Question

When both a base class and a derived class have constructors, the base class's constructor is called ______________ (first/last).

Short answer

Answer: When creating an instance of a derived class, the base class's constructor is called first. Then, the derived class's constructor is called.

Step-by-step solution

Understand base and derived classes

In object-oriented programming, a base class is a class from which other classes (derived classes) inherit properties and methods. The derived class continues to have all of its properties and methods while also owning the ones inherited from the base class.

Explain example context

Let's consider an example where we have two classes, a base class "Animal" and a derived class "Cat".

Create base class with a constructor

We will create a "Animal" class with a constructor. The constructor is a special method that is called when an object is instantiated. ``` class Animal { public Animal() { System.out.println("Animal constructor called"); } } ```

Create derived class with a constructor

Now, we will create a "Cat" class that inherits from "Animal" class and also has a constructor. ``` class Cat extends Animal { public Cat() { System.out.println("Cat constructor called"); } } ```

Instantiate the derived class and observe the constructor call order

Finally, we will create an instance of the derived class "Cat". When we do this, we will see the order in which the constructors are called. ``` public class Main { public static void main(String[] args) { Cat myCat = new Cat(); } } ``` When we run the "Main" class, the console will output: ``` Animal constructor called Cat constructor called ``` From the output, we can conclude that the base class's constructor is called #tag_answer#first#tag_answer#.

Key concepts

Base Class

In object-oriented programming (OOP), a base class serves as a foundational building block for other classes. Think of it like the blueprint of a house. Just as you may use a single blueprint to build multiple houses, a base class can be extended by many derived classes to build unique but related objects. The base class encapsulates common attributes and behaviors that its derived classes inherit.

For instance, consider a base class called "Animal". This class defines general properties and methods such as

• Attributes like number of legs, type of diet (carnivore, herbivore), and habitat.
• Methods (or functions) like sleep(), eat(), and move().

Using a base class refines your code by promoting reuse and reducing redundancy. Instead of rewriting the same methods for every new animal, they get shared through inheritance with "Animal" acting as a single source of truth for what all animals should have and do.

Derived Class

A derived class in OOP is developed using an existing base class, and it inherits all properties and behaviors from this base class. However, the magic of a derived class lies in its ability to extend or override features of the base class to accommodate more specific needs. In simpler terms, the derived class is like a tailored suit; it starts with the base design but introduces alterations that make it uniquely its own.

To illustrate this, let’s take the derived class "Cat", which extends from the base class "Animal". The derived class “Cat” inherits the basic attributes and behaviors of "Animal" (like sleeping and eating), while also adding its own characteristics:

• Attributes like purring, sharp claws, and hunting skill.
• Methods such as purr(), climb(), and scratch().

This inheritance structure allows "Cat" to seamlessly build off of "Animal", gaining all the benefits of the base class with added unique capabilities.

Constructor Order in Inheritance

In OOP, when a derived class is instantiated, constructors from both the derived class and base class are called to set up the object. However, the order in which these constructors execute is crucial for the correct initialization of the object.

When constructing an object, the base class constructor is always called first, followed by the derived class constructor. This sequence ensures that all foundational properties are set up before any enhancements or modifications are made by the derived class.

Let's revisit our "Animal" and "Cat" example:

• When "Cat" is instantiated, the "Animal" constructor runs, initializing common attributes shared across all animals.
• After the "Animal" constructor, the "Cat" constructor executes, setting up cat-specific features.

This order guarantees that by the time the "Cat" constructor runs, the object is fully ready to accept its cat-centric attributes, laying a reliable base for further specialized behavior.