Apex オブジェクトのAPI名一覧取得

public String getAllFieldNamess(String sobjName){
    String strFieldKeys='';
    Schema.SObjectType sobjType = Schema.getGlobalDescribe().get(sobjName);
    Map<String, Schema.SObjectField> fmap = sobjType.getDescribe().fields.getMap();
    for (String fieldKey : fmap.keySet()) {
        Schema.SObjectField f = fmap.get(fieldKey);
        if(strFieldKeys == ''){
            strFieldKeys = fieldKey;
        }else{
            strFieldKeys +=  ',' + fieldKey;
        }
    }
    return strFieldKeys;
}