How does a subclass use the functionality of its parent class?

Through inheritance: a derived class receives the public and protected members of the parent class.

• Direct access: Public and protected fields and methods of the parent can be accessed directly by their name within the derived class.
• super keyword: Used to call the parent's constructor or to access overridden methods/fields of the parent when the derived class has members with the same name.
• Overriding: Parent methods (not final) can be overridden in the derived class, changing their behavior.

// Example of a parent class
class Parent {
    protected int parentValue;

    public Parent(int value) {
        this.parentValue = value;
    }

    public void displayParentValue() {
        System.out.println("Parent value: " + parentValue);
    }

    public final void cannotBeOverridden() {
        System.out.println("This method cannot be overridden.");
    }
}

// Example of a derived class
class Child extends Parent {
    private int childValue;

    public Child(int parentValue, int childValue) {
        super(parentValue); // Call to the parent's constructor
        this.childValue = childValue;
    }

    // Overriding the parent's method
    @Override
    public void displayParentValue() {
        System.out.println("Overridden parent value: " + parentValue); // Access to the parent's protected field
    }

    public void displayChildValue() {
        System.out.println("Child value: " + childValue);
    }

    public void callParentMethod() {
        super.displayParentValue(); // Call to the parent's method via super
    }
}