Add examples supporting BAEL-2273 article

ddd/src/main/java/com/baeldung/ddd/order/doubledispatch/AmountBasedDiscountPolicy.java

@@ -0,0 +1,15 @@
+package com.baeldung.ddd.order.doubledispatch;
+
+import org.joda.money.CurrencyUnit;
+import org.joda.money.Money;
+
+public class AmountBasedDiscountPolicy implements DiscountPolicy {
+    @Override
+    public double discount(Order order) {
+        if (order.totalCost()
+            .isGreaterThan(Money.of(CurrencyUnit.USD, 500.00))) {
+            return 0.10;
+        } else
+            return 0;
+    }
+}

ddd/src/main/java/com/baeldung/ddd/order/doubledispatch/DiscountPolicy.java

@@ -0,0 +1,5 @@
+package com.baeldung.ddd.order.doubledispatch;
+
+public interface DiscountPolicy {
+    double discount(Order order);
+}

ddd/src/main/java/com/baeldung/ddd/order/doubledispatch/FlatDiscountPolicy.java

@@ -0,0 +1,8 @@
+package com.baeldung.ddd.order.doubledispatch;
+
+public class FlatDiscountPolicy implements DiscountPolicy {
+    @Override
+    public double discount(Order order) {
+        return 0.01;
+    }
+}

ddd/src/main/java/com/baeldung/ddd/order/doubledispatch/Order.java

@@ -0,0 +1,29 @@
+package com.baeldung.ddd.order.doubledispatch;
+
+import java.math.RoundingMode;
+import java.util.List;
+
+import org.joda.money.Money;
+
+import com.baeldung.ddd.order.OrderLine;
+import com.baeldung.ddd.order.doubledispatch.visitor.OrderVisitor;
+import com.baeldung.ddd.order.doubledispatch.visitor.Visitable;
+
+public class Order extends com.baeldung.ddd.order.Order implements Visitable<OrderVisitor> {
+    public Order(List<OrderLine> orderLines) {
+        super(orderLines);        
+    }
+
+    public Money totalCost(SpecialDiscountPolicy discountPolicy) {
+        return totalCost().multipliedBy(1 - applyDiscountPolicy(discountPolicy), RoundingMode.HALF_UP);
+    }
+
+    protected double applyDiscountPolicy(SpecialDiscountPolicy discountPolicy) {
+        return discountPolicy.discount(this);
+    }
+    
+    @Override
+    public void accept(OrderVisitor visitor) {
+        visitor.visit(this);        
+    }
+}

ddd/src/main/java/com/baeldung/ddd/order/doubledispatch/SpecialDiscountPolicy.java

@@ -0,0 +1,5 @@
+package com.baeldung.ddd.order.doubledispatch;
+
+public interface SpecialDiscountPolicy extends DiscountPolicy {
+    double discount(SpecialOrder order);
+}

ddd/src/main/java/com/baeldung/ddd/order/doubledispatch/SpecialOrder.java

@@ -0,0 +1,36 @@
+package com.baeldung.ddd.order.doubledispatch;
+
+import java.util.List;
+
+import com.baeldung.ddd.order.OrderLine;
+import com.baeldung.ddd.order.doubledispatch.visitor.OrderVisitor;
+
+public class SpecialOrder extends Order {
+
+    private boolean eligibleForExtraDiscount;
+
+    public SpecialOrder(List<OrderLine> orderLines) {
+        super(orderLines);
+        this.eligibleForExtraDiscount = false;
+    }
+
+    public SpecialOrder(List<OrderLine> orderLines, boolean eligibleForSpecialDiscount) {
+        super(orderLines);
+        this.eligibleForExtraDiscount = eligibleForSpecialDiscount;
+    }
+
+    public boolean isEligibleForExtraDiscount() {
+        return eligibleForExtraDiscount;
+    }
+
+    @Override
+    protected double applyDiscountPolicy(SpecialDiscountPolicy discountPolicy) {
+        return discountPolicy.discount(this);
+    }
+
+    @Override
+    public void accept(OrderVisitor visitor) {
+        visitor.visit(this);
+    }
+
+}

ddd/src/main/java/com/baeldung/ddd/order/doubledispatch/visitor/HtmlOrderViewCreator.java

@@ -0,0 +1,24 @@
+package com.baeldung.ddd.order.doubledispatch.visitor;
+
+import com.baeldung.ddd.order.doubledispatch.Order;
+import com.baeldung.ddd.order.doubledispatch.SpecialOrder;
+
+public class HtmlOrderViewCreator implements OrderVisitor {
+    
+    private String html;
+    
+    public String getHtml() {
+        return html;
+    }
+
+    @Override
+    public void visit(Order order) {
+        html = String.format("<p>Regular order total cost: %s</p>", order.totalCost());
+    }
+
+    @Override
+    public void visit(SpecialOrder order) {
+        html = String.format("<h1>Special Order</h1><p>total cost: %s</p>", order.totalCost());
+    }
+
+}

ddd/src/main/java/com/baeldung/ddd/order/doubledispatch/visitor/OrderVisitor.java

@@ -0,0 +1,9 @@
+package com.baeldung.ddd.order.doubledispatch.visitor;
+
+import com.baeldung.ddd.order.doubledispatch.Order;
+import com.baeldung.ddd.order.doubledispatch.SpecialOrder;
+
+public interface OrderVisitor {
+    void visit(Order order);
+    void visit(SpecialOrder order);
+}

ddd/src/main/java/com/baeldung/ddd/order/doubledispatch/visitor/Visitable.java

@@ -0,0 +1,5 @@
+package com.baeldung.ddd.order.doubledispatch.visitor;
+
+public interface Visitable<V> {
+    void accept(V visitor);
+}

ddd/src/main/java/com/baeldung/ddd/order/jpa/JpaOrder.java

@@ -92,7 +92,7 @@ class JpaOrder {
     }
 
     void removeLineItem(int line) {
-        JpaOrderLine removedLine = orderLines.remove(line);
+        orderLines.remove(line);
     }
 
     void setCurrencyUnit(String currencyUnit) {

ddd/src/main/java/com/baeldung/ddd/order/jpa/JpaProduct.java

@@ -15,7 +15,7 @@ class JpaProduct {
     public JpaProduct(BigDecimal price, String currencyUnit) {
         super();
         this.price = price;
-        currencyUnit = currencyUnit;
+        this.currencyUnit = currencyUnit;
     }
 
     @Override

ddd/src/test/java/com/baeldung/ddd/order/OrderFixtureUtils.java

@@ -0,0 +1,17 @@
+package com.baeldung.ddd.order;
+
+import java.util.Arrays;
+import java.util.List;
+
+import org.joda.money.CurrencyUnit;
+import org.joda.money.Money;
+
+public class OrderFixtureUtils {
+    public static List<OrderLine> anyOrderLines() {
+        return Arrays.asList(new OrderLine(new Product(Money.of(CurrencyUnit.USD, 100)), 1));
+    }
+
+    public static List<OrderLine> orderLineItemsWorthNDollars(int totalCost) {
+        return Arrays.asList(new OrderLine(new Product(Money.of(CurrencyUnit.USD, totalCost)), 1));
+    }
+}

ddd/src/test/java/com/baeldung/ddd/order/doubledispatch/DoubleDispatchDiscountPolicyUnitTest.java

@@ -0,0 +1,77 @@
+package com.baeldung.ddd.order.doubledispatch;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.joda.money.CurrencyUnit;
+import org.joda.money.Money;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+import com.baeldung.ddd.order.OrderFixtureUtils;
+
+public class DoubleDispatchDiscountPolicyUnitTest {
+    // @formatter:off
+    @DisplayName(
+        "given regular order with items worth $100 total, " +
+        "when apply 10% discount policy, " +
+        "then cost after discount is $90"
+        )
+    // @formatter:on
+    @Test
+    void test() throws Exception {
+        // given
+        Order order = new Order(OrderFixtureUtils.orderLineItemsWorthNDollars(100));
+        SpecialDiscountPolicy discountPolicy = new SpecialDiscountPolicy() {
+
+            @Override
+            public double discount(Order order) {
+                return 0.10;
+            }
+
+            @Override
+            public double discount(SpecialOrder order) {
+                return 0;
+            }
+        };
+
+        // when
+        Money totalCostAfterDiscount = order.totalCost(discountPolicy);
+
+        // then
+        assertThat(totalCostAfterDiscount).isEqualTo(Money.of(CurrencyUnit.USD, 90));
+    }
+
+    // @formatter:off
+    @DisplayName(
+        "given special order eligible for extra discount with items worth $100 total, " +
+        "when apply 20% discount policy for extra discount orders, " +
+        "then cost after discount is $80"
+        )
+    // @formatter:on
+    @Test
+    void test1() throws Exception {
+        // given
+        boolean eligibleForExtraDiscount = true;
+        Order order = new SpecialOrder(OrderFixtureUtils.orderLineItemsWorthNDollars(100), eligibleForExtraDiscount);
+        SpecialDiscountPolicy discountPolicy = new SpecialDiscountPolicy() {
+
+            @Override
+            public double discount(Order order) {
+                return 0;
+            }
+
+            @Override
+            public double discount(SpecialOrder order) {
+                if (order.isEligibleForExtraDiscount())
+                    return 0.20;
+                return 0.10;
+            }
+        };
+
+        // when
+        Money totalCostAfterDiscount = order.totalCost(discountPolicy);
+
+        // then
+        assertThat(totalCostAfterDiscount).isEqualTo(Money.of(CurrencyUnit.USD, 80.00));
+    }
+}

ddd/src/test/java/com/baeldung/ddd/order/doubledispatch/HtmlOrderViewCreatorUnitTest.java

@@ -0,0 +1,43 @@
+package com.baeldung.ddd.order.doubledispatch;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.util.Arrays;
+import java.util.List;
+
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+import com.baeldung.ddd.order.doubledispatch.Order;
+import com.baeldung.ddd.order.OrderFixtureUtils;
+import com.baeldung.ddd.order.OrderLine;
+import com.baeldung.ddd.order.doubledispatch.visitor.HtmlOrderViewCreator;
+
+public class HtmlOrderViewCreatorUnitTest {
+    // @formatter:off 
+    @DisplayName(
+            "given collection of regular and special orders, " +
+            "when create HTML view using visitor for each order, " +
+            "then the dedicated view is created for each order"   
+        )
+    // @formatter:on
+    @Test
+    void test() throws Exception {
+        // given
+        List<OrderLine> anyOrderLines = OrderFixtureUtils.anyOrderLines();
+        List<Order> orders = Arrays.asList(new Order(anyOrderLines), new SpecialOrder(anyOrderLines));
+        HtmlOrderViewCreator htmlOrderViewCreator = new HtmlOrderViewCreator();
+
+        // when
+        orders.get(0)
+            .accept(htmlOrderViewCreator);
+        String regularOrderHtml = htmlOrderViewCreator.getHtml();
+        orders.get(1)
+            .accept(htmlOrderViewCreator);
+        String specialOrderHtml = htmlOrderViewCreator.getHtml();
+
+        // then
+        assertThat(regularOrderHtml).containsPattern("<p>Regular order total cost: .*</p>");
+        assertThat(specialOrderHtml).containsPattern("<h1>Special Order</h1><p>total cost: .*</p>");
+    }
+}

ddd/src/test/java/com/baeldung/ddd/order/doubledispatch/MethodOverloadExampleUnitTest.java

@@ -0,0 +1,50 @@
+package com.baeldung.ddd.order.doubledispatch;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.util.List;
+
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+import com.baeldung.ddd.order.doubledispatch.Order;
+import com.baeldung.ddd.order.OrderFixtureUtils;
+import com.baeldung.ddd.order.OrderLine;
+import com.baeldung.ddd.order.doubledispatch.SpecialDiscountPolicy;
+import com.baeldung.ddd.order.doubledispatch.SpecialOrder;
+
+public class MethodOverloadExampleUnitTest {
+// @formatter:off
+@DisplayName(
+    "given discount policy accepting special orders, " +
+    "when apply the policy on special order declared as regular order, " +
+    "then regular discount method is used"
+    )
+// @formatter:on    
+    @Test
+    void test() throws Exception {
+        // given
+        SpecialDiscountPolicy specialPolicy = new SpecialDiscountPolicy() {
+            @Override
+            public double discount(Order order) {
+                return 0.01;
+            }
+
+            @Override
+            public double discount(SpecialOrder order) {
+                return 0.10;
+            }
+        };
+        Order specialOrder = new SpecialOrder(anyOrderLines());
+
+        // when
+        double discount = specialPolicy.discount(specialOrder);
+
+        // then
+        assertThat(discount).isEqualTo(0.01);
+    }
+
+    private List<OrderLine> anyOrderLines() {
+        return OrderFixtureUtils.anyOrderLines();
+    }
+}

ddd/src/test/java/com/baeldung/ddd/order/doubledispatch/SingleDispatchDiscountPolicyUnitTest.java

@@ -0,0 +1,37 @@
+package com.baeldung.ddd.order.doubledispatch;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+import com.baeldung.ddd.order.OrderFixtureUtils;
+
+public class SingleDispatchDiscountPolicyUnitTest {
+    // @formatter:off
+    @DisplayName(
+        "given two discount policies, " +
+        "when use these policies, " +
+        "then single dispatch chooses the implementation based on runtime type"
+        )
+    // @formatter:on    
+    @Test
+    void test() throws Exception {
+        // given
+        DiscountPolicy flatPolicy = new FlatDiscountPolicy();
+        DiscountPolicy amountPolicy = new AmountBasedDiscountPolicy();
+        Order orderWorth501Dollars = orderWorthNDollars(501);
+
+        // when
+        double flatDiscount = flatPolicy.discount(orderWorth501Dollars);
+        double amountDiscount = amountPolicy.discount(orderWorth501Dollars);
+
+        // then
+        assertThat(flatDiscount).isEqualTo(0.01);
+        assertThat(amountDiscount).isEqualTo(0.1);
+    }
+
+    private Order orderWorthNDollars(int totalCost) {
+        return new Order(OrderFixtureUtils.orderLineItemsWorthNDollars(totalCost));
+    }
+}