Java8 Stream
Java 8 API添加了一个新的抽象称为流Stream，可以让你以一种声明的方式处理数据。
Stream 使用一种类似用 SQL 语句从数据库查询数据的直观方式来提供一种对 Java 集合运算和表达的高阶抽象。
Stream API可以极大提高Java程序员的生产力，让程序员写出高效率、干净、简洁的代码。
这种风格将要处理的元素集合看作一种流， 流在管道中传输， 并且可以在管道的节点上进行处理， 比如筛选， 排序，聚合等。
元素流在管道中经过中间操作（intermediate operation）的处理，最后由最终操作(terminal operation)得到前面处理的结果。
+--------------------+ +------+ +------+ +---+ +-------+
| stream of elements +-----> |filter+-> |sorted+-> |map+-> |collect|
+--------------------+ +------+ +------+ +---+ +-------+
学习代码记录
List<Long> nums = Arrays.asList(3L, 3L, 39L, 32L, 123L, 534L, 46542L, 3211L);
nums.stream()
.map(integer -> integer * integer + " ")
.distinct()
.collect(Collectors.toList())
.forEach(System.out::print);
System.out.println();
System.out.println("----------------------------------------------");
long count = nums.stream().filter(i -> i.equals(3L)).count();
System.out.println("包含3L的有 = " + count+"个");
System.out.println("----------------------------------------------");
Random random = new Random();
System.out.println("随机数排序前");
random.ints().limit(10).forEach(System.out::println);
System.out.println("----------------------------------------------");
System.out.println("随机数排序后");
random.ints().limit(10).sorted().forEach(System.out::println);
System.out.println("----------------------------------------------");
List<String> strings = Arrays.asList("acas", "", "sda", "sdadsaddsad", "dwadas");
long count1 = strings.parallelStream()
.filter(s -> s.isEmpty()).count();
System.out.println("在"+strings+"中包含空字符串有" + count1 + "个");
System.out.println("----------------------------------------------");
List<String> collect = strings.stream()
.filter(s -> !s.isEmpty())
.collect(Collectors.toList());
System.out.println("去除空后的筛选列表"+collect);
System.out.println("----------------------------------------------");
String collect1 = strings.stream()
.filter(s -> !s.isEmpty())
.collect(Collectors.joining(","));
System.out.println("将"+strings+"组合成一个string后：" + collect1);
System.out.println("----------------------------------------------");
List<Integer> integers = Arrays.asList(1, 2, 3, 4, 5, 6, 11, 111, 22, 12);
IntSummaryStatistics intSummaryStatistics = integers.stream()
.mapToInt((x) -> x).summaryStatistics();
System.out.println("最大值:"+intSummaryStatistics.getMax());
System.out.println("最小值:"+intSummaryStatistics.getMin());
System.out.println("所有数之和:"+intSummaryStatistics.getSum());
System.out.println("平均数:"+intSummaryStatistics.getAverage());
打印结果
9 1521 1024 15129 285156 2166157764 10310521
----------------------------------------------
包含3L的有 = 2个
----------------------------------------------
随机数排序前
1399523865
374116917
-1454199312
1925671743
580107117
-2137427526
237695867
-55497742
-1502376427
1398197257
----------------------------------------------
随机数排序后
-2130838075
-1574466058
-1469617767
-534793213
-158077868
-143294047
234751172
1147396663
1216434490
2103427929
----------------------------------------------
在[acas, , sda, sdadsaddsad, dwadas]中包含空字符串有1个
----------------------------------------------
去除空后的筛选列表[acas, sda, sdadsaddsad, dwadas]
----------------------------------------------
将[acas, , sda, sdadsaddsad, dwadas]组合成一个string后：acas,sda,sdadsaddsad,dwadas
----------------------------------------------
最大值:111
最小值:1
所有数之和:177
平均数:17.7
Process finished with exit code 0
6.5更新
利用steam流对学生成绩进行分区
学生实体类
import lombok.Data;
@Data
public class Student {
private String name;
private int score;
public Student(String name, int score) {
this.name = name;
this.score = score;
}
}
处理
public static void main(String[] args) {
List<Student> students = Arrays.asList(
new Student("小红", 61),
new Student("小绿", 51),
new Student("小蓝", 60),
new Student("小紫", 99),
new Student("小粉", 100),
new Student("小灰", 59)
);
Map<Boolean, List<Student>> collect = students.stream()
.collect(Collectors.partitioningBy(student -> student.getScore() > 59));
System.out.println("collect = " + collect);
System.out.println("及格的");
System.out.println(collect.get(true));
System.out.println("不及格的");
System.out.println(collect.get(false));
}
打印结果
collect = {false=[Student(name=小绿, score=51), Student(name=小灰, score=59)], true=[Student(name=小红, score=61), Student(name=小蓝, score=60), Student(name=小紫, score=99), Student(name=小粉, score=100)]}
及格的
[Student(name=小红, score=61), Student(name=小蓝, score=60), Student(name=小紫, score=99), Student(name=小粉, score=100)]
不及格的
[Student(name=小绿, score=51), Student(name=小灰, score=59)]
Process finished with exit code 0
