Inheritance (extends), super Keyword & Constructor Delegation

1Concept

Inheritance models an IS-A relationship where a subclass inherits state and behavior from a superclass. Subclasses use `super(...)` to invoke the superclass constructor, which MUST execute before subclass field initialization to guarantee proper parent state setup.

2Architecture Diagram

Hierarchy:
    [ Superclass: Device ]  (fields: id, brand)
             ^
             |
    [ Subclass: Laptop ]    (fields: ramGb, os) ---> Invokes super(id, brand)

3Code Example

public class InheritanceDemo {
    static class Device {
        protected String brand;

        public Device(String brand) {
            this.brand = brand;
            System.out.println("Superclass constructor: Device initialized.");
        }
    }

    static class Laptop extends Device {
        private int ramGb;

        public Laptop(String brand, int ramGb) {
            super(brand); // Delegates to parent constructor
            this.ramGb = ramGb;
            System.out.println("Subclass constructor: Laptop initialized with " + ramGb + "GB RAM.");
        }
    }

    public static void main(String[] args) {
        Laptop laptop = new Laptop("Dell", 32);
    }
}

4Expected Output

Superclass constructor: Device initialized.
Subclass constructor: Laptop initialized with 32GB RAM.

5Key Takeaways

• ✓Java supports single class inheritance; a class can only extend one direct superclass.
• ✓If parent has no no-arg constructor, child constructor MUST explicitly invoke `super(args)`.
• ✓Constructors are NOT inherited by subclasses.