Encapsulation: private, Getters and Setters

Keep the data safe

If anyone can change a field directly, they can put nonsense in it, like a negative bank balance. Encapsulation means hiding the fields with private and only letting other code change them through methods that check the values first.

Think of it like this: A bank doesn't let you walk into the vault and take money. You go to the counter (a method), and the cashier checks that you have enough first.

class BankAccount {
    private String owner;
    private double balance;

    BankAccount(String owner) {
        this.owner = owner;
    }

    public double getBalance() {
        return balance;
    }

    public void deposit(double amount) {
        if (amount > 0) {
            balance += amount;
        }
    }

    public boolean withdraw(double amount) {
        if (amount > 0 && amount <= balance) {
            balance -= amount;
            return true;
        }
        return false;
    }
}

public class Main {
    public static void main(String[] args) {
        BankAccount acc = new BankAccount("Asha");
        acc.deposit(500);
        System.out.println(acc.withdraw(800));
        System.out.println(acc.withdraw(200));
        System.out.println(acc.getBalance());
    }
}

Output

false
true
300.0

Now nobody can write acc.balance = -1000:

class BankAccount {
    private double balance;
}

public class Main {
    public static void main(String[] args) {
        BankAccount acc = new BankAccount();
        acc.balance = -1000;
    }
}

Output (the program does not compile)

Main.java:8: error: balance has private access in BankAccount
        acc.balance = -1000;
           ^

Access modifiers

Tip: A good habit: make fields private, and add a getX() method to read them and a setX() method (with checks) only if they really need to change.