Encapsulation

Here is the thing about fields in a class — if you make them public, anyone can change them to whatever they want. That might sound fine until someone sets the age of a person to -5.

Encapsulation is the idea of keeping the internal data of an object private and only allowing controlled access through methods. It is one of the core principles of object-oriented programming.

Private Fields

The private keyword means the field can only be accessed from within the same class. No other class can touch it directly.

public class Person {
  private String name;
  private int age;
}

If you try to do person.name = "John" from another class, Java will refuse to compile it. That field is locked down.

Getters and Setters

If the fields are private, how do you read or change them? You write getter and setter methods. These are public methods that control access to the private fields.

public class Person {
  private String name;
  private int age;

  public String getName() {
    return name;
  }

  public void setName(String name) {
    this.name = name;
  }

  public int getAge() {
    return age;
  }

  public void setAge(int age) {
    if (age > 0) {
      this.age = age;
    }
  }
}

public class Main {
  public static void main(String[] args) {
    Person p = new Person();
    p.setName("Amina");
    p.setAge(25);
    System.out.println(p.getName() + " is " + p.getAge());
  }
}

Look at setAge(). It checks that the age is positive before setting it. That is the whole point of encapsulation — you can add validation, logging, or any logic you want before allowing changes to the data.

Access Modifiers

Java gives you a few options for controlling access to your classes, fields, and methods:

• private — only accessible within the same class
• default (no keyword) — accessible within the same package
• protected — accessible within the same package and by subclasses
• public — accessible from anywhere

public class BankAccount {
  private double balance;

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

  public double getBalance() {
    return balance;
  }
}

The balance is private. Nobody can just reach in and change it. They have to go through the deposit() method, which makes sure they are depositing a positive amount. That is data hiding in action.