In the EF-Pi there are several interfaces working with physical quantities. For all of these quantities there are multiple units you could use: seconds, minutes, hours, joules, kilowatt hours... One programmer might prefer working with joules while another might prefer working with kilowatt hours. In order to prevent confusion and cumbersome manual conversion we have adopted the javax.measure packages from the JScience project. These packages provide classes and interfaces which combine numbers and units in a measurable object. These objects are so commonly used in the EF-Pi that they are part of the flexiblepower.api bundle.
The package provides a set of SI-units (such as second and joule) and NonSI-units (such as hour and Kilowatt hour). Measurables are typed with the generic Measurable interface.
// Static imports make working with units easier.
import static javax.measure.unit.SI.*;
import static javax.measure.unit.NonSI.*;
// Create a measurable which stores an amount of power
Measurable<Power> power = Measure.valueOf(50, WATT);
// Create a measurable which stores a duration
Measurable<Duration> hour = Measure.valueOf(1, HOUR);
// Compare two measurables. This results in true.
boolean res = Measure.valueOf(1000, WATT).equals(Measure.valueOf(1, KILO(WATT)))
// Compare two measurables. This results in a value < 0.
int compare = Measure.valueOf(1, JOULE).compareTo(Measure.valueOf(1, KWH));
// Get the value of a measurable as a double. This results in 3600.
double seconds = Measure.valueOf(1, HOUR).doubleValue(SECOND);The Java API provides a Date class to represent a time. Since we work a lot with Measurable in the EF-Pi, there is a utility class called TimeUtil which provides some methods for combining the two.
Date now = new Date();
Date oneHourInTheFuture = TimeUtil.add(now, Measure.valueOf(1, HOUR));
Date oneHourInThePast = TimeUtil.subtract(now, Measure.valueOf(1, HOUR));
Measurable<Duration> twoHours = TimeUtil.difference(oneHourInThePast, oneHourInTheFuture);Note: as you will read in the TimeSource section we don't recommand using new Date() in the EF-Pi. This is just for demonstration purposes.