0 - Invoice number

src/main/java/pl/edu/agh/mwo/invoice/Invoice.java

@@ -7,7 +7,9 @@
 import pl.edu.agh.mwo.invoice.product.Product;
 
 public class Invoice {
-    private Map<Product, Integer> products = new HashMap<Product, Integer>();
+    private Map<Product, Integer> products = new HashMap<>();
+    private static int nextNumber = 0;
+    private final int number = ++nextNumber;
 
     public void addProduct(Product product) {
         addProduct(product, 1);
@@ -41,4 +43,8 @@ public BigDecimal getGrossTotal() {
         }
         return totalGross;
     }
+
+    public int getNumber() {
+        return number;
+    }
 }

src/test/java/pl/edu/agh/mwo/invoice/InvoiceTest.java

@@ -125,4 +125,29 @@ public void testInvoiceWithNegativeQuantity() {
     public void testAddingNullProduct() {
         invoice.addProduct(null);
     }
+
+    @Test
+    public void testInvoiceHasNumberGreaterThan0() {
+        int number = invoice.getNumber();
+        Assert.assertThat(number, Matchers.greaterThan(0));
+    }
+
+    @Test
+    public void testTwoInvoicesHaveDifferentNumbers() {
+        int number1 = new Invoice().getNumber();
+        int number2 = new Invoice().getNumber();
+        Assert.assertNotEquals(number1, number2);
+    }
+
+    @Test
+    public void testInvoiceDoesNotChangeItsNumber() {
+        Assert.assertEquals(invoice.getNumber(), invoice.getNumber());
+    }
+
+    @Test
+    public void testTheFirstInvoiceNumberIsLowerThanTheSecond() {
+        int number1 = new Invoice().getNumber();
+        int number2 = new Invoice().getNumber();
+        Assert.assertThat(number1, Matchers.lessThan(number2));
+    }
 }