Understanding Java Access Modifiers: Public, Protected, Package-Private, and Private

What are the differences between public, protected, package-private, and private access modifiers in Java, and when should each be used?

In Java, access modifiers control the visibility and accessibility of classes, methods, and other members. Understanding the differences and appropriate use cases for the access modifiers: public, protected, package-private (default), and private is crucial for effective Java programming.

// Example in Java
class Parent {
    public int publicValue = 1;
    protected int protectedValue = 2;
    int packagePrivateValue = 3; // Default access
    private int privateValue = 4;
}

class Child extends Parent {
    void displayValues() {
        System.out.println(publicValue);         // Accessible
        System.out.println(protectedValue);      // Accessible
        System.out.println(packagePrivateValue); // Accessible in the same package only
        // System.out.println(privateValue);  // Not accessible
    }
} 

public class Main {
    public static void main(String[] args) {
        Parent parent = new Parent();
        System.out.println(parent.publicValue);   // Accessible
        // System.out.println(parent.protectedValue); // Not accessible in Main class
        // System.out.println(parent.packagePrivateValue); // Not accessible
        // System.out.println(parent.privateValue); // Not accessible
    }
} 

// In a different package or class Try to access parent values appropriately based on access modifier rules.

Causes

• Different access levels prevent unauthorized access and modifications to class members.
• Access modifiers help encapsulate data, enabling better code management and security.

Solutions

• **Public**: Members declared as public can be accessed from any other class or package. Use public when you want to provide access to your class or method across all packages.
• **Protected**: Members marked as protected can be accessed within the same package or subclasses across different packages. Use this modifier when you want to allow extended classes to access certain members while restricting outside access.
• **Package-Private**: Also known as default, this access modifier has no keyword. Members without any modifier are accessible only within the same package. It is useful for classes that should not be exposed beyond their package, promoting encapsulation.
• **Private**: Members declared private are accessible only within the class they are defined. Use private for data that should not be accessible from outside the class, ensuring strict encapsulation.

Mistake: Using public for everything without considering encapsulation.

Solution: Evaluate the necessity of exposing class members; prefer private or protected judiciously.

Mistake: Not understanding that package-private is the default in Java without an explicit modifier.

Solution: Remember to specify access modifiers to improve code clarity and intention.

Mistake: Attempting to access private members of a parent class from a child class.

Solution: Use protected or public modifiers for members that need to be accessed in subclasses.