String-->JSONArray-->JSONObject

String putcurrent = model.get("json_put_current_str");

BigDecimal equipamt = BigDecimal.ZERO;
JSONArray array = new JSONArray(rentplan);
for (int i = 0; i < array.length(); i++) {
     JSONObject obj = array.getJSONObject(i);
     equipamt = equipamt.add(new BigDecimal(obj.optString("planmoney")));
}