@Override
    public int getAvailableDeviceCount() {
        return availableDevices.size();
    }
@Test
    public final void testGetAvailableDeviceCount() {
        assertEquals("initialy empty", 0, deviceStore.getAvailableDeviceCount());

        putDevice(DID1, SW1);
        putDevice(DID2, SW2);

        deviceStore.markOffline(DID1);

        assertEquals("expect 1 available device", 1, deviceStore.getAvailableDeviceCount());

        deviceStore.markOnline(DID1);

        assertEquals("expect 2 available devices", 2, deviceStore.getAvailableDeviceCount());
    }