Why Is It Not Possible to Reduce Method Visibility in a Java Subclass?

Why can't you reduce the visibility of a method in a Java subclass?

class Parent {
    protected void display() { 
        System.out.println("Parent display");
    }
}
class Child extends Parent {
    // Attempting to reduce visibility here will cause a compile-time error
    // private void display() { 
    //     System.out.println("Child display");
    // }
}

In Java, the visibility (or access level) of a method in a subclass cannot be less restrictive than that of the method in its parent class. This restriction ensures that the contract established by the parent class is not violated, allowing subclasses to maintain polymorphic behavior correctly.

// Child class demonstrating limited access
class Child extends Parent {
    // This method is new but does not override 'display'
    public void show() { 
        System.out.println("Child show");
    }
}

Causes

• Java's access modifiers include public, protected, and private, with varying levels of accessibility.
• The principle of 'least privilege' ensures that when a subclass inherits a method, it maintains the visibility level of the superclass.
• Reducing visibility would prevent the superclass from being able to call the overridden method, which could compromise the integrity of the class hierarchy.

Solutions

• If you need to restrict access to certain methods, consider using private methods in the parent class and provide public or protected methods that act as access points.
• You can define a new method in the subclass with a different name to provide restricted functionality, rather than overriding the existing method.

Mistake: Attempting to change the access modifier from protected to private when overriding a method.

Solution: Always ensure that the access modifier is either the same or more permissive than the original method.

Mistake: Believing that private methods in the parent class can be accessed in the subclass.

Solution: Understand that private methods are not visible to subclasses; consider protected methods for subclass access.