Object-Oriented Design• Patterns: Strategy, Factory

Splitwise (Expense Sharing App)

Hard

Problem Summary

Design Splitwise to add expenses, share balances among friends, and optimize debt simplification.

Functional Scope

Users can log group expenses split equally, unequally, or by percentages.
Automatically compute balances between users.
Provide a debt-simplification algorithm (minimize transactions between members).

Entity-Relationship (ER) Schema

Group [1] <---> [*] User
Group [1] <---> [*] Expense
Expense [1] <---> [*] Split
User [1] <---> [1] BalanceSheet

Design Approach

Compute net balance sheets for all users. Run a greedy optimization algorithm (using a Min Heap for debtors and a Max Heap for creditors) to iteratively settle the largest debts, minimizing the total number of transactions.

Core Classes & Models

User (Name, Email, Balance Sheet)Expense (Base class for EqualExpense, UnequalExpense)Split (Maps User to split amount)Group (Tracks users and expenses list)

Code Blueprint

public abstract class Expense {
    private double totalAmount;
    public abstract boolean validate();
}