这里有新鲜出炉的Java并发编程示例,程序狗速度看过来!
java 是一种可以撰写跨平台应用软件的面向对象的程序设计语言,是由Sun Microsystems公司于1995年5月推出的Java程序设计语言和Java平台(即JavaEE(j2ee), JavaME(j2me), JavaSE(j2se))的总称。
下面小编就为大家带来一篇浅谈Java之Map 按值排序 (Map sort by value)。小编觉得挺不错的,现在就分享给大家,也给大家做个参考。一起跟随小编过来看看吧
Map是键值对的集合,又叫作字典或关联数组等,是最常见的数据结构之一。在java如何让一个map按value排序呢? 看似简单,但却不容易!
比如,Map中key是String类型,表示一个单词,而value是int型,表示该单词出现的次数,现在我们想要按照单词出现的次数来排序:
- Map map = new TreeMap();
- map.put("me", 1000);
- map.put("and", 4000);
- map.put("you", 3000);
- map.put("food", 10000);
- map.put("hungry", 5000);
- map.put("later", 6000);
按值排序的结果应该是:
- key value
- me 1000
- you 3000
- and 4000
- hungry 5000
- later 6000
- food 10000
首先,不能采用SortedMap结构,因为SortedMap是按键排序的Map,而不是按值排序的Map,我们要的是按值排序的Map。
Couldn't you do this with a SortedMap?
No, because the map are being sorted by its keys.
方法一:
如下Java代码:
- import java.util.Iterator;
- import java.util.Set;
- import java.util.TreeSet;
- public class Main {
- public static void main(String[] args) {
- Set set = new TreeSet();
- set.add(new Pair("me", "1000"));
- set.add(new Pair("and", "4000"));
- set.add(new Pair("you", "3000"));
- set.add(new Pair("food", "10000"));
- set.add(new Pair("hungry", "5000"));
- set.add(new Pair("later", "6000"));
- set.add(new Pair("myself", "1000"));
- for (Iterator i = set.iterator(); i.hasNext();)
- System.out.println(i.next());
- }
- }
- class Pair implements Comparable {
- private final String name;
- private final int number;
- public Pair(String name, int number) {
- this.name = name;
- this.number = number;
- }
- public Pair(String name, String number) throws NumberFormatException {
- this.name = name;
- this.number = Integer.parseInt(number);
- }
- public int compareTo(Object o) {
- if (o instanceof Pair) {
- int cmp = Double.compare(number, ((Pair) o).number);
- if (cmp != 0) {
- return cmp;
- }
- return name.compareTo(((Pair) o).name);
- }
- throw new ClassCastException("Cannot compare Pair with " + o.getClass().getName());
- }
- public String toString() {
- return name + ' ' + number;
- }
- }
类似的C++代码:
- typedef pair < string,
- int > PAIR;
- int cmp(const PAIR & x, const PAIR & y) {
- return x.second > y.second;
- }
- map < string,
- int > m;
- vector < PAIR > vec;
- for (map < wstring, int > ::iterator curr = m.begin(); curr != m.end(); ++curr) {
- vec.push_back(make_pair(curr - >first, curr - >second));
- }
- sort(vec.begin(), vec.end(), cmp);
上面方法的实质意义是:将Map结构中的键值对(Map.Entry)封装成一个自定义的类(结构),或者直接用Map.Entry类。自定义类知道自己应该如何排序,也就是按值排序,具体为自己实现Comparable接口或构造一个Comparator对象,然后不用Map结构而采用有序集合(SortedSet, TreeSet是SortedSet的一种实现),这样就实现了Map中sort by value要达到的目的。就是说,不用Map,而是把Map.Entry当作一个对象,这样问题变为实现一个该对象的有序集合或对该对象的集合做排序。既可以用SortedSet,这样插入完成后自然就是有序的了,又或者用一个List或数组,然后再对其做排序(Collections.sort() or Arrays.sort())。
Encapsulate the information in its own class. Either implement
Comparable and write rules for the natural ordering or write a
Comparator based on your criteria. Store the information in a sorted
collection, or use the Collections.sort() method.
方法二:
You can also use the following code to sort by value:
- public static Map sortByValue(Map map) {
- List list = new LinkedList(map.entrySet());
- Collections.sort(list, new Comparator() {
- public int compare(Object o1, Object o2) {
- return ((Comparable) ((Map.Entry) (o1)).getValue())
- .compareTo(((Map.Entry) (o2)).getValue());
- }
- });
- Map result = new LinkedHashMap();
- for (Iterator it = list.iterator(); it.hasNext();) {
- Map.Entry entry = (Map.Entry) it.next();
- result.put(entry.getKey(), entry.getValue());
- }
- return result;
- }
- public static Map sortByValue(Map map, final boolean reverse) {
- List list = new LinkedList(map.entrySet());
- Collections.sort(list, new Comparator() {
- public int compare(Object o1, Object o2) {
- if (reverse) {
- return -((Comparable) ((Map.Entry) (o1)).getValue())
- .compareTo(((Map.Entry) (o2)).getValue());
- }
- return ((Comparable) ((Map.Entry) (o1)).getValue())
- .compareTo(((Map.Entry) (o2)).getValue());
- }
- });
- Map result = new LinkedHashMap();
- for (Iterator it = list.iterator(); it.hasNext();) {
- Map.Entry entry = (Map.Entry) it.next();
- result.put(entry.getKey(), entry.getValue());
- }
- return result;
- }
- Map map = new HashMap();
- map.put("a", 4);
- map.put("b", 1);
- map.put("c", 3);
- map.put("d", 2);
- Map sorted = sortByValue(map);
- System.out.println(sorted);
- // output : {b=1, d=2, c=3, a=4}
- 或者还可以这样:
- Map map = new HashMap();
- map.put("a", 4);
- map.put("b", 1);
- map.put("c", 3);
- map.put("d", 2);
- Set<Map.Entry<String, Integer>> treeSet = new TreeSet<Map.Entry<String, Integer>>(
- new Comparator<Map.Entry<String, Integer>>() {
- public int compare(Map.Entry<String, Integer> o1,
- Map.Entry<String, Integer> o2) {
- Integer d1 = o1.getValue();
- Integer d2 = o2.getValue();
- int r = d2.compareTo(d1);
- if (r != 0)
- return r;
- else
- return o2.getKey().compareTo(o1.getKey());
- }
- });
- treeSet.addAll(map.entrySet());
- System.out.println(treeSet);
- // output : [a=4, c=3, d=2, b=1]
另外,Groovy 中实现 sort map by value,当然本质是一样的,但却很简洁 :
用 groovy 中 map 的 sort 方法(需要 groovy 1.6),
- def result = map.sort(){ a, b ->
- b.value.compareTo(a.value)
- }
如:
["a":3,"b":1,"c":4,"d":2].sort{ a,b -> a.value - b.value }
结果为: [b:1, d:2, a:3, c:4]
Python中也类似:
- h = {"a":2,"b":1,"c":3}
- i = h.items() // i = [('a', 2), ('c', 3), ('b', 1)]
- i.sort(lambda (k1,v1),(k2,v2): cmp(v2,v1) ) // i = [('c', 3), ('a', 2), ('b', 1)]
以上这篇浅谈Java之Map 按值排序 (Map sort by value)就是小编分享给大家的全部内容了,希望能给大家一个参考,也希望大家多多支持PHPERZ。
来源: http://www.phperz.com/article/17/1121/360221.html