When we look at inheritance in object-oriented programming (OOP), access modifiers play a big role. These modifiers can really change how classes behave and who can use them. Understanding access modifiers helps us build better systems in OOP.

Let’s start by explaining what access modifiers are. In languages like Java and C#, access modifiers tell us who can access classes, methods, and attributes. There are four main types of access modifiers:

1. Public: Anyone can access public members, no restrictions here!

2. Protected: Protected members can be accessed by the class itself, by classes that are derived from it, and by classes in the same package. It balances privacy and inheritance.

3. Private: Private members can only be accessed within the class they are declared in. This keeps them safe from outside classes.

4. Default (Package-Private): If no access modifier is given, the member is only accessible to classes in the same package.

Now, let’s see how these modifiers affect how inheritance works.

Base Class Accessibility
When you create a base class (the main class), the access modifier you choose affects what derived classes (the classes that come from the base class) can do with it.

For example, if you set something as private in the base class, derived classes cannot access it directly. This helps keep things organized and prevents derived classes from messing with the base class’s structure. Developers often need to make methods protected or public to allow derived classes to access certain private members.

Here’s an example:

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

   protected String getName() {
       return name; // Access through a protected method
   }
}

class Dog extends Animal {
   public void bark() {
       System.out.println(this.getName() + " says Woof!"); 
   }
}

In this example, the Dog class uses the getName() method, which is protected, to get the name. But it can’t directly access the name variable. This shows how a private setting for name keeps the derived classes following the rules set by the base class.

Inheritance of Access Modifiers
When a derived class inherits from a base class, the access modifiers of the base class are very important. Here’s how it works:

• Public members stay public in the derived class.
• Protected members stay protected.
• Private members are hidden and not inherited.
• Default members are only accessible within the package.

This means if you want derived classes to have access to certain features, you need to be clear about how you set them up in the base class.

For example, if a company wants multiple developers to work together, they might use protected members to allow collaboration while still keeping an eye on things. Making some parts protected helps derived classes add new features without losing control of the base class.

Composition Over Inheritance
Sometimes, access modifiers might make developers rethink using inheritance. Instead, they might prefer using composition. This means they use or refer to base classes instead of directly inheriting from them, avoiding access level issues.

For example, instead of extending a class filled with private members, you can create a new class that includes an instance of another class:

class Engine {
   private int horsepower;
   
   public Engine(int horsepower) {
       this.horsepower = horsepower;
   }
}

class Car {
   private Engine engine; // No inheritance here
   
   public Car(int horsepower) {
       this.engine = new Engine(horsepower);
   }
}

In this case, the Car class has an Engine, which means it can use the engine without showing any sensitive data. The choice between inheritance and composition depends on how closely the classes should be connected and is often guided by access modifiers.

Access Modifiers and the Liskov Substitution Principle
When we talk about inheritance, we need to consider principles like the Liskov Substitution Principle (LSP). This principle says that you should be able to replace a base class with a derived class without breaking the program.

If a derived class changes a public method from the base class to a private one, it breaks the rule. Users of the base class expect the same behavior, whether they are using the base class or the derived class. Changing access levels incorrectly can cause errors or unexpected results.

So, keeping access modifiers consistent across classes helps support design principles like LSP and makes the code easier to maintain and understand.

Protected Members and Inheritance Depth
When we go deeper into inheritance, things can get tricky, especially with protected members. In a setup with multiple levels of inheritance, having protected members can be both useful and restrictive.

Here’s an example with three classes:

class Vehicle {
   protected int speed;

   public Vehicle(int speed) {
       this.speed = speed;
   }
}

class Car extends Vehicle {
   protected int wheels;

   public Car(int speed, int wheels) {
       super(speed);
       this.wheels = wheels;
   }
}

class SportsCar extends Car {
   private String mode;

   public SportsCar(int speed, int wheels, String mode) {
       super(speed, wheels);
       this.mode = mode; 
   }
   
   public void display() {
       System.out.println("Speed: " + speed + ", Wheels: " + wheels + ", Mode: " + mode);
   }
}

In this example, the speed variable is protected, so both Car and SportsCar can access it. This raises some important questions:

• Should a derived class use the protected members?
• Might there be a risk of misusing these members as the hierarchy gets larger?

Protected members allow easy access but can also lead to mistakes in more complex systems. This highlights the need to think carefully about visibility with member access.

Bridging Interfaces and Access Modifiers
Access modifiers also connect with interfaces in OOP. Interfaces are usually public and act as agreements that classes must follow. The methods in interfaces can influence how the class’s implementation can be accessed.

When a method is defined in an interface, the class that implements it must keep the same access level. For example, if an interface method is public, the class must also make it public:

interface Drawable {
   void draw();
}

class Shape implements Drawable {
   public void draw() {
       System.out.println("Drawing a shape.");
   }
}

If the class tries to make the draw() method private instead:

class Shape implements Drawable {
   private void draw() { // Error: Can't make it less visible
       System.out.println("Drawing a shape.");
   }
}

This shows that planning access carefully when using interfaces can lead to better design decisions.

Conclusion
Looking at access modifiers in relation to inheritance helps us understand control, visibility, and design choices. These modifiers shape how classes interact and ensure that derived classes follow the rules set by base classes.

They make us think about balancing access and protection, guiding us to structure our code better. A thoughtful hierarchy respects these modifiers and enjoys the benefits of good organization. Each choice we make impacts how easily the code can be maintained and understood.

Therefore, understanding how to use inheritance while following access rules helps us navigate the complexities of object-oriented programming. It leads us to create solid and lasting design patterns that work well over time.