Access Modifiers

Access modifiers control which parts of your code are allowed to see or use a field, method, or class. They're what make encapsulation possible: the practice of hiding an object's internal details and only exposing what other code actually needs.

The Four Access Levels

Modifier	Same Class	Same Package	Subclass	Everywhere
public	✅	✅	✅	✅
protected	✅	✅	✅	❌
(none, package-private)	✅	✅	❌	❌
private	✅	❌	❌	❌

• public: Accessible from anywhere in the program.
• protected: Accessible within the same package, and by subclasses (see the Inheritance section).
• Package-private (no keyword at all): Accessible only within the same package.
• private: Accessible only within the same class.

Why Hide Fields?

It might seem easier to just make every field public so any code can read or change it directly. However, this makes it very easy for other code to put an object into an invalid state. For example, imagine a Flywheel class with a public voltage field:

flywheel.voltage = 9999.0; // Nothing is stopping this from happening!

Nothing prevents another part of the codebase from accidentally setting a dangerously high voltage. By marking voltage as private, the field can no longer be accessed directly from outside the class, forcing all interaction to go through methods that the class controls.

Getters and Setters

Since private fields can't be accessed directly from outside their class, classes typically expose public methods, called getters and setters, to read and modify them safely.

Show code

class Flywheel {
    private double voltage;

    public double getVoltage() {
        return voltage;
    }

    public void setVoltage(double newVoltage) {
        if (newVoltage <= 12.0) {
            voltage = newVoltage;
        }
    }
}

Flywheel shooterWheel = new Flywheel();
shooterWheel.setVoltage(9999.0); // Ignored, since it fails the check
shooterWheel.setVoltage(12.0);

System.out.println(shooterWheel.getVoltage());

Output:

12.0

Now, the setVoltage() method can validate incoming data before it's ever stored, something that would be impossible if voltage were a plain public field.

Naming Conventions

• Getter methods are named get followed by the field name in PascalCase: getVoltage()
• Setter methods are named set followed by the field name in PascalCase: setVoltage()
• Boolean getters are often named is followed by the field name instead of get: isSpinning()