List 过滤、排序、校验等处理方法

List filter：过滤、map：映射、sorted：排序、forEach、collect：聚合、statistics：统计、parallelStream：并行流

 
 
public class Example{
    private String name;
    private Double score;
 
    public Example(String name, Double score) {
        this.name = name;
        this.score = score;
    }
 
    public String getName() {
        return name;
    }
 
    public void setName(String name) {
        this.name = name;
    }
 
    public Double getScore() {
        return score;
    }
 
    public void setScore(Double score) {
        this.score = score;
    }
 
    @Override
    public String toString() {
        return "Example{" +
                "name='" + name + '\'' +
                ", score=" + score +
                '}';
    }
}

+--------------------+       +------+   +------+   +---+   +-------+
| stream of elements +-----> |filter+-> |sorted+-> |map+-> |collect|
+--------------------+       +------+   +------+   +---+   +-------+
 

处理测试类

import java.util.ArrayList;
import java.util.Comparator;
import java.util.DoubleSummaryStatistics;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
 
 
 
public class StreamTest {
 
 
    public static void main(String args[]){
 
        List<Example> list = new ArrayList<>();
        list.add(new Example("乔峰", 20.d));
        list.add(new Example("虚竹", 40.d));
        list.add(new Example("段誉", 80.d));
        list.add(new Example("扫地僧", 50.d));
        list.add(new Example("方丈", null));
        list.add(new Example("王语嫣", 30.d));
 
        long count = 0;
        List<Example> filterList = null;
 
 
        // filter 过滤器的使用
        // 筛选出成绩不为空的学生人数
        count = list.stream().filter(p -> null != p.getScore()).count();
        System.out.println("参加考试的学生人数：" + count);
 
        // collect
        // 筛选出成绩不为空的学生集合
        filterList = list.stream().filter(p -> null != p.getScore()).collect(Collectors.toList());
        System.out.println("参加考试的学生信息：");
        filterList.stream().forEach(System.out::println);
 
        // map 将集合映射为另外一个集合
        // 取出所有学生的成绩
        List<Double> scoreList = list.stream().map(p -> p.getScore()).collect(Collectors.toList());
        System.out.println("所有学生的成绩集合：" + scoreList);
 
        // 将学生姓名集合串成字符串，用逗号分隔
        String nameString = list.stream().map(p -> p.getName()).collect(Collectors.joining(","));
        System.out.println("所有学生的姓名字符串：" + nameString);
 
        // sorted排序
        // 按学生成绩逆序排序 正序则不需要加.reversed()
        filterList = list.stream().filter(p -> null != p.getScore()).sorted(Comparator.comparing(UserPo::getScore).reversed()).collect(Collectors.toList());
        System.out.println("所有学生的成绩集合，逆序排序：");
        filterList.stream().forEach(System.out::println);
 
        System.out.println("按学生成绩归集：");
        Map<Double, List<UserPo>> groupByScoreMap = list.stream().filter(p -> null != p.getScore())
                .collect(Collectors.groupingBy(UserPo::getScore));
        for (Map.Entry<Double, List<UserPo>> entry : groupByScoreMap.entrySet()) {
            System.out.println("成绩：" + entry.getKey() + " 人数：" + entry.getValue().size());
        }
 
        // forEach
        filterList.stream().forEach(p -> p.setScore(p.getScore() + 10));
        System.out.println("及格人数太少，给每个人加10分");
        filterList.stream().forEach(System.out::println);
 
        // count
        count = filterList.stream().filter(p -> p.getScore() >= 60).count();
        System.out.println("最后及格人数" + count);
 
        DoubleSummaryStatistics statistics = filterList.stream().mapToDouble(p -> p.getScore()).summaryStatistics();
        System.out.println("列表中最大的数 : " + statistics.getMax());
        System.out.println("列表中最小的数 : " + statistics.getMin());
        System.out.println("所有数之和 : " + statistics.getSum());
        System.out.println("平均数 : " + statistics.getAverage());
 
        // 并行流 使用
        count = list.parallelStream().filter(p -> null != p.getScore()).count();
        System.out.println("test：" + count);
 
    }
 
}