StringBuffer jsonStrBuf = new StringBuffer("[");
        String[] arr = { "2011-11", "2012-02", "2012-03", "2012-04", "2012-05", "2012-06", "2012-07" };
        List<String> list = Arrays.asList(arr);
        Map<String, List<String>> map = new HashMap<String, List<String>>();
        for (String str : list) {
            String y = str.substring(0, 4);
            String m = str.substring(5);
            if (map.containsKey(y)) {
                map.get(y).add(m);
            } else {
                List<String> l = new ArrayList<String>();
                l.add(m);
                map.put(y, l);
            }
        }
        for (String key : map.keySet()) {
            jsonStrBuf.append("{'year':" + key);
            jsonStrBuf.append(",'month':" + Arrays.toString((map.get(key).toArray(new String[0]))) + "},");
        }
        System.out.println(jsonStrBuf.delete(jsonStrBuf.length() - 1, jsonStrBuf.length()).append("]"));

------解决方案--------------------

for example
Java code
list = [2011-11, 2012-02, 2012-03, 2012-04, 2012-05, 2012-06, 2012-07];

Map<String, List<String>> map = new HashMap<String, List<String>>();
for (String s : list) { //把list解析，结果存到map里
    String[] sa = s.split("-");
    if (!map.containsKey(sa[0])) map.put(sa[0], new ArrayList<String>());
    map.get(sa[0]).add(sa[1]);
}

StringBuilder buf = new StringBuilder();
for (Map.Entry<String, List<String>> e : map.entrySet()) {
    buf.append("{'year:").append(e.getKey()).append(",'month':[");
    for (String s : e.getValue()) {
        buf.append("'").append(s).append("',");
    }
    buf.delete(buf.length()-1, buf.length());
    buf.append("},");
}
buf.delete(buf.length()-1, buf.length());
System.out.println(buf);