splitwise

Splitwise System (LLD)

Problem Statement

Design and implement a Splitwise System that allows users to split expenses among groups and individuals. The system should handle expense tracking, balance calculations, and settlement of debts between users.

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Requirements

1. User Management:

   • Create and manage user profiles
   • Track user balances
   • Handle user relationships

2. Group Management:

   • Create and manage groups
   • Add/remove members
   • Track group expenses

3. Expense Management:

   • Add expenses to groups or individuals
   • Support different split types (EQUAL, EXACT, PERCENTAGE)
   • Track expense history

4. Balance Management:

   • Calculate balances between users
   • Track who owes whom
   • Handle settlements

5. Transaction Management:

   • Record transactions
   • Track payment status
   • Generate balance reports

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Core Entities

1. SplitwiseService

• Fields: List users, List groups, List expenses
• Methods:
  • addUser()
  • createGroup()
  • addExpense()
  • getBalance()
  • settleExpense()

2. User

• Fields: String id, String name, String email, Map<User, Double> balances
• Methods:
  • updateProfile()
  • getBalance()
  • addBalance()
  • subtractBalance()

3. Group

• Fields: String id, String name, List members, List expenses
• Methods:
  • addMember()
  • removeMember()
  • addExpense()
  • getBalances()

4. Expense

• Fields: String id, String description, double amount, User paidBy, List paidFor, SplitType splitType
• Methods:
  • calculateSplits()
  • getAmount()
  • getPaidBy()

5. Transaction

• Fields: String id, User from, User to, double amount
• Methods:
  • execute()
  • getStatus()

6. SplitType (Enum)

• Values: EQUAL, EXACT, PERCENTAGE

UML Class Diagram

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Example Usage

SplitwiseService service = new SplitwiseService();

// Create users
User user1 = service.addUser("John", "john@example.com");
User user2 = service.addUser("Jane", "jane@example.com");

// Create a group
Group group = service.createGroup("Trip to Paris");
group.addMember(user1);
group.addMember(user2);

// Add an expense
Expense expense = service.addExpense(
    "Dinner", 
    100.0, 
    user1, 
    Arrays.asList(user1, user2), 
    SplitType.EQUAL
);

// Get balances
double balance = service.getBalance(user1, user2);

// Settle expense
service.settleExpense(user2, user1, 50.0);

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Demo

See SplitwiseDemo.java for a sample usage and simulation of the Splitwise system.

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Extending the Framework

• Add expense categories: Categorize expenses (food, travel, etc.)
• Add recurring expenses: Support for regular payments
• Add expense comments: Allow users to add notes to expenses
• Add expense attachments: Support for receipts and documents
• Add payment integration: Integrate with payment gateways
• Add notification system: Send reminders for pending payments

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Design Patterns Used

• Singleton Pattern: For the Splitwise service instance
• Factory Pattern: For creating different types of splits
• Strategy Pattern: For different expense splitting strategies
• Observer Pattern: For balance updates and notifications

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Exception Handling

• InvalidAmountException: Thrown when expense amount is invalid
• InvalidSplitException: Thrown when split details are invalid
• UserNotFoundException: Thrown when user is not found
• GroupNotFoundException: Thrown when group is not found
• InsufficientBalanceException: Thrown when user has insufficient balance

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