Structure: Access Control & Class Design

Encapsulation Structure: A well-encapsulated class has three layers of access control.
BankAccount
- accountNumber: String
- balance: Money
- status: AccountStatus
+ deposit(amount: Money): void
+ withdraw(amount: Money): Result
+ getBalance(): Money
- validateAmount(amount: Money): bool
- updateBalance(delta: Money): void
Access Levels:
First, Private Data (Red): balance, accountNumber, status cannot be accessed from outside. This prevents invalid state like negative balances or unauthorized status changes.
Second, Public Interface (Yellow): deposit(), withdraw(), getBalance() form the contract. These methods enforce business rules and maintain invariants.
Third, Private Helpers (Blue): validateAmount(), updateBalance() are implementation details. They can be refactored without affecting clients.
Method Implementation Pattern:
public withdraw(amount: Money): Result {
  // Validate input
  if (!this.validateAmount(amount)) {
    return Result.failure("Invalid amount")
  }
  // Check business rule
  if (this.balance < amount) {
    return Result.failure("Insufficient funds")
  }
  // Update state through private method
  this.updateBalance(-amount)
  return Result.success()
}
Interview Tip: When designing classes, start by identifying invariants (rules that must always be true), then make all fields private that participate in those invariants.

💡 Key Takeaways

✓Private fields prevent direct manipulation of internal state

✓Public methods form the contract and enforce business rules

✓Private helper methods encapsulate implementation details

✓Validation logic centralizes constraint checking

✓Access modifiers create explicit boundaries between interface and implementation

📌 Interview Tips

1BankAccount ensuring balance never goes negative

2Parking lot ensuring spot capacity is never exceeded

3Library preventing checkout of already borrowed books

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