java - 如何使用流计算组内的值

Question

我正在尝试计算一个范围内的值，我越来越接近了，但我不知道如何输出 0。这是我的目标:

input: 12,34
output: 
0 (no values from 0-9)
1 (12 is between 10-19)
0
1 (34 between 30-39)
... (my values are guaranteed to be between 0-100)

到目前为止我的代码:

package javaapplication2;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Map;
import java.util.concurrent.ThreadLocalRandom;
import java.util.stream.Collectors;

public class JavaApplication2 {

    public static void main(String[] args){
        //generating sample data------------------------------    
        class Temp{
            Long vote=ThreadLocalRandom.current().nextLong(100);
            public long getVote(){
                System.out.println(vote);
                return vote;
            }
        }

        ArrayList<Temp> t = new ArrayList<>();
        t.add(new Temp());
        t.add(new Temp());
        t.add(new Temp());
        //end generating data------------------------------------
        Map<Integer, Long> counters = t.stream()
            .collect(Collectors.groupingBy(p -> ((int)p.getVote())/10, 
                     Collectors.counting()));

        Collection<Long> values = counters.values();
        Long[] res = values.toArray(new Long[values.size()]);
        for(Long l:res)
            System.out.println(l.toString());
    }
}

Answer 1

要使结果包含 0，您可以将范围从 0 到 10 的流映射到您的实际结果:

Map<Integer, Long> map = t.stream()
    .collect(Collectors.groupingBy(p -> ((int) p.getVote())/10, 
Collectors.counting()));
Map<Integer, Long> result = IntStream.range(0, 10).boxed()
    .collect(Collectors.toMap(Function.identity(), i -> map.getOrDefault(i, 0L)));