private fields

Learn field hiding with private.

private fields

Encapsulation protects a class's fields from outside access. By making fields private, you prevent direct access. Data can then only be changed through the class's own methods; external interference is prevented.

Example: class Account { private double balance; }. Here balance is private. If you write Account a = new Account(); a.balance = 100; you get a compile error; because balance cannot be accessed from outside the class. Access is provided through getter and setter methods; these come in later lessons. Encapsulation protects data integrity.

Why private?

Making fields private keeps data under control. It prevents unwanted value assignments from outside. For example, you can stop a balance field from receiving a negative value.

Without encapsulation, a.balance = -50; can be written and the account drops to a negative balance. With private you prevent this; the balance is only changed through a setter method, and the setter rejects negative values. This ensures the data always stays valid. Encapsulation hides the class's internal design from outside; changes happen inside the class, outside code is not affected.

Exercise Add a private field to Account

Define a class Account. Add a private double balance field. Write a void deposit(double amount) method; its body runs balance += amount;. Add a double getBalance() method returning balance. In main create an Account instance, call deposit(100) and print the result of getBalance().

Show example solution

class Account {
    private double balance;
    void deposit(double amount) {
        balance += amount;
    }
    double getBalance() {
        return balance;
    }
    public static void main(String[] args) {
        Account a = new Account();
        a.deposit(100);
        System.out.println(a.getBalance());
    }
}

Test yourself

What happens when you access a private double balance field from outside?
- The value resets to zero
- A compile error occurs
- The value is assigned
- The class is deleted
Show answer

A compile error occurs — private fields cannot be accessed from outside; a compile error occurs.

Make the field private

class Account {
    ____ double balance;
}

Show answer

class Account {
    private double balance;
}

Match access modifiers

private

Class only
public

Everywhere
protected

Subclasses
(no modifier)

Same package

Next lesson: Getter

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