Expense Sharing Application (Backend)

A simplified backend implementation of an expense sharing application (similar to Splitwise) built using Spring Boot. This project focuses on clean design, correct business logic, and balance simplification.

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Video Link( API Testing on Postman)

Watch the video

Features

• Create users

• Create groups and manage group members

• Add shared expenses

• Supported split types:

  • Equal split
  • Exact amount split
  • Percentage split

• Track balances (who owes whom)

• Simplified net balances

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Tech Stack

• Java 17
• Spring Boot
• Spring Web
• Spring Data JPA
• H2 In-Memory Database
• Lombok

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High-Level Architecture

The application follows a layered architecture:

• Controller Layer – Handles REST APIs
• Service Layer – Contains business logic
• Repository Layer – Database access using JPA
• Model Layer – Domain entities

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System Design (Mermaid Diagram)

classDiagram
    direction LR

    class User {
        Long id
        String name
        String email
    }

    class Group {
        Long id
        String name
    }

    class Expense {
        Long id
        String description
        double amount
        SplitType splitType
    }

    class ExpenseSplit {
        Long id
        double amount
    }

    class Balance {
        Long id
        double amount
    }

    User "many" -- "many" Group : member_of
    Group "one" -- "many" Expense : has
    Expense "one" -- "many" ExpenseSplit : split_into
    ExpenseSplit "many" -- "one" User : owes
    Balance "many" -- "one" User : debtor
    Balance "many" -- "one" User : creditor

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Core Domain Model

• User Represents an individual using the application.

• Group Represents a collection of users sharing expenses.

• Expense Represents a transaction paid by one user for a group.

• ExpenseSplit Stores how much each user owes for a specific expense.

• Balance Stores net dues between users in simplified form.

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Split Strategies

Equal Split

The total expense amount is divided equally among selected users.

Exact Split

Each user pays an exact amount specified in the request.

Percentage Split

Each user pays a percentage of the total expense.

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Balance Simplification

Balances are stored in net form to avoid redundant entries.

Example:

• A owes B ₹500
• B owes A ₹200

Final balance:

• A owes B ₹300

Only one directional entry is stored.

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API Overview

User APIs

• POST /users – Create user

Group APIs

• POST /groups – Create group
• POST /groups/{groupId}/users – Add users to group

Expense APIs

• POST /expenses – Add an expense

Balance APIs

• GET /balances – View all balances

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Sample Expense Request

{
  "groupId": 1,
  "paidBy": 1,
  "amount": 600,
  "description": "Hotel",
  "splitType": "EQUAL",
  "splits": {
    "2": 0,
    "3": 0
  }
}

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Running the Application

1. Clone the repository
2. Run the Spring Boot application
3. Test APIs using Postman
4. H2 Console is available at /h2-console

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Notes

• Uses H2 in-memory database for simplicity
• Designed for clarity and easy extension
• Can be enhanced with authentication, settlement APIs, and persistent databases

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