diff --git a/pom.xml b/pom.xml
index 2614c6984..2d40c3e8a 100644
--- a/pom.xml
+++ b/pom.xml
@@ -21,8 +21,8 @@
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-compiler-plugin</artifactId>
                 <configuration>
-                    <source>1.8</source>
-                    <target>1.8</target>
+                    <source>17</source>
+                    <target>17</target>
                 </configuration>
             </plugin>
         </plugins>
diff --git a/src/test/java/pl/edu/agh/mwo/invoice/InvoiceTest.java b/src/test/java/pl/edu/agh/mwo/invoice/InvoiceTest.java
new file mode 100644
index 000000000..7f4b6f795
--- /dev/null
+++ b/src/test/java/pl/edu/agh/mwo/invoice/InvoiceTest.java
@@ -0,0 +1,128 @@
+package pl.edu.agh.mwo.invoice;
+
+import java.math.BigDecimal;
+
+import org.hamcrest.Matchers;
+import org.junit.Assert;
+import org.junit.Before;
+import org.junit.Test;
+
+import pl.edu.agh.mwo.invoice.Invoice;
+import pl.edu.agh.mwo.invoice.product.DairyProduct;
+import pl.edu.agh.mwo.invoice.product.OtherProduct;
+import pl.edu.agh.mwo.invoice.product.Product;
+import pl.edu.agh.mwo.invoice.product.TaxFreeProduct;
+
+public class InvoiceTest {
+    private Invoice invoice;
+
+    @Before
+    public void createEmptyInvoiceForTheTest() {
+        invoice = new Invoice();
+    }
+
+    @Test
+    public void testEmptyInvoiceHasEmptySubtotal() {
+        Assert.assertThat(BigDecimal.ZERO, Matchers.comparesEqualTo(invoice.getSubtotal()));
+    }
+
+    @Test
+    public void testEmptyInvoiceHasEmptyTaxAmount() {
+        Assert.assertThat(BigDecimal.ZERO, Matchers.comparesEqualTo(invoice.getTax()));
+    }
+
+    @Test
+    public void testEmptyInvoiceHasEmptyTotal() {
+        Assert.assertThat(BigDecimal.ZERO, Matchers.comparesEqualTo(invoice.getTotal()));
+    }
+
+    @Test
+    public void testInvoiceSubtotalWithTwoDifferentProducts() {
+        Product onions = new TaxFreeProduct("Warzywa", new BigDecimal("10"));
+        Product apples = new TaxFreeProduct("Owoce", new BigDecimal("10"));
+        invoice.addProduct(onions);
+        invoice.addProduct(apples);
+        Assert.assertThat(new BigDecimal("20"), Matchers.comparesEqualTo(invoice.getSubtotal()));
+    }
+
+    @Test
+    public void testInvoiceSubtotalWithManySameProducts() {
+        Product onions = new TaxFreeProduct("Warzywa", BigDecimal.valueOf(10));
+        invoice.addProduct(onions, 100);
+        Assert.assertThat(new BigDecimal("1000"), Matchers.comparesEqualTo(invoice.getSubtotal()));
+    }
+
+    @Test
+    public void testInvoiceHasTheSameSubtotalAndTotalIfTaxIsZero() {
+        Product taxFreeProduct = new TaxFreeProduct("Warzywa", new BigDecimal("199.99"));
+        invoice.addProduct(taxFreeProduct);
+        Assert.assertThat(invoice.getTotal(), Matchers.comparesEqualTo(invoice.getSubtotal()));
+    }
+
+    @Test
+    public void testInvoiceHasProperSubtotalForManyProducts() {
+        invoice.addProduct(new TaxFreeProduct("Owoce", new BigDecimal("200")));
+        invoice.addProduct(new DairyProduct("Maslanka", new BigDecimal("100")));
+        invoice.addProduct(new OtherProduct("Wino", new BigDecimal("10")));
+        Assert.assertThat(new BigDecimal("310"), Matchers.comparesEqualTo(invoice.getSubtotal()));
+    }
+
+    @Test
+    public void testInvoiceHasProperTaxValueForManyProduct() {
+        // tax: 0
+        invoice.addProduct(new TaxFreeProduct("Pampersy", new BigDecimal("200")));
+        // tax: 8
+        invoice.addProduct(new DairyProduct("Kefir", new BigDecimal("100")));
+        // tax: 2.30
+        invoice.addProduct(new OtherProduct("Piwko", new BigDecimal("10")));
+        Assert.assertThat(new BigDecimal("10.30"), Matchers.comparesEqualTo(invoice.getTax()));
+    }
+
+    @Test
+    public void testInvoiceHasProperTotalValueForManyProduct() {
+        // price with tax: 200
+        invoice.addProduct(new TaxFreeProduct("Maskotki", new BigDecimal("200")));
+        // price with tax: 108
+        invoice.addProduct(new DairyProduct("Maslo", new BigDecimal("100")));
+        // price with tax: 12.30
+        invoice.addProduct(new OtherProduct("Chipsy", new BigDecimal("10")));
+        Assert.assertThat(new BigDecimal("320.30"), Matchers.comparesEqualTo(invoice.getTotal()));
+    }
+
+    @Test
+    public void testInvoiceHasPropoerSubtotalWithQuantityMoreThanOne() {
+        // 2x kubek - price: 10
+        invoice.addProduct(new TaxFreeProduct("Kubek", new BigDecimal("5")), 2);
+        // 3x kozi serek - price: 30
+        invoice.addProduct(new DairyProduct("Kozi Serek", new BigDecimal("10")), 3);
+        // 1000x pinezka - price: 10
+        invoice.addProduct(new OtherProduct("Pinezka", new BigDecimal("0.01")), 1000);
+        Assert.assertThat(new BigDecimal("50"), Matchers.comparesEqualTo(invoice.getSubtotal()));
+    }
+
+    @Test
+    public void testInvoiceHasPropoerTotalWithQuantityMoreThanOne() {
+        // 2x chleb - price with tax: 10
+        invoice.addProduct(new TaxFreeProduct("Chleb", new BigDecimal("5")), 2);
+        // 3x chedar - price with tax: 32.40
+        invoice.addProduct(new DairyProduct("Chedar", new BigDecimal("10")), 3);
+        // 1000x pinezka - price with tax: 12.30
+        invoice.addProduct(new OtherProduct("Pinezka", new BigDecimal("0.01")), 1000);
+        Assert.assertThat(new BigDecimal("54.70"), Matchers.comparesEqualTo(invoice.getTotal()));
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void testInvoiceWithZeroQuantity() {
+        invoice.addProduct(new TaxFreeProduct("Tablet", new BigDecimal("1678")), 0);
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void testInvoiceWithNegativeQuantity() {
+        invoice.addProduct(new DairyProduct("Zsiadle mleko", new BigDecimal("5.55")), -1);
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void testAddingNullProduct() {
+        invoice.addProduct(null);
+    }
+}
diff --git a/src/test/java/pl/edu/agh/mwo/invoice/product/ProductTest.java b/src/test/java/pl/edu/agh/mwo/invoice/product/ProductTest.java
new file mode 100644
index 000000000..9b3c5bd6e
--- /dev/null
+++ b/src/test/java/pl/edu/agh/mwo/invoice/product/ProductTest.java
@@ -0,0 +1,57 @@
+package pl.edu.agh.mwo.invoice.product;
+
+import java.math.BigDecimal;
+
+import org.hamcrest.Matchers;
+import org.junit.Assert;
+import org.junit.Test;
+
+import pl.edu.agh.mwo.invoice.product.Product;
+
+public class ProductTest {
+    @Test
+    public void testProductNameIsCorrect() {
+        Product product = new OtherProduct("buty", new BigDecimal("100.0"));
+        Assert.assertEquals("buty", product.getName());
+    }
+
+    @Test
+    public void testProductPriceAndTaxWithDefaultTax() {
+        Product product = new OtherProduct("Ogorki", new BigDecimal("100.0"));
+        Assert.assertThat(new BigDecimal("100"), Matchers.comparesEqualTo(product.getPrice()));
+        Assert.assertThat(new BigDecimal("0.23"), Matchers.comparesEqualTo(product.getTaxPercent()));
+    }
+
+    @Test
+    public void testProductPriceAndTaxWithDairyProduct() {
+        Product product = new DairyProduct("Szarlotka", new BigDecimal("100.0"));
+        Assert.assertThat(new BigDecimal("100"), Matchers.comparesEqualTo(product.getPrice()));
+        Assert.assertThat(new BigDecimal("0.08"), Matchers.comparesEqualTo(product.getTaxPercent()));
+    }
+
+    @Test
+    public void testPriceWithTax() {
+        Product product = new DairyProduct("Oscypek", new BigDecimal("100.0"));
+        Assert.assertThat(new BigDecimal("108"), Matchers.comparesEqualTo(product.getPriceWithTax()));
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void testProductWithNullName() {
+        new OtherProduct(null, new BigDecimal("100.0"));
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void testProductWithEmptyName() {
+        new TaxFreeProduct("", new BigDecimal("100.0"));
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void testProductWithNullPrice() {
+        new DairyProduct("Banany", null);
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void testProductWithNegativePrice() {
+        new TaxFreeProduct("Mandarynki", new BigDecimal("-1.00"));
+    }
+}