Java HashMap 中常用方法

原文地址：http://www.programcreek.com/2009/02/a-simple-treeset-example/

当需要计数器时，HashMap 非常有用。

HashMap<String, Integer> countMap = new HashMap<String, Integer>();

//.... a lot of a's like the following
if(countMap.keySet().contains(a)){
    countMap.put(a, countMap.get(a)+1);
}else{
    countMap.put(a, 1);
}

1. 循环遍历 HashMap

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());
}

2. 打印 HashMap

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
    }
}

3. 按值排序 HashMap

下面的示例代码利用了一个 TreeMap 的构造函数。

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
    }
}

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);

有不同的方法来排序 HashMap，这种方式在 stackoverflow 中投票最多。