How to concatenate values into one to display a summary

Setup, Configuration and Use Scripts & Rules
10

At least form me, it gets more complicated:

import java.Math.BigDecimal
import org.openhab.core.library.types.*

rule "HeatingDescription"
when
	Item Office_Heat_Left_ActTemp received update or
	Item Office_Heat_Left_TargetTemp received update or
	Item Office_Heat_Left_Fault received update or
	Item Office_Heat_Left_StateBat received update or
	Item Office_Heat_Left_Valve received update or
	Item Office_Heat_Left_Boost received update or
	Item Office_Heat_Left_LowBat received update or 
	System started
then
	val BigDecimal Heat_Left_ActTemp = if (Office_Heat_Left_ActTemp.state instanceof DecimalType) (Office_Heat_Left_ActTemp.state as DecimalType).toBigDecimal else new BigDecimal(0)
	val BigDecimal Heat_Left_TargetTemp = if (Office_Heat_Left_TargetTemp.state instanceof DecimalType) (Office_Heat_Left_TargetTemp.state as DecimalType).toBigDecimal else BigDecimal(0)
	var String formattedString = String::format("%1$.0f°C / %2$.0f°C", Heat_Left_ActTemp, Heat_Left_TargetTemp)
	logInfo(">>> HeatingDescription <<<", formattedString)
	Office_Heat_Left.postUpdate(new StringType(formattedString))
end

leads to:

2016-01-23 14:57:30.947 [ERROR] [m.r.internal.engine.RuleEngine] - Error during the execution of startup rule 'HeatingDescription': Could not invoke method: java.lang.String.format(java.util.Locale,java.lang.String,java.lang.Object[]) on instance: null