Understand the idea

Encapsulation lets an object control how its state changes. Private fields and meaningful methods can keep invalid values out. Adding getters and setters alone is not the goal; preserving a useful invariant is.

Explore the details

Encapsulation

The meaning of Encapsulation, is to make sure that "sensitive" data is hidden from users. To achieve this, you must:

• declare class variables/attributes as private
• provide public get and set methods to access and update the value of a private variable

Get and Set

You learned from the previous chapter that private variables can only be accessed within the same class (an outside class has no access to it). However, it is possible to access them if we provide public get and set methods.

The get method returns the variable value, and the set method sets the value.

Syntax for both is that they start with either get or set, followed by the name of the variable, with the first letter in upper case:

Example explained

The get method returns the value of the variable name.

The set method takes a parameter (newName) and assigns it to the name variable. The this keyword is used to refer to the current object.

However, as the name variable is declared as private, we cannot access it from outside this class:

If the variable was declared as public, we would expect the following output:

However, as we try to access a private variable, we get an error:

Instead, we use the getName() and setName() methods to access and update the variable:

Why Encapsulation?

• Better control of class attributes and methods
• Class attributes can be made read-only (if you only use the get method), or write-only (if you only use the set method)
• Flexible: the programmer can change one part of the code without affecting other parts
• Increased security of data

class BankAccount {
    private double balance;  // hidden — direct access blocked

    BankAccount(double initial) { this.balance = initial; }

    public double getBalance()           { return balance; }
    public void deposit(double amount)   { if (amount > 0) balance += amount; }
    public void withdraw(double amount)  { if (amount <= balance) balance -= amount; }
}

BankAccount acc = new BankAccount(1000);
acc.deposit(500);
acc.withdraw(200);
System.out.println(acc.getBalance());  // 1300.0
// acc.balance = -9999;  // ERROR: balance is private

A complete runnable example

This complete Main.java program focuses on the lesson idea. Reference fragments above may demonstrate individual statements; this example includes the imports, entry point, and supporting types it needs.

public class Main {
    public static void main(String[] args) throws Exception {
        Score score = new Score();
        score.setPoints(80);
        System.out.println(score.getPoints());
    }
}
class Score {
    private int points;
    int getPoints() { return points; }
    void setPoints(int value) {
        if (value < 0 || value > 100) throw new IllegalArgumentException("Use 0 to 100");
        points = value;
    }
}

80

Change it. Run it. Understand it.

Load this lesson example into the editor. Change one value or statement, predict the result, then press RUN to execute Java.

Make it your own

Your learning checkpoint

• Explain the example without reading the output first.
• Make the change requested in the practice task.
• Describe why the result changed—or why compilation failed.

Ready to move on? Follow the next lesson in course order, or return to the curriculum to review a prerequisite.