Encapsulation

Definition

Encapsulation means keeping an object's data private and letting other code change it only through methods. The object can then protect its own rules, so it never ends up in an invalid state.

Example code

A bank account that protects its balance

class BankAccount {
    private long balanceCents;                  // hidden: nobody can set it directly

    void deposit(long cents) {
        if (cents <= 0) throw new IllegalArgumentException("Deposit must be positive");
        balanceCents += cents;
    }

    void withdraw(long cents) {
        if (cents > balanceCents) throw new IllegalStateException("Not enough money");
        balanceCents -= cents;
    }

    long balance() { return balanceCents; }
}

class Demo {
    public static void main(String[] args) {
        BankAccount account = new BankAccount();
        account.deposit(5_000);
        account.withdraw(1_200);
        System.out.println(account.balance());
        try {
            account.withdraw(10_000);
        } catch (IllegalStateException e) {
            System.out.println(e.getMessage());
        }
    }
}

// Output:
// 3800
// Not enough money