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；
本文转自风一样的码农博客园博客，原文链接：http://www.cnblogs.com/chenpi/p/5493678.html，如需转载请自行联系原作者
