Java Tutorial · Object-Oriented Java · Lesson 20 of 37

Java Encapsulation and Modifiers

Access modifiers, getters and setters, static and final.

Access modifiers

Modifier	Visible to
`public`	everyone
`protected`	same package and subclasses
(none)	same package only
`private`	the class itself

Encapsulation means keeping fields private and exposing safe methods, so an object can enforce its own rules:

Example 1

public class Main {
    public static void main(String[] args) {
        BankAccount acct = new BankAccount("Ada");
        acct.deposit(100);
        acct.withdraw(30);
        System.out.println(acct.getOwner() + ": " + acct.getBalance());
        try {
            acct.withdraw(500);
        } catch (IllegalArgumentException e) {
            System.out.println("Rejected: " + e.getMessage());
        }
    }
}

class BankAccount {
    private final String owner;
    private double balance;

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

    public String getOwner() { return owner; }
    public double getBalance() { return balance; }

    public void deposit(double amount) {
        if (amount <= 0) throw new IllegalArgumentException("Deposit must be positive");
        balance += amount;
    }

    public void withdraw(double amount) {
        if (amount > balance) throw new IllegalArgumentException("Insufficient funds");
        balance -= amount;
    }
}

Output

Ada: 70.0
Rejected: Insufficient funds

Run this on your computer — see “Get Started” for setup.

static and final

static fields and methods belong to the class, shared by all instances.
final fields can be set only once; final classes can't be extended; final methods can't be overridden.

Example 2

public class Main {
    public static void main(String[] args) {
        new Ticket();
        new Ticket();
        Ticket t = new Ticket();
        System.out.println("Ticket #" + t.number + " of " + Ticket.issued);
        System.out.println(Ticket.PREFIX + t.number);
    }
}

class Ticket {
    static final String PREFIX = "TKT-";
    static int issued = 0;
    final int number;

    Ticket() {
        issued++;
        number = issued;
    }
}

Output

Ticket #3 of 3
TKT-3

Run this on your computer — see “Get Started” for setup.

Test yourself

Which access modifier makes a field visible only inside its own class?

public
protected
private
static

Show answer

Answer: private