Inside My PayPay Android Interview: Questions, Challenges, and Key Takeaways

Part 2 — PayPay Interview Series (Part 2): Coding, Security & System Design

Intro Hook

This is Part 2 of my PayPay interview journey. If you missed Part 1 (Experience & Deep Dive), you can read it here.

After a conversational first round, Round 2 was different. No warm-ups. No small talk. Just:

“We’ll discuss some code, do an algorithm challenge, talk security, and wrap up with a system design exercise.”

Game on.

Algorithm Challenge — 3Sum Problem

The problem: Find all unique triplets in an array that sum to zero.

Currency Converter App Discussion

We moved into app-specific scenarios:

• Exchange Rate Storage → One JSON string in a single DB row for simplicity.
• Screen Rotation → ViewModel to keep state intact.
• Invalid Inputs → Input filters to prevent multiple decimals or invalid characters.

Security Discussion

In fintech, this is where you can’t fake knowledge. I explained how I’d protect API tokens:

• Store build-time secrets in Gradle.
• Authenticate with JWT.
• Enforce HTTPS + certificate pinning.
• Encrypt sensitive data at rest.

System Design — QR Code Payments

Goal: Design a secure QR code payment feature.

• QR Code Content → Token only, no sensitive info.
• Token Generation → Random auto-increment ID to stop replay attacks.
• Offline Mode? Not for token generation — online required.
• Payload → Amount, merchant ID, token.
• Payment Confirmation → HTTPS for the request + WebSocket for real-time updates.

Takeaways

• Security first — in fintech, every feature is a security feature.
• Think out loud — interviewers want your reasoning, not just your solution.
• System design isn’t just backend — mobile devs need to design for scale too.

⬅ Missed Part 1? Read PayPay Interview — Experience & Deep Dive

Tags: Android Interview PayPay System Design Kotlin QR Code Payments Mobile Security Programming Interviews