Map实现按照Key的升降序和按照Value的升降序

小鱼儿 2021-09-26 16:16 348阅读 0赞

1.TreeMap按照Key排序，默认为升序  
2.将TreeMap按照Key降序则需要传入Comparator对象，重写compare方法。  
3.按照Value排序则需要将map.entrySet()转换为list。

import java.util.ArrayList;
    import java.util.Collections;
    import java.util.Comparator;
    import java.util.List;
    import java.util.Map;
    import java.util.Map.Entry;
    import java.util.TreeMap;
    
    public class mapSort { 
    
        /** * @param args */
        public static void main(String[] args) {
            // TODO Auto-generated method stub
            mapSort m = new mapSort();
            m.sortByKey();
            m.sortByKey_DES();
            m.sortByValue();
            m.sortByValue_DES();
        }
        //按照key进行排序 采用TreeMap 默认升序
        public void sortByKey() {
            System.out.println("测试按照Key升序");
            Map<String,Integer> map = new TreeMap<String,Integer>();
            map.put("hello",33);
            map.put("awake",320);
            map.put("yeah",78);
            for (Entry<String, Integer> entry : map.entrySet()) {
                System.out.println(entry.getKey()+" "+entry.getValue());
            }
        }
        //按照key进行排序 采用TreeMap 降序
        public void sortByKey_DES() {
            System.out.println("测试按照Key降序");
            Map<String,Integer> map = new TreeMap<String,Integer>(new Comparator<String>() {
                @Override
                public int compare(String o1, String o2) {
                    // TODO Auto-generated method stub
                    return o2.compareTo(o1);
                }
            });
            map.put("hello",33);
            map.put("awake",320);
            map.put("yeah",78);
            for (Entry<String, Integer> entry : map.entrySet()) {
                System.out.println(entry.getKey()+" "+entry.getValue());
            }
        }
        //按照value进行排序 采用TreeMap 升序
        public void sortByValue() {
            System.out.println("测试按照Value升序");
            Map<String,Integer> map = new TreeMap<String,Integer>();
            map.put("hello",33);
            map.put("awake",320);
            map.put("yeah",78);
            //将map.entrySet()转换为list
            List<Map.Entry<String, Integer>> list = new ArrayList<Map.Entry<String, Integer>>(map.entrySet());
            //通过集合的比较其来实现排序
            Collections.sort(list,new Comparator<Map.Entry<String, Integer>>() {
    
                @Override
                public int compare(Entry<String, Integer> o1,
                        Entry<String, Integer> o2) {
                    // TODO Auto-generated method stub
                    return o1.getValue().compareTo(o2.getValue());
                }
            });
            for (Entry<String, Integer> entry :list) {
                System.out.println(entry.getKey()+" "+entry.getValue());
            }
        }
    
        //按照value进行排序 采用TreeMap 降序
        public void sortByValue_DES() {
            System.out.println("测试按照Value降序");
            Map<String,Integer> map = new TreeMap<String,Integer>();
            map.put("hello",33);
            map.put("awake",320);
            map.put("yeah",78);
            //将map.entrySet()转换为list
            List<Map.Entry<String, Integer>> list = new ArrayList<Map.Entry<String, Integer>>(map.entrySet());
            //通过集合的比较其来实现排序
            Collections.sort(list,new Comparator<Map.Entry<String, Integer>>() {
    
                @Override
                public int compare(Entry<String, Integer> o1,
                        Entry<String, Integer> o2) {
                    // TODO Auto-generated method stub
                    return o2.getValue().compareTo(o1.getValue());
                }
            });
            for (Entry<String, Integer> entry : list) {
                System.out.println(entry.getKey()+" "+entry.getValue());
            }
        }
    }