Open closed principle

src/main/java/com/example/Basket.java

@@ -5,7 +5,13 @@
 import java.util.List;
 
 public final class Basket {
+
     private final List<Item> items = new LinkedList<>();
+    private final Tax tax;
+
+    Basket(final Tax tax) {
+        this.tax = tax;
+    }
 
     public void add(final Item item) {
         items.add(item);
@@ -20,8 +26,6 @@ public BigDecimal subTotal() {
     }
 
     public BigDecimal total() {
-        return subTotal()
-                .multiply(BigDecimal.valueOf(1.15))
-                .setScale(2, BigDecimal.ROUND_DOWN);
+        return tax.applyTo(subTotal());
     }
 }

src/main/java/com/example/Tax.java

@@ -0,0 +1,7 @@
+package com.example;
+
+import java.math.BigDecimal;
+
+public interface Tax {
+    BigDecimal applyTo(final BigDecimal subTotal);
+}

src/main/java/com/example/Tax15PCRoundedDown.java

@@ -0,0 +1,13 @@
+package com.example;
+
+import java.math.BigDecimal;
+
+public final class Tax15PCRoundedDown implements Tax {
+    @Override
+    public BigDecimal applyTo(final BigDecimal subTotal) {
+        return subTotal
+                .multiply(BigDecimal.valueOf(1.15))
+                .setScale(2, BigDecimal.ROUND_DOWN);
+
+    }
+}

src/test/java/com.example/BasketTests.java

@@ -2,22 +2,25 @@
 
 import org.junit.Test;
 
-import java.math.BigDecimal;
-
+import static com.example.BigDecimalHelpers.bd;
 import static org.junit.Assert.assertEquals;
 
 public final class BasketTests {
 
+    private Basket givenBasket() {
+        return new Basket(new Tax15PCRoundedDown());
+    }
+
     @Test
     public void canAddToBasketAndGetSubtotal() {
-        final Basket basket = new Basket();
+        final Basket basket = givenBasket();
         basket.add(new Item(bd(1.23)));
         assertEquals(bd(1.23), basket.subTotal());
     }
 
     @Test
     public void canAddTwoItemsToBasketAndGetSubtotal() {
-        final Basket basket = new Basket();
+        final Basket basket = givenBasket();
         final double val = 1.23;
         basket.add(new Item(bd(val)));
         basket.add(new Item(bd(2.34)));
@@ -26,21 +29,15 @@ public void canAddTwoItemsToBasketAndGetSubtotal() {
 
     @Test
     public void canGetTotalWith15PercentTax() {
-        final Basket basket = new Basket();
+        final Basket basket = givenBasket();
         basket.add(new Item(bd(10)));
         assertEquals(bd(11.5), basket.total());
     }
 
     @Test
     public void canGetTotalWith15PercentTaxRoundedDown() {
-        final Basket basket = new Basket();
+        final Basket basket = givenBasket();
         basket.add(new Item(bd(1.10)));
         assertEquals(bd(1.26), basket.total());
     }
-
-    private BigDecimal bd(final double val) {
-        return BigDecimal
-                .valueOf(val)
-                .setScale(2, BigDecimal.ROUND_UNNECESSARY);
-    }
 }

src/test/java/com.example/BigDecimalHelpers.java

@@ -0,0 +1,12 @@
+package com.example;
+
+import java.math.BigDecimal;
+
+public final class BigDecimalHelpers {
+
+    public static BigDecimal bd(final double val) {
+        return BigDecimal
+                .valueOf(val)
+                .setScale(2, BigDecimal.ROUND_UNNECESSARY);
+    }
+}