Build a service that processes financial transactions (like payments) with the following requirements:

Core Requirements:

1. Idempotency: The same transaction should not be processed multiple times, even if the request is sent multiple times
2. Double-charging prevention: Ensure a customer is never charged twice for the same transaction
3. Transaction states: Track transaction lifecycle (PENDING → SUCCESS/FAILED)
4. Concurrent request handling: Handle multiple simultaneous requests safely

Features to implement:

• processTransaction(Transaction txn) - Process a new transaction
• getTransactionStatus(String txnId) - Get status of a transaction
• cancelTransaction(String txnId) - Cancel a pending transaction (optional)

Key considerations:

• Use idempotency keys (unique transaction IDs)
• Handle race conditions when same transaction is submitted concurrently
• Implement proper locking/synchronization
• Handle different failure scenarios

Go ahead and code your solution in Java

• Idempotency implementation
• Thread safety
• Design patterns used
• Edge case handling