Encapsulation in Practice

Beyond simply making fields private, real encapsulation means the getters and setters actually enforce invariants rather than just passing values through unchanged:

class BankAccount {
    private double balance;

    public void setBalance(double balance) {
        if (balance < 0) {
            throw new IllegalArgumentException("Balance cannot be negative");
        }
        this.balance = balance;
    }

    public double getBalance() {
        return balance;
    }
}

Not every field needs both a getter and a setter — an immutable class (see Records, in Level 3) only exposes getters, and every field is set once, in the constructor, and never changed again.

Common mistakes

• Generating a setter for every field automatically (IDEs offer to do this) without considering whether that field should actually be changeable from outside the class at all.