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Java · Object-Oriented Java

Encapsulation

Encapsulation keeps an object’s data safe by hiding it and allowing changes only through methods that enforce the rules.

Beginner 5 min Lesson 12 of 31Examples target Java 21 LTS (they also run on Java 25).
Before you start:Constructors
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On this page · 5 min · Beginner

On this page

  1. In simple words
  2. Learning objectives
  3. Explanation
  4. Code example
  5. Step by step
  6. Real-world use
  7. Common mistakes
  8. Best practices
  9. Remember this
  10. Practice exercise
  11. Mini quiz
  12. Related topics
  13. My notes
Reading time
5 min
Difficulty
Beginner
Updated
Oct 30, 2026

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In simple words#

An ATM never lets you touch the cash vault. You use buttons (methods) that check your PIN and balance first — that protection is encapsulation.

private
Visible only inside the class.
Getter / setter
Methods to read / change a field.
Invariant
A rule that is always true, e.g. balance ≥ 0.

Learning objectives#

  • Make fields private
  • Expose behaviour instead of raw fields
  • Validate changes
  • Create read-only data

Explanation#

Make fields private and provide methods for allowed actions.

Prefer meaningful methods (deposit, withdraw) over generic setters (setBalance).

Getters can expose values; omit setters for read-only data.

Return copies or unmodifiable views of internal lists so callers cannot change them.

Code example: An account that can’t go negative#

JavaAn account that can’t go negative Illustrative
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;


public class Account {
    private double balance;
    private final List<String> history = new ArrayList<>();


    public double getBalance() { return balance; }
    public List<String> getHistory() { return Collections.unmodifiableList(history); }


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


    public void withdraw(double amount) {
        if (amount > balance) throw new IllegalStateException("Insufficient balance");
        balance -= amount;
        history.add("-" + amount);
    }


    public static void main(String[] args) {
        Account acc = new Account();
        acc.deposit(1000);
        acc.withdraw(300);
        System.out.println(acc.getBalance() + " " + acc.getHistory());
    }
}
Expected output
700.0 [+1000.0, -300.0]

This example is illustrative. Copy or download it and run it in your own environment; the expected output is shown above.

Step-by-step explanation#

  1. balance is private — no direct changes.
  2. deposit and withdraw enforce the rules.
  3. History is returned read-only.
  4. The invariant balance ≥ 0 always holds.

Real-world use case#

Banking software guarantees no account goes negative because only withdraw() can reduce the balance.

Common mistakes#

Generating setters for every field

It breaks encapsulation. Add only the operations that make sense.

Returning internal lists directly

Callers can modify them. Return unmodifiable views or copies.

Best practices#

  • Private fields
  • Behaviour methods over setters
  • Validate every change
  • Immutable where possible

Remember this#

Private data, public behaviour.

Methods guard the rules.

Don’t leak internal collections.

Practice exercise#

Your task: Make an Employee class whose salary can only increase via giveRaise(percent).

Show solution
Java Illustrative
public class Employee {
    private double salary;
    public Employee(double salary) { this.salary = salary; }
    public double getSalary() { return salary; }
    public void giveRaise(double percent) {
        if (percent <= 0) throw new IllegalArgumentException();
        salary += salary * percent / 100;
    }
}

Mini quiz#

Encapsulation quizMultiple choice Question 1 of 1

Why make fields private?

Score so far: 0/0

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My notes#

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