[#3] Use test to express the design, as Kent Beck suggested in his book

src/main/java/org/thoughtworks/linesh/multicurrencymoney/Bank.java

@@ -0,0 +1,7 @@
+package org.thoughtworks.linesh.multicurrencymoney;
+
+public class Bank {
+    public Money reduced(Expression money, String currency) {
+        return Money.dollar(10);
+    }
+}

src/main/java/org/thoughtworks/linesh/multicurrencymoney/Expression.java

@@ -0,0 +1,5 @@
+package org.thoughtworks.linesh.multicurrencymoney;
+
+public interface Expression {
+    Expression plus(Money money);
+}

src/main/java/org/thoughtworks/linesh/multicurrencymoney/Money.java

@@ -1,6 +1,6 @@
 package org.thoughtworks.linesh.multicurrencymoney;
 
-public class Money {
+public class Money implements Expression {
     protected int amount;
     public Money(int amount, String currency) {
         this.amount = amount;
@@ -40,7 +40,9 @@ public Money times(int multiplier) {
         return new Money(this.amount * multiplier, currency);
     }
 
-    public Money plus(Money addend) {
-        return Money.dollar(this.amount + addend.getAmount());
+
+    @Override
+    public Expression plus(Money money) {
+        return Money.dollar(this.amount + money.getAmount());
     }
 }

src/test/java/org/thoughtworks/linesh/multicurrencymoney/MoneyTest.java

@@ -51,8 +51,11 @@ void should_test_different_class_equality() {
     class should_test_simple_addition {
         @Test
         void should_get_10_dollars_when_adding_5_dollars_with_5_dollars() {
-            Money sum = Money.dollar(5).plus(Money.dollar(5));
-            assertEquals(Money.dollar(10), sum);
+            Money fiveDollars = Money.dollar(5);
+            Expression sum = fiveDollars.plus(fiveDollars);
+            Bank bank = new Bank();
+            Money reduced = bank.reduced(sum, "USD");
+            assertEquals(Money.dollar(10), reduced);
         }
     }
 }