（9）Map按键和按键值排序

桃扇骨 2022-07-29 09:03 224阅读 0赞

Map按键升序排序可以直接new一个TreeMap对象就可以实现了，要按键降序的话，需要重构TreeMap的Comparator比较器。这里给出了两种Map按值排序的方式。

import java.util.Collections;
    import java.util.Comparator;
    import java.util.HashMap;
    import java.util.LinkedHashMap;
    import java.util.LinkedList;
    import java.util.Map;
    import java.util.TreeMap;
    
    public class SortMap {
    
        public static void main(String args[]){       
            //Map按键排序
            //Map中的TreeMap默认按照键升序排序
            Map<String, String> map1 = new TreeMap<String, String>();
            map1.put("b", "ccccc");
            map1.put("d", "aaaaa");
            map1.put("c", "bbbbb");
            map1.put("a", "ddddd");
            System.out.println("按键升序排序："+map1);
            //通过是比较器Comparator,可以使Map按照键降序排序
            Map<String, String> map2 = new TreeMap<String, String>(
                new Comparator<String>() {
                    public int compare(String obj1, String obj2) {
                        // 降序排序
                        return obj2.compareTo(obj1);
                    }
                });
            map2.putAll(map1);
            System.out.println("按键降序排序："+map2);      
            System.out.println("按键值升序排序："+sortByValue(map1));   
            //按键值降序排序
            HashMap<String,Double> map = new HashMap<String,Double>();  
            ValueComparator bvc =  new ValueComparator(map);  
            TreeMap<String,Double> sorted_map = new TreeMap<String,Double>(bvc);   
            map.put("A",99.5);  
            map.put("B",67.4);  
            map.put("C",67.4);  
            map.put("D",67.3);    
            System.out.println("unsorted map: "+map);   
            sorted_map.putAll(map);          
            System.out.println("results: "+sorted_map);  
            System.out.println(sortByValue(map));
        }
    
        public static <K, V extends Comparable<? super V>> Map<K, V> sortByValue( Map<K, V> map){  
            //将map.entrySet()转换成list
            LinkedList<Map.Entry<K, V>> list =  new LinkedList<Map.Entry<K, V>>( map.entrySet() );  
            //然后通过比较器来实现排序
            Collections.sort( list, new Comparator<Map.Entry<K, V>>() {  
                ////升序排序
                public int compare( Map.Entry<K, V> o1, Map.Entry<K, V> o2 ){  
                    return (o1.getValue()).compareTo( o2.getValue() ); 
                    //return (o2.getValue()).compareTo( o1.getValue() );//降序排序
                }  
            } );  
    
            Map<K, V> result = new LinkedHashMap<K, V>();  
            for (Map.Entry<K, V> entry : list) {  
                result.put( entry.getKey(), entry.getValue() );  
            }  
            return result;  
        }  
    
        static class ValueComparator implements Comparator<String> {  
    
            Map<String, Double> base;  
            public ValueComparator(Map<String, Double> base) {  
                this.base = base;  
            }  
    
            // 由大到小排序 
            public int compare(String a, String b) {  
                if (base.get(a) >= base.get(b)) {  
                    return -1;  
                } else {  
                    return 1;  
                } // returning 0 would merge keys 
            }  
        }  
    
    }