docs/examples/units-extension/UnitsExtensionDemo.java

import org.checkerframework.checker.units.qual.Prefix;
import org.checkerframework.checker.units.qual.s;
import org.checkerframework.checker.units.util.UnitsTools;
import qual.Frequency;
import qual.Hz;
import qual.kHz;

public class UnitsExtensionDemo {
  @Hz int frq;

  void bad() {
    // Error! Unqualified value assigned to a @Hz value.
    // :: error: (assignment)
    frq = 5;

    // suppress all warnings issued by the units checker for the d1 assignment statement
    @SuppressWarnings("units")
    @Hz int d1 = 9;

    // specifically suppress warnings related to any frequency units for the d2 assigment
    // statement
    @SuppressWarnings("frequency")
    @Hz int d2 = 10;
  }

  // specifically suppresses warnings for the hz annotation for the toHz method
  @SuppressWarnings("hz")
  static @Hz int toHz(int hz) {
    return hz;
  }

  void good() {
    frq = toHz(9);

    @s double time = 5 * UnitsTools.s;
    @Hz double freq2 = 20 / time;
  }

  void auto(@s int time) {
    // The @Hz annotation is automatically added to the result
    // of the division, because we provide class FrequencyRelations.
    frq = 99 / time;
  }

  public static void main(String[] args) {
    @Hz int hertz = toHz(20);
    @s int seconds = 5 * UnitsTools.s;

    @SuppressWarnings("units")
    @s(Prefix.milli) int millisec = 10;

    @SuppressWarnings("hz")
    @kHz int kilohertz = 30;

    @Hz int resultHz = hertz + 20 / seconds;
    System.out.println(resultHz);

    @kHz int resultkHz = kilohertz + 50 / millisec;
    System.out.println(resultkHz);

    // this demonstrates the type hierarchy resolution: the common supertype of Hz and kHz is
    // Frequency, so this statement will pass
    @Frequency int okTernaryAssign = seconds > 10 ? hertz : kilohertz;

    // on the other hand, this statement expects the right hand side to be a Hz, so it will fail
    // :: error: (assignment)
    @Hz int badTernaryAssign = seconds > 10 ? hertz : kilohertz;
  }
}