Add method for substracting EffortDurations

FEA: ItEr60S19TimeUnitDataType

navalplanner-business/src/main/java/org/navalplanner/business/workingday/EffortDuration.java

@@ -168,4 +168,20 @@ public class EffortDuration implements Comparable<EffortDuration> {
         return seconds == 0;
     }
 
+    /**
+     * Substracts two {@link EffortDuration}. Because {@link EffortDuration
+     * durations} cannot be negative <code>this</code> must be bigger than the
+     * parameter or the same
+     *
+     * @param duration
+     * @return the result of substracting the two durations
+     * @throws IllegalArgumentException
+     *             if the parameter is bigger than <code>this</code>
+     */
+    public EffortDuration minus(EffortDuration duration) {
+        Validate.isTrue(this.compareTo(duration) >= 0,
+                "minued must not be smaller than subtrahend");
+        return new EffortDuration(seconds - duration.seconds);
+    }
+
 }

navalplanner-business/src/test/java/org/navalplanner/business/test/workingday/EffortDurationTest.java

@@ -149,4 +149,16 @@ public class EffortDurationTest {
         assertThat(hours(3).divideBy(4), equalTo(minutes(45)));
     }
 
+    @Test(expected = IllegalArgumentException.class)
+    public void effortDurationCannotBeSubstractedIfMinuedIsSmallerThanSubtrahend() {
+        EffortDuration threeHours = hours(3);
+        threeHours.minus(threeHours.and(1, Granularity.SECONDS));
+    }
+
+    @Test
+    public void effortDurationCanBeSubstracted() {
+        assertThat(hours(2).minus(minutes(120)), equalTo(EffortDuration.zero()));
+        assertThat(hours(2).minus(minutes(60)), equalTo(hours(1)));
+    }
+
 }