Java8 对Map(key/value)排序后取TopN

Myth丶恋晨 2022-10-01 09:56 301阅读 0赞

import com.alibaba.fastjson.JSON;
    import java.util.*;
    import java.util.stream.Collectors;
    
    /**
     * Hello world!
     *
     */
    public class App {
        public static void main( String[] args ) {
            Map<String,Integer> mapRepeat = new HashMap<>();
            mapRepeat.put("aa", 1);
            mapRepeat.put("bb", 45);
            mapRepeat.put("cc", 32);
            mapRepeat.put("dd", 226);
            mapRepeat.put("ee", 16);
            mapRepeat.put("ff", 320);
            mapRepeat.put("gg", 99);
    
            // 1.8以后 使用lambda表达式和Stream处理
            // 1.对Map的value进行降序排序，并取前5个key
            List<String> mobileList = mapRepeat.entrySet().stream()
                    .sorted((Map.Entry<String, Integer> e1, Map.Entry<String, Integer> e2) -> e2.getValue() - e1.getValue())
                    .map(entry -> entry.getKey()).collect(Collectors.toList())
                    .subList(0, 5);
            System.out.println(JSON.toJSONString(mobileList));
    
    
            // 2、正向对Map的value排序并将结果输出到LinkedHashMap
            final Map<String, Integer> sortedByCount1 = mapRepeat.entrySet()
                    .stream()
                    .sorted(Map.Entry.comparingByValue())
                    .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (e1, e2) -> e1, LinkedHashMap::new));
            System.out.println(JSON.toJSONString(sortedByCount1));
    
            // 3、反向 reversed对Map的value排序并将结果输出到LinkedHashMap
            final Map<String, Integer> sortedByCount2 = mapRepeat.entrySet()
                    .stream()
                    .sorted((Map.Entry.<String, Integer>comparingByValue().reversed()))
                    .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (e1, e2) -> e1, LinkedHashMap::new));
            System.out.println(JSON.toJSONString(sortedByCount2));
    
    
            // 4.正向 Comparator 对Map的value排序并将结果输出到LinkedHashMap
            final Map<String, Integer> sortedByCount3 = mapRepeat.entrySet()
                    .stream()
                    .sorted((e1, e2) -> e1.getValue().compareTo(e2.getValue()))
                    .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (e1, e2) -> e1, LinkedHashMap::new));
            System.out.println(JSON.toJSONString(sortedByCount3));
    
    
            //反向 Comparator 对Map的value排序并将结果输出到LinkedHashMap
            final Map<String, Integer> sortedByCount4 = mapRepeat.entrySet()
                    .stream()
                    .sorted((e1, e2) -> e2.getValue().compareTo(e1.getValue()))
                    .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (e1, e2) -> e1, LinkedHashMap::new));
            System.out.println(JSON.toJSONString(sortedByCount4));
    
            // 1.8以前
            List<Map.Entry<String, Integer>> list1 = new ArrayList<>();
            list1.addAll(mapRepeat.entrySet());
            Collections.sort(list1, new Comparator<Map.Entry<String, Integer>>() {
                @Override
                public int compare(Map.Entry<String, Integer> o1, Map.Entry<String, Integer> o2) {
                    return o1.getValue() - o2.getValue();
                }
            });
            for (Map.Entry<String, Integer> entry : list1) {
                System.out.println("key:" + entry.getKey() + ",value:" + entry.getValue());
            }
        }
    }