Java 8 Stream

定义

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。

生成流

在 Java 8 中, 集合接口有两个方法来生成流：

• stream() − 为集合创建串行流。
  
• parallelStream() − 为集合创建并行流。

函数式编程

与匿名类不同

import java.util.Arrays;
import java.util.IntSummaryStatistics;
import java.util.List;
import java.util.Random;
import java.util.stream.Collectors;
public class StreamTest {
    public static void main(String[] args) {
        System.out.println("Use Java8:");
        List<String> strings = Arrays.asList("abc", "", "bc", "efg", "abcd", "", "jkl");
        System.out.println("List: "+ strings);
        long count=strings.stream().filter(string->string.isEmpty()).count();
        System.out.println("Count of empty string: " + count);
        count = strings.stream().filter(string->string.length()==3).count();
        System.out.println("Count of string with length 3: " + count);
        List<String> filtered = strings.stream().filter(string -> !string.isEmpty()).collect(Collectors.toList());
        System.out.println("After filter: " + filtered);
        String mergedString = strings.stream().filter(string->!string.isEmpty()).collect(Collectors.joining(", "));
        System.out.println("After merged: " + mergedString);
        List<Integer> numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5);
        List<Integer> squaresList = numbers.stream().map(i ->i*i).distinct().collect(Collectors.toList());
        System.out.println("Squares List: "+ squaresList);
        List<Integer> integers = Arrays.asList(1,2,13,4,15,6,17,8,19);
        System.out.println("List:" + integers);
        IntSummaryStatistics stats = integers.stream().mapToInt((x)->x).summaryStatistics();
        System.out.println("Max: " + stats.getMax());
        System.out.println("Min: " + stats.getMin());
        System.out.println("Sum: " + stats.getSum());
        System.out.println("Average: " + stats.getAverage());
        System.out.println("Random:");
        Random random = new Random();
        random.ints(0,100).limit(10).sorted().forEach(System.out::println);
        count = strings.parallelStream().filter(string->string.isEmpty()).count();
        System.out.println("Count of empty string: " + count);
    }
}