question: @ Override public void execute ( MetricTimeSeries timeSeries , FunctionValueMap functionValueMap ) { if ( timeSeries . size ( ) <= 0 ) { functionValueMap . add ( this , Double . NaN ) ; return ; } timeSeries . sort ( ) ; double firstValue = timeSeries . getValue ( 0 ) ; double lastValue = timeSeries . getValue ( timeSeries . size ( ) - 1 ) ; functionValueMap . add ( this , Math . abs ( firstValue - lastValue ) ) ; }
target:  Calculate the difference between the first and the last value of a given time series