Map is a collection interface of key-value pairs. Its implementation classes mainly include: HashMap, TreeMap, Hashtable and LinkedHashMap, etc.
•TreeMap: A NavigableMap implementation based on a Red-Black tree, which is sorted according to the natural ordering of its keys, or according to the Comparator provided when the map is created, depending on the Construction method.
•The value of HashMap is not in order. It is implemented according to the HashCode of the key. How do we implement sorting for this unordered HashMap? Refer to the value sorting of TreeMap.
Map.Entry returns the Collections view.
Sort by key
TreeMap is in ascending order by default. If we need to change the sorting method, we need to use a comparator: Comparator. Comparator is a comparator interface that can sort collection objects or arrays. Sorting can be achieved by implementing the public compare(T o1, To2) method of this interface.
Note: The following codes have been tested and passed in Jdk1.6
TreeMap is sorted by key in ascending order by default
public static void keyUpSort() {
// 默认情况，TreeMap按key升序排序
Map<String, Integer> map = new TreeMap<String, Integer>();
map.put("acb1", 5);
map.put("bac1", 3);
map.put("bca1", 20);
map.put("cab1", 80);
map.put("cba1", 1);
map.put("abc1", 10);
map.put("abc2", 12);
// 默认情况下，TreeMap对key进行升序排序
System.out.println("------------正常情况，TreeMap按key升序排序--------------------");
for (Map.Entry<String, Integer> entry : map.entrySet()) {
System.out.println(entry.getKey() + ":" + entry.getValue());
}
}
Modify the sorting method of TreeMap, Sort by key in descending order
public static void keyDownSort() {
// TreeMap,按key降序排序
// 降序排序比较器
Comparator<String> keyComparator = new Comparator<String>() {
@Override
public int compare(String o1, String o2) {
// TODO Auto-generated method stub
return o2.compareTo(o1);
}
};
Map<String, Integer> map = new TreeMap<String, Integer>(keyComparator);
map.put("acb1", 5);
map.put("bac1", 3);
map.put("bca1", 20);
map.put("cab1", 80);
map.put("cba1", 1);
map.put("abc1", 10);
map.put("abc2", 12);
System.out.println("------------TreeMap按key降序排序--------------------");
for (Map.Entry<String, Integer> entry : map.entrySet()) {
System.out.println(entry.getKey() + ":" + entry.getValue());
}
}
Sort by Value
The following only demonstrates sorting by Value in ascending order by TreeMap, which also applies to HashMap.
Modify the sorting method of TreeMap and sort in ascending order of Value
Note: Under normal circumstances, Map cannot be sorted using the Collections.sort() method, but it can be converted into a list. Sort again.
public static void valueUpSort() {
// 默认情况，TreeMap按key升序排序
Map<String, Integer> map = new TreeMap<String, Integer>();
map.put("acb1", 5);
map.put("bac1", 3);
map.put("bca1", 20);
map.put("cab1", 80);
map.put("cba1", 1);
map.put("abc1", 10);
map.put("abc2", 12);
// 升序比较器
Comparator<Map.Entry<String, Integer>> valueComparator = 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()-o2.getValue();
}
};
// map转换成list进行排序
List<Map.Entry<String, Integer>> list = new ArrayList<Map.Entry<String,Integer>>(map.entrySet());
// 排序
Collections.sort(list,valueComparator);
// 默认情况下，TreeMap对key进行升序排序
System.out.println("------------map按照value升序排序--------------------");
for (Map.Entry<String, Integer> entry : list) {
System.out.println(entry.getKey() + ":" + entry.getValue());
}
}
Test results
------------正常情况，TreeMap按key升序排序-------------------- abc1:10 abc2:12 acb1:5 bac1:3 bca1:20 cab1:80 cba1:1 ------------TreeMap按key降序排序-------------------- cba1:1 cab1:80 bca1:20 bac1:3 acb1:5 abc2:12 abc1:10 ------------map按照value升序排序-------------------- cba1:1 bac1:3 acb1:5 abc1:10 abc2:12 bca1:20 cab1:80
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