oop-encapsulation

# OOP Encapsulation

Master encapsulation and information hiding to create robust, maintainable object-oriented systems. This skill focuses on controlling access to object internals and exposing well-defined interfaces.

## Understanding Encapsulation

Encapsulation is the bundling of data and methods that operate on that data within a single unit, while restricting direct access to some of the object's components. This principle protects object integrity and reduces coupling.

### Java Encapsulation

```java
// Strong encapsulation with validation
public class BankAccount {
    private String accountNumber;
    private BigDecimal balance;
    private final List<Transaction> transactions;

    public BankAccount(String accountNumber, BigDecimal initialBalance) {
        if (accountNumber == null || accountNumber.isEmpty()) {
            throw new IllegalArgumentException("Account number required");
        }
        if (initialBalance.compareTo(BigDecimal.ZERO) < 0) {
            throw new IllegalArgumentException("Initial balance cannot be negative");
        }

        this.accountNumber = accountNumber;
        this.balance = initialBalance;
        this.transactions = new ArrayList<>();
    }

    // Read-only access to account number
    public String getAccountNumber() {
        return accountNumber;
    }

    // Read-only access to balance
    public BigDecimal getBalance() {
        return balance;
    }

    // Defensive copy for collection
    public List<Transaction> getTransactions() {
        return Collections.unmodifiableList(transactions);
    }

    // Controlled mutation with validation
    public void deposit(BigDecimal amount) {
        if (amount.compareTo(BigDecimal.ZERO) <= 0) {
            throw new IllegalArgumentException("Deposit amount must be positive");
        }

        balance = balance.add(amount);
        transactions.add(new Transaction(TransactionType.DEPOSIT, amount));
    }

    // Controlled mutation with business logic
    p