当需要对元素进行计数时，HashMap非常有用，如下例子，统计一个字符串中每个字符出现的次数：

package simplejava;

import java.util.HashMap;
import java.util.Map.Entry;

public class Q12 {

    public static void main(String[] args) {
        HashMap<Integer, Integer> countMap = new HashMap<Integer, Integer>();
        // .... a lot of a’s like the following

        String chars = "abcabcabcghgk";

        for(int i = 0; i < chars.length(); i++){
            int a = chars.charAt(i);
            if (countMap.keySet().contains(a)) {
                countMap.put(a, countMap.get(a) + 1);
            } else {
                countMap.put(a, 1);
            }
        }

        for(Entry<Integer, Integer> e : countMap.entrySet()){
            System.out.println((char)(int)e.getKey() + " " + e.getValue());
        }

    }

}

输出结果：

g 2
b 3
c 3
a 3
k 1
h 1

HashMap遍历

        Map<Integer, Integer> mp = new HashMap<Integer, Integer>();
        Iterator it = mp.entrySet().iterator();
        while (it.hasNext()) {
            Map.Entry pairs = (Map.Entry) it.next();
            System.out.println(pairs.getKey() + " = " + pairs.getValue());
        }

        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
        for (Map.Entry<Integer, Integer> entry : map.entrySet()) {
            System.out.println("Key = " + entry.getKey() + ", Value = " + entry.getValue());
        }

打印HashMap的元素

    public static void printMap(Map mp) {
        Iterator it = mp.entrySet().iterator();
        while (it.hasNext()) {
            Map.Entry pairs = (Map.Entry) it.next();
            System.out.println(pairs.getKey() + " = " + pairs.getValue());
            it.remove(); // avoids a ConcurrentModificationException
        }
    }

根据键值对的value排序

以下代码往TreeMap的构造函数传入一个比较器，来对map进行排序：

package simplejava;

import java.util.Comparator;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.TreeMap;

class ValueComparator implements Comparator<String> {
    Map<String, Integer> base;

    public ValueComparator(Map<String, Integer> base) {
        this.base = base;
    }

    public int compare(String a, String b) {
        if (base.get(a) >= base.get(b)) {
            return -1;
        } else {
            return 1;
        } // returning 0 would merge keys
    }
}

public class Q12 {
    public static void printMap(Map mp) {
        Iterator it = mp.entrySet().iterator();
        while (it.hasNext()) {
            Map.Entry pairs = (Map.Entry) it.next();
            System.out.println(pairs.getKey() + " = " + pairs.getValue());
            it.remove(); // avoids a ConcurrentModificationException
        }
    }

    public static void main(String[] args) {
        HashMap<String, Integer> countMap = new HashMap<String, Integer>();
        // add a lot of entries
        countMap.put("a", 10);
        countMap.put("b", 20);
        ValueComparator vc = new ValueComparator(countMap);
        TreeMap<String, Integer> sortedMap = new TreeMap<String, Integer>(vc);
        sortedMap.putAll(countMap);
        printMap(sortedMap);

    }

}

虽然有很多种方法来对HashMap进行排序，但以上这种方法在stackoverflow中是最被推崇的；

注：使用了一个比较器Comparator对TreeMap排序，该比较器比较key的方式是取出key对应的value进行大小比较；

译文地址：http://www.programcreek.com/2013/04/frequently-used-methods-of-java-hashmap/