Java8-Stream笔记

我会带着你远行 2021-10-15 23:57 216阅读 0赞

## 分批处理数据 ##

在程序经常会处理集合数据，有时直接处理整个集合并不是很好的主意，通常需要分批次处理。比如在批量查询/插入数据到数据库的时候。

import com.google.common.collect.Iterables;
    import org.apache.commons.collections4.CollectionUtils;
    
    import java.util.Collection;
    import java.util.List;
    import java.util.stream.Stream;
    import java.util.stream.StreamSupport;
    
    public static <T> Stream<List<T>> partitionStream(Collection<T> coll, int size) { 
      if (CollectionUtils.isEmpty(coll)) { 
        return Stream.empty();
      }
      return StreamSupport.stream(Iterables.partition(coll, size).spliterator(), false);
    }

//Guava集合分组
    List<String> list = new ArrayList<>();
    //按每50个一组分割
    List<List<String>> parts = Lists.partition(list, 50);

## 分组统计 ##

有时候我们拿到一个集合，里面有重复的元素，想统计每个元素的个数，如何做？

### 简单集合 ###

List<String> items = Arrays.asList("a", "b", "b", "b", "c", "c", "c");
    Map<String, Long> result2 = items.stream().collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));
    System.out.println(result2);//{a=1, b=3, c=3}

### 实体集合 ###

通过用TreeMap接收结果实现排序功能

List<VisitorLogs> visitorLogs = Lists.newArrayList(
                    new VisitorLogs("2020-06-27", 8),
                    new VisitorLogs("2020-06-27", 9),
                    new VisitorLogs("2020-06-30", 7),
                    new VisitorLogs("2020-06-30", 9),
                    new VisitorLogs("2020-06-30", 1),
                    new VisitorLogs("2020-07-05", 1),
                    new VisitorLogs("2020-07-05", 8),
                    new VisitorLogs("2020-07-05", 6));
    
    TreeMap<String, Integer> collect = visitorLogs.stream().collect(Collectors.groupingBy(d -> d.getTime(), TreeMap::new, Collectors.summingInt(o -> o.getCount())));
    System.out.println(collect);//{2020-06-27=13, 2020-06-30=17, 2020-07-05=15}
    
    @Data
    @NoArgsConstructor
    @AllArgsConstructor
    public class VisitorLogs implements Comparable<VisitorLogs> { 
        private String time;
        private int count;
    
        @Override
        public int compareTo(VisitorLogs o) { 
            return time.compareTo(o.time);
        }
    }