Expense Tracker

Introduction

Build an expense tracker with Spring Boot — REST API for adding expenses, categorising by tag, monthly summaries and CSV export. Covers CRUD, JPA, validation and reporting with Streams.

Informative example

Monthly summary endpoint:

@GetMapping("/expenses/summary")
public Map<String, BigDecimal> monthlySummary(
    @RequestParam YearMonth month
) {
    return repo.findByDateBetween(month.atDay(1), month.atEndOfMonth())
        .stream()
        .collect(groupingBy(Expense::category,
            reducing(ZERO, Expense::amount, BigDecimal::add)));
}

Best practices

• Use records for Expense DTOs.
• Add pagination on the list endpoint from day one.
• Write MockMvc tests for every endpoint.

Purpose of this lesson

Spring Boot REST API for tracking expenses with categories and monthly reports.

Step-by-step explanation

1. Understand the core idea behind Expense Tracker.
2. Walk through the runnable example and tweak it in the playground.
3. Apply the pattern in a small Spring Boot or CLI exercise of your own.
4. Re-read the common mistakes and interview Q&A to lock the concept in.

Interactive workflow diagram

Useful template

@Entity
public class Expense {
  @Id @GeneratedValue Long id;
  BigDecimal amount;
  String category;
  LocalDate date;
}

Debugging tips

• Read the full stack trace — Java's exception messages name the offending class and line.
• Reproduce in the smallest possible main() method before fixing in the real app.
• Use IntelliJ's debugger breakpoints and 'Evaluate Expression' rather than scattering System.out.

Optimization strategies

• Profile before optimizing — JFR (Java Flight Recorder) and async-profiler reveal real hotspots.
• Prefer immutable data and stream pipelines over hand-rolled loops when readability matters.
• Reach for the right JDK collection (ArrayList vs LinkedList vs ArrayDeque) before writing custom data structures.

Enterprise example

Teams at Netflix, Uber and Goldman Sachs apply Expense Tracker daily — usually wrapped behind Spring Boot services with observability hooks (Micrometer + OpenTelemetry).

Interview questions & answers

Summary

In this lesson you learned Expense Tracker — the concept, syntax, a runnable example, and the production pitfalls to avoid. Apply it in the playground before moving on.