StockMonitorTest.java

public class StockMonitorTest {

    @Test
    public void alertSentWhenProductNeedsReordering() {
        Alert alert = mock(Alert.class);
        ReorderLevel reorderLevel = product1 -> 10;
        Product product = new Product(811, 11, 14);

        Warehouse warehouse = productId -> product;
        StockMonitor monitor = new StockMonitor(alert, warehouse, reorderLevel);
        monitor.productSold(811, 1);
        verify(alert).send(product);
    }
}