
.forEach(System.out::print);
}
}

takeWhile方法获取a，b和c的所有值，然后，一旦字符串为空，它将停止执行。

abc

dropWhile语法 

default Stream<T> dropWhile(Predicate<? super T> predicate)

dropWhile 方法和 takeWhile 作用相反的，使用一个断言作为参数，直到断言语句第一次返回 true 才返回给定 Stream 的子集。

import java.util.stream.Stream;
public class Tester {
public static void main(String[] args) {
Stream.of("a","b","c","","e","f").dropWhile(s-> !s.isEmpty())
.forEach(System.out::print);
  </span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">();</span><span class="pln">
  </span><span class="typ">Stream</span><span class="pun">.</span><span class="kwd">of</span><span class="pun">(</span><span class="str">"a"</span><span class="pun">,</span><span class="str">"b"</span><span class="pun">,</span><span class="str">"c"</span><span class="pun">,</span><span class="str">""</span><span class="pun">,</span><span class="str">"e"</span><span class="pun">,</span><span class="str">""</span><span class="pun">,</span><span class="str">"f"</span><span class="pun">).</span><span class="pln">dropWhile</span><span class="pun">(</span><span class="pln">s</span><span class="pun">-&gt;</span><span class="pln"> </span><span class="pun">!</span><span class="pln">s</span><span class="pun">.</span><span class="pln">isEmpty</span><span class="pun">())</span><span class="pln">
     </span><span class="pun">.</span><span class="pln">forEach</span><span class="pun">(</span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">::</span><span class="kwd">print</span><span class="pun">);</span><span class="pln">

}
}

dropWhile方法删除a，b和c值，然后，一旦字符串为空，它将获取所有值。

ef
ef

Iterate语法

static <T> Stream<T> iterate(T seed, Predicate<? super T> hasNext, UnaryOperator<T> next)

iterate方法现在具有hasNext断言作为参数，一旦hasNext断言返回false，它就会停止循环。

import java.util.stream.IntStream;
public class Tester {
public static void main(String[] args) {
IntStream.iterate(3, x -> x < 10, x -> x+ 3).forEach(System.out::println);
}
}

输出

3
6
9

ofNullable语法 

static <T> Stream<T> ofNullable(T t)

引入了ofNullable方法以防止NullPointerExceptions并避免对Stream进行空检查。 此方法返回一个包含单个元素的顺序Stream，否则返回一个空Stream。

import java.util.stream.Stream;
public class Tester {
public static void main(String[] args) {
long count=Stream.ofNullable(100).count();
System.out.println(count);
  count</span><span class="pun">=</span><span class="typ">Stream</span><span class="pun">.ofNullable(</span><span class="kwd">null</span><span class="pun">).</span><span class="pln">count</span><span class="pun">();</span><span class="pln">
  </span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="pln">count</span><span class="pun">);</span><span class="pln">

}
}

输出

1
0

参考链接

www.learnfk.com/java/java9-…