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Ilkka Seppälä
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money/README.md
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title: "Money Pattern in Java: Encapsulating Monetary Values with Currency Consistency"
shortTitle: Money
description: "Learn how the Money design pattern in Java ensures currency safety, precision handling, and maintainable financial operations. Explore examples, applicability, and benefits of the pattern."
category: Behavioral
category: Structural
language: en
tag:
- Encapsulation
- Business
- Domain
- Encapsulation
- Immutable
---
## Also known as
@@ -14,147 +17,115 @@ tag:
## Intent of Money Design Pattern
The Money design pattern provides a robust way to encapsulate monetary values and their associated currencies. It ensures precise calculations, currency consistency, and maintainability of financial logic in Java applications.
Encapsulate monetary values and their associated currency in a domain-specific object.
## Detailed Explanation of Money Pattern with Real-World Examples
### Real-world example
Real-world example
> Imagine an e-commerce platform where customers shop in their local currencies. The platform needs to calculate order totals, taxes, and discounts accurately while handling multiple currencies seamlessly.
> Imagine an online gift card system, where each gift card holds a specific balance in a particular currency. Instead of just using a floating-point value for the balance, the system uses a Money object to precisely track the amount and currency. Whenever someone uses the gift card, it updates the balance with accurate calculations that avoid floating-point rounding errors, ensuring the domain logic stays consistent and accurate.
In this example:
- Each monetary value (like a product price or tax amount) is encapsulated in a `Money` object.
- The `Money` class ensures that only values in the same currency are combined and supports safe currency conversion for global operations.
### In plain words
In plain words
> The Money pattern encapsulates both an amount and its currency, ensuring financial operations are precise, consistent, and maintainable.
### Wikipedia says
Wikipedia says
> "The Money design pattern encapsulates a monetary value and its currency, allowing for safe arithmetic operations and conversions while preserving accuracy and consistency in financial calculations."
> The Money design pattern encapsulates a monetary value and its currency, allowing for safe arithmetic operations and conversions while preserving accuracy and consistency in financial calculations.
Mind map
![Money Pattern Mind Map](./etc/money-mind-map.png)
Flowchart
![Money Pattern Flowchart](./etc/money-flowchart.png)
## Programmatic Example of Money Pattern in Java
### Money Class
In this example, we're creating a `Money` class to demonstrate how monetary values can be encapsulated along with their currency. This approach helps avoid floating-point inaccuracies, ensures arithmetic operations are handled consistently, and provides a clear domain-centric way of working with money.
```java
/**
* Represents a monetary value with an associated currency.
* Provides operations for basic arithmetic (addition, subtraction, multiplication),
* as well as currency conversion while ensuring proper rounding.
*/
@AllArgsConstructor
@Getter
public class Money {
private @Getter double amount;
private @Getter String currency;
private double amount;
private String currency;
public Money(double amnt, String curr) {
this.amount = amnt;
this.currency = curr;
}
private double roundToTwoDecimals(double value) {
return Math.round(value * 100.0) / 100.0;
}
public void addMoney(Money moneyToBeAdded) throws CannotAddTwoCurrienciesException {
if (!moneyToBeAdded.getCurrency().equals(this.currency)) {
throw new CannotAddTwoCurrienciesException("You are trying to add two different currencies");
public Money(double amnt, String curr) {
this.amount = amnt;
this.currency = curr;
}
this.amount = roundToTwoDecimals(this.amount + moneyToBeAdded.getAmount());
}
public void subtractMoney(Money moneyToBeSubtracted) throws CannotSubtractException {
if (!moneyToBeSubtracted.getCurrency().equals(this.currency)) {
throw new CannotSubtractException("You are trying to subtract two different currencies");
} else if (moneyToBeSubtracted.getAmount() > this.amount) {
throw new CannotSubtractException("The amount you are trying to subtract is larger than the amount you have");
private double roundToTwoDecimals(double value) {
return Math.round(value * 100.0) / 100.0;
}
this.amount = roundToTwoDecimals(this.amount - moneyToBeSubtracted.getAmount());
}
public void multiply(int factor) {
if (factor < 0) {
throw new IllegalArgumentException("Factor must be non-negative");
public void addMoney(Money moneyToBeAdded) throws CannotAddTwoCurrienciesException {
if (!moneyToBeAdded.getCurrency().equals(this.currency)) {
throw new CannotAddTwoCurrienciesException("You are trying to add two different currencies");
}
this.amount = roundToTwoDecimals(this.amount + moneyToBeAdded.getAmount());
}
this.amount = roundToTwoDecimals(this.amount * factor);
}
public void exchangeCurrency(String currencyToChangeTo, double exchangeRate) {
if (exchangeRate < 0) {
throw new IllegalArgumentException("Exchange rate must be non-negative");
public void subtractMoney(Money moneyToBeSubtracted) throws CannotSubtractException {
if (!moneyToBeSubtracted.getCurrency().equals(this.currency)) {
throw new CannotSubtractException("You are trying to subtract two different currencies");
} else if (moneyToBeSubtracted.getAmount() > this.amount) {
throw new CannotSubtractException("The amount you are trying to subtract is larger than the amount you have");
}
this.amount = roundToTwoDecimals(this.amount - moneyToBeSubtracted.getAmount());
}
public void multiply(int factor) {
if (factor < 0) {
throw new IllegalArgumentException("Factor must be non-negative");
}
this.amount = roundToTwoDecimals(this.amount * factor);
}
public void exchangeCurrency(String currencyToChangeTo, double exchangeRate) {
if (exchangeRate < 0) {
throw new IllegalArgumentException("Exchange rate must be non-negative");
}
this.amount = roundToTwoDecimals(this.amount * exchangeRate);
this.currency = currencyToChangeTo;
}
this.amount = roundToTwoDecimals(this.amount * exchangeRate);
this.currency = currencyToChangeTo;
}
}
```
By encapsulating all money-related logic in a single class, we reduce the risk of mixing different currencies, improve clarity of the codebase, and facilitate future modifications such as adding new currencies or refining rounding rules. This pattern ultimately strengthens the domain model by treating money as a distinct concept rather than just another numeric value.
## When to Use the Money Pattern
The Money pattern should be used in scenarios where:
* When financial calculations or money manipulations are part of the business logic
* When precise handling of currency amounts is required to avoid floating-point inaccuracies
* When domain-driven design principles and strong typing are desired
1. **Currency-safe arithmetic operations**
To ensure that arithmetic operations like addition, subtraction, and multiplication are performed only between amounts in the same currency, preventing inconsistencies or errors in calculations.
## Real-World Applications of Monad Pattern in Java
2. **Accurate rounding for financial calculations**
Precise rounding to two decimal places is critical to maintain accuracy and consistency in financial systems.
* JSR 354 (Java Money and Currency) library in Java
* Custom domain models in e-commerce and accounting systems
3. **Consistent currency conversion**
When handling international transactions or displaying monetary values in different currencies, the Money pattern facilitates easy and reliable conversion using exchange rates.
4. **Encapsulation of monetary logic**
By encapsulating all monetary operations within a dedicated class, the Money pattern improves maintainability and reduces the likelihood of errors.
5. **Preventing errors in financial operations**
Strict validation ensures that operations like subtraction or multiplication are only performed when conditions are met, safeguarding against misuse or logical errors.
6. **Handling diverse scenarios in financial systems**
Useful in complex systems like e-commerce, banking, and payroll applications where precise and consistent monetary value handling is crucial.
---
## Benefits and Trade-offs of Money Pattern
### Benefits
1. **Precision and Accuracy**
The Money pattern ensures precise handling of monetary values, reducing the risk of rounding errors.
Benefits
2. **Encapsulation of Business Logic**
By encapsulating monetary operations, the pattern enhances maintainability and reduces redundancy in financial systems.
* Provides a single, type-safe representation of monetary amounts and currency
* Encourages encapsulation of related operations such as addition, subtraction, and formatting
* Avoids floating-point errors by using integers or specialized decimal libraries
3. **Currency Safety**
It ensures operations are performed only between amounts of the same currency, avoiding logical errors.
Trade-offs
4. **Improved Readability**
By abstracting monetary logic into a dedicated class, the code becomes easier to read and maintain.
5. **Ease of Extension**
Adding new operations, handling different currencies, or incorporating additional business rules is straightforward.
### Trade-offs
1. **Increased Complexity**
Introducing a dedicated `Money` class can add some overhead, especially for small or simple projects.
2. **Potential for Misuse**
Without proper validation and handling, incorrect usage of the Money pattern may introduce subtle bugs.
3. **Performance Overhead**
Precision and encapsulation might slightly affect performance in systems with extremely high transaction volumes.
---
* Requires additional classes and infrastructure to handle currency conversions and formatting
* Might introduce performance overhead when performing large numbers of money operations
## Related Design Patterns
1. **Value Object**
Money is a classic example of the Value Object pattern, where objects are immutable and define equality based on their value.
Link:https://martinfowler.com/bliki/ValueObject.html
2. **Factory Method**
Factories can be employed to handle creation logic, such as applying default exchange rates or rounding rules.
Link:https://www.geeksforgeeks.org/factory-method-for-designing-pattern/
---
* [Value Object](https://java-design-patterns.com/patterns/value-object/): Money is typically a prime example of a domain-driven design value object.
## References and Credits
- [Patterns of Enterprise Application Architecture](https://martinfowler.com/eaaCatalog/money.html) by Martin Fowler
- [Design Patterns: Elements of Reusable Object-Oriented Software](https://amzn.to/3w0pvKI)
* [Domain-Driven Design: Tackling Complexity in the Heart of Software](https://amzn.to/3wlDrze)
* [Implementing Domain-Driven Design](https://amzn.to/4dmBjrB)
* [Patterns of Enterprise Application Architecture](https://amzn.to/3WfKBPR)
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money/src/main/java/com/iluwatar/Money.java
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*/
package com.iluwatar;
import lombok.AllArgsConstructor;
import lombok.Getter;
/**
@@ -31,21 +32,11 @@ import lombok.Getter;
* (addition, subtraction, multiplication), as well as currency conversion while ensuring proper
* rounding.
*/
@AllArgsConstructor
@Getter
public class Money {
private @Getter double amount;
private @Getter String currency;
/**
* Constructs a Money object with the specified amount and currency.
*
* @param amnt the amount of money (as a double).
* @param curr the currency code (e.g., "USD", "EUR").
*/
public Money(double amnt, String curr) {
this.amount = amnt;
this.currency = curr;
}
private double amount;
private String currency;
/**
* Rounds the given value to two decimal places.
money/src/test/java/com/iluwater/money/MoneyTest.java
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@@ -61,9 +61,7 @@ class MoneyTest {
assertThrows(
CannotAddTwoCurrienciesException.class,
() -> {
money1.addMoney(money2);
});
() -> money1.addMoney(money2));
}
@Test
@@ -85,9 +83,7 @@ class MoneyTest {
assertThrows(
CannotSubtractException.class,
() -> {
money1.subtractMoney(money2);
});
() -> money1.subtractMoney(money2));
}
@Test
@@ -98,9 +94,7 @@ class MoneyTest {
assertThrows(
CannotSubtractException.class,
() -> {
money1.subtractMoney(money2);
});
() -> money1.subtractMoney(money2));
}
@Test
@@ -120,9 +114,7 @@ class MoneyTest {
assertThrows(
IllegalArgumentException.class,
() -> {
money.multiply(-2);
});
() -> money.multiply(-2));
}
@Test
@@ -143,17 +135,13 @@ class MoneyTest {
assertThrows(
IllegalArgumentException.class,
() -> {
money.exchangeCurrency("EUR", -0.85);
});
() -> money.exchangeCurrency("EUR", -0.85));
}
@Test
void testAppExecution() {
assertDoesNotThrow(
() -> {
App.main(new String[] {});
},
() -> App.main(new String[] {}),
"App execution should not throw any exceptions");
}
}