Implement add() and subtract() methos in Bandwidth

Change-Id: I93a382cff1a441fb9c9929fc4fab82c956ccdd0e

@@ -90,6 +90,26 @@ public final class Bandwidth implements RichComparable<Bandwidth> {
         return bps;
     }
 
+    /**
+     * Returns a Bandwidth whose value is (this + value).
+     *
+     * @param value value to be added to this Frequency
+     * @return this + value
+     */
+    public Bandwidth add(Bandwidth value) {
+        return bps(this.bps + value.bps);
+    }
+
+    /**
+     * Returns a Bandwidth whose value is (this - value).
+     *
+     * @param value value to be added to this Frequency
+     * @return this - value
+     */
+    public Bandwidth subtract(Bandwidth value) {
+        return bps(this.bps - value.bps);
+    }
+
     @Override
     public int compareTo(Bandwidth other) {
         return ComparisonChain.start()

@@ -43,6 +43,26 @@ public class BandwidthTest {
     }
 
     /**
+     * Tests add operation of two Bandwidths.
+     */
+    @Test
+    public void testAdd() {
+        Bandwidth expected = Bandwidth.kbps(1100.0);
+
+        assertThat(small.add(large), is(expected));
+    }
+
+    /**
+     * Tests subtract operation of two Bandwidths.
+     */
+    @Test
+    public void testSubtract() {
+        Bandwidth expected = Bandwidth.kbps(900.0);
+
+        assertThat(large.subtract(small), is(expected));
+    }
+
+    /**
      * Tests if the first object is less than the second object.
      */
     @Test