public static void main(String[] args) {
        String str = "ahadcdffhachaasdfaweadbastawetaesartaw";

        // 准备
        char[] cTarget = str.toLowerCase().toCharArray(); // 转为char数组
        int[] cnts = new int[26]; // 计数器

        // 检索
        for (char c : cTarget) {
            cnts[c - 'a']++;
        }

        // 排序前准备下用于记录位置的数组
        char[] cs = new char[26];
        for (char i = 0; i < cs.length; i++) {
            cs[i] = (char) ('a' + i);
        }
        // 排序
        for (int i = 0; i < cnts.length; i++) {
            for (int j = i + 1; j < cs.length; j++) {
                if (cnts[j] < cnts[i]) {
                    int a = cnts[j];
                    cnts[j] = cnts[i];
                    cnts[i] = a;
                    char c = cs[j];
                    cs[j] = cs[i];
                    cs[i] = c;
                }
            }
        }

        // 最后输出
        for (int i = 0; i < cs.length; i++) {
            if (cnts[i] > 0) {
                System.out.println(cs[i] + ":" + cnts[i] + "\t");
            }
        }
    }

------解决方案--------------------

期待更短的。。。

Java code

String str = "ahadcdffhacha";
        Map map = new HashMap();
        for(int i = 0;i < str.length();i++)
        {
            if(map.containsKey(str.charAt(i)))
            {
                map.put(str.charAt(i), Integer.parseInt(map.get(str.charAt(i)).toString())+1);
            }
            else
            {
                map.put(str.charAt(i), 1);
            }
        }
        
        List list = new ArrayList();
        Set set = map.entrySet();
        for(Iterator i = set.iterator();i.hasNext();)
        {
            Map.Entry entry = (Map.Entry)i.next();
            list.add(entry);
        }
        
        Collections.sort(list,new Comparator()
        {
            public int compare(Object o1, Object o2) {
                Map.Entry e1 = (Map.Entry)o1;
                Map.Entry e2 = (Map.Entry)o2;
                return Integer.parseInt(e1.getValue().toString())-Integer.parseInt(e2.getValue().toString());
            }
        });

        System.out.println(list);