Java Stream 使用

of()接收数组并创建流，然后map()将每个元素都获取平方，distinct()去重，再使用collect()将流中的元素变成List

@Test
public void mapTest(){
    List<Integer> squaresList = Stream.of(3, 2, 2, 3, 7, 3, 5)
            .map(e -> e*e)
            .distinct()
            .collect(Collectors.toList());
}

Collection可使用stream()获取流，filter()过滤出空字符串，count()计数

@Test
public void filerTest(){
    List<String> stringList = Arrays.asList("abc", "d", "ef", "ghi", "","123", "bcd","");
    long count = stringList.stream().filter(string -> string.isEmpty()).count();
    // 2
}

filter()过滤出非空字符串，skip()过滤掉前三个，并使用逗号间隔生成String

@Test
public void collectTest(){
    List<String> stringList = Arrays.asList("abc", "d", "ef", "ghi", "","123", "bcd","");
    String mergedString = stringList.stream().filter(string -> !string.isEmpty()).skip(3).collect(Collectors.joining(", "));
    // ghi, 123, bcd
}

一些产生统计结果的收集器也非常有用。
它们主要用于int、double、long等基本类型上，它们可以用来产生类似如下的统计结果。

@Test
public void statTest(){
    List<Integer> numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5);

    IntSummaryStatistics stats = numbers.stream().mapToInt((x) -> x).summaryStatistics();

    System.out.println("列表中最大的数 : " + stats.getMax());//7
    System.out.println("列表中最小的数 : " + stats.getMin());//2
    System.out.println("所有数之和 : " + stats.getSum());//25
    System.out.println("平均数 : " + stats.getAverage());//3.5714285714285716
}

并行执行，顺序可能会与原来的顺序不同。
filter()过滤出非空字符串，limit()过滤出前两个

@Test
public void parallelStreamTest(){
    List<String> stringList = Arrays.asList("abc", "d", "ef", "ghi", "","123", "bcd","");
    stringList = stringList.parallelStream().filter(string -> !string.isEmpty()).limit(2).collect(Collectors.toList());
    // [abc, d]
}

合并两条未关闭的流，如果有个流先使用count(),collect()或forEach()，就会关闭流，那么合并时会报错
stream has already been operated upon or closed

@Test
public void concatTest(){
    Stream<Integer> stream1 = Stream.of(1,2,3);
    Stream<Integer> stream2 = Stream.of(4,5,6);

    // 4,5,6,
    Stream.concat(stream1, stream2).filter(e -> e > 3).forEach(e -> System.out.print(e + ","));
}

参考：
Java 8 Stream
Java 之 Stream 流
了解Java8中的parallelStream