Map<String,  String> maps = new HashMap<String, String>();

maps.put("_id", "id") ;

maps.put("_name", "name");

maps.put("_age", "age");

String str1 = "";

for(String s : maps.keySet()) {

　　str1 += s + ",";

}

str1 = str1.substring(0, str1.length() - 1);

System.out.println(str1);

输出为：_id,_name,_age