openhab mqtt configuration files for receiving temperature values and displaying the temperature and the time of receipt in openhab

default.items

String   Test_Temperature_Str "Test String  [%s]"  {channel="mqtt:topic:test_temperature:test_temperature_channel"}
Number   Test_Temperature   "Temperature [%.2f °C]"      <temperature>   (Balcony, gTemperature)   ["Temperature"]
DateTime Test_DateTime "last transmission [%1$tA, %1$tH:%1$tM]" <time>   (Balcony, gTemperature)   ["Temperature"]

Group    Balcony               "Balcony"          <terrace>       (Home)                    ["Outdoor"]
Group:Number:AVG   gTemperature   "Temperatur"   <temperature>   (Home)   ["Temperature"]

temperature.rules

rule "Convert String to Item Number"

 when
    Item Test_Temperature_Str changed
 then
    // use the transformation service to retrieve the value
    val temp_str = Test_Temperature_Str.state.toString

    if(temp_str != "1000.00"){
        // post the new value to the Number Item
        Test_Temperature.postUpdate(temp_str)
        Test_DateTime.postUpdate(new DateTimeType())
        logInfo("temp", temp_str)
    }
 end

All you have to do to display a temperature value to openHab is to add a "Generic MQTT Thing" in the PaperUI of openHab and to add a channel to this ,which has the topic of the temperature sensor as "MQTT state topic"(e.g. "/home/balcony/temperature").
Note that the id of the "Generic MQTT Thing" and the id of the channel are the same as in the string "Test_Temperature_Str"(mqtt:topic:test_temperature:test_temperature_channel) .

You also need to add the default.items file to the /etc/openhab2/items folder and the temperature.rules file to the /etc/openhab2/rules folder.