Access Modifiers

TL;DR

Access modifiers determine the visibility/accessibility of classes, methods, and variables.

• public: Accessible from anywhere.
• protected: Accessible within the same package, and by subclasses in other packages.
• default (no keyword): Accessible only within the same package.
• private: Accessible only within the exact same class.

Concept

Modifiers are the core of Encapsulation. They allow you to define exactly who is allowed to see or modify your data.

Modifier	Same Class	Same Package	Subclass (Diff Pkg)	Global (Anywhere)
public	✅	✅	✅	✅
protected	✅	✅	✅	❌
default	✅	✅	❌	❌
private	✅	❌	❌	❌

Examples

// file: packageA/Person.java
package packageA;

public class Person {
    public String name;       // Anyone can read this
    protected int age;        // Package-private + child classes
    String ssn;               // Package-private (default)
    private double salary;    // Only Person methods can read this
    
    public double getSalary() {
        return salary; // Class itself can access private fields
    }
}

// file: packageB/Employee.java
package packageB;
import packageA.Person;

public class Employee extends Person {
    public void printInfo() {
        System.out.println(name); // Allowed (public)
        System.out.println(age);  // Allowed (protected allows subclasses in diff packages)
        // System.out.println(ssn); // ERROR: default is restricted to packageA
        // System.out.println(salary); // ERROR: private is restricted to Person class
    }
}

Interview Questions

Q: Can a top-level Class be private or protected?
A: No. A top-level class can only be public or default (package-private). If a top-level class were private, it would be completely invisible to everything, making it useless. However, Inner classes (classes defined inside other classes) can be private or protected.

Q: What is the default access modifier if you don’t specify one?
A: It is “package-private” (often just called “default”). It means the member is accessible to any other class in the exact same package, but completely hidden from classes outside that package (even subclasses).