Encapsulation

class PersonBad {
    public String name;
    public int age;  // dangerous — anyone can set age = -99
}

class PersonGood {
    private String name;    // hidden — only accessible via methods
    private int age;

PersonGood(String name, int age) {
        this.name = name;
        setAge(age);  // validate even in constructor
    }

void setAge(int age) {
        if (age < 0 || age > 150) throw new IllegalArgumentException("Invalid age: " + age);
        this.age = age;
    }

int getAge()    { return age; }
    String getName(){ return name; }
}

public class Main {
    public static void main(String[] args) {
        PersonBad bad = new PersonBad();
        bad.age = -99;  // Java allows this — very wrong!
        System.out.println("Bad age: " + bad.age);

PersonGood good = new PersonGood("Alex", 25);
        System.out.println("Good age: " + good.getAge());

try {
            good.setAge(-1);  // throws!
        } catch (IllegalArgumentException e) {
            System.out.println("Caught: " + e.getMessage());
        }
    }
}

Output

Click Run to execute, or Check to validate.

Java Tutorial: Encapsulation

Access modifiers, private fields, getters and setters, data validation, and why hiding implementation details matters.

Protect your data

Encapsulation is one of the four pillars of OOP. By hiding internal state and controlling access through methods, you prevent invalid data from ever entering an object.

A `BankAccount` with a `public balance` field is dangerous — anyone can set it to anything. Make it `private` and only allow changes through `deposit()` and `withdraw()` — which can validate and log.

What you'll learn in this Java encapsulation tutorial

This interactive Java tutorial has 6 hands-on exercises. Estimated time: 18 minutes.

The problem with public fields — Public fields can be set to anything — including impossible values:
Access modifiers — Java has four access levels:
Getters and setters — Convention for private fields:
Immutable objects with final fields — An **immutable** object's state cannot change after construction. Make fields `final` and provide only getters — no sett…
Builder pattern for complex objects — When a class has many optional fields, a constructor with 10 parameters is unreadable.
Build a validated bank account — Final challenge — no starter code.

Java Encapsulation concepts covered

Protect your data

Discussion

UuByte

Encapsulation

class PersonBad {
    public String name;
    public int age;  // dangerous — anyone can set age = -99
}

class PersonGood {
    private String name;    // hidden — only accessible via methods
    private int age;

PersonGood(String name, int age) {
        this.name = name;
        setAge(age);  // validate even in constructor
    }

void setAge(int age) {
        if (age < 0 || age > 150) throw new IllegalArgumentException("Invalid age: " + age);
        this.age = age;
    }

int getAge()    { return age; }
    String getName(){ return name; }
}

PersonGood good = new PersonGood("Alex", 25);
        System.out.println("Good age: " + good.getAge());

try {
            good.setAge(-1);  // throws!
        } catch (IllegalArgumentException e) {
            System.out.println("Caught: " + e.getMessage());
        }
    }
}

Output

Click Run to execute, or Check to validate.

Java Tutorial: Encapsulation

Access modifiers, private fields, getters and setters, data validation, and why hiding implementation details matters.

Protect your data

Encapsulation is one of the four pillars of OOP. By hiding internal state and controlling access through methods, you prevent invalid data from ever entering an object.

What you'll learn in this Java encapsulation tutorial

This interactive Java tutorial has 6 hands-on exercises. Estimated time: 18 minutes.

The problem with public fields — Public fields can be set to anything — including impossible values:
Access modifiers — Java has four access levels:
Getters and setters — Convention for private fields:
Immutable objects with final fields — An **immutable** object's state cannot change after construction. Make fields `final` and provide only getters — no sett…
Builder pattern for complex objects — When a class has many optional fields, a constructor with 10 parameters is unreadable.
Build a validated bank account — Final challenge — no starter code.

Java Encapsulation concepts covered

Protect your data

Discussion