Default constructor in Java - Mini Project: Build & Apply

Default constructor

📖 Scenario: Imagine you are creating a simple program to represent a book in a library. Each book has a title and an author. Sometimes, you want to create a book without giving any details first, and then add details later.

🎯 Goal: You will create a Book class with a default constructor that sets the title and author to default values. Then, you will create an object of this class and print its details.

📋 What You'll Learn

Create a class called Book

Add a default constructor to Book that sets title to "Unknown Title" and author to "Unknown Author"

Create an object of Book using the default constructor

Print the title and author of the created Book object

💡 Why This Matters

🌍 Real World

Default constructors are used to create objects with default settings before setting specific details later.

💼 Career

Understanding constructors is essential for creating and initializing objects in Java, a key skill for software development.

1

Create the Book class with title and author fields

Create a class called Book with two String fields: title and author.

Java

public class Book {
    // Your code here
}

Hint

Fields are variables inside a class. Use String title; and String author; inside the class.

2

Add a default constructor to Book

Add a default constructor to the Book class that sets title to "Unknown Title" and author to "Unknown Author".

Java

public class Book {
    String title;
    String author;

    // Your code here
}

Hint

A default constructor has no parameters and sets the fields inside its body.

3

Create a Book object using the default constructor

In a Main class, create a Book object called myBook using the default constructor.

Java

public class Book {
    String title;
    String author;

    public Book() {
        title = "Unknown Title";
        author = "Unknown Author";
    }
}

public class Main {
    public static void main(String[] args) {
        // Create Book object here
        // Your code here
    }
}

Hint

Use Book myBook = new Book(); to create the object.

4

Print the title and author of the Book object

In the main method, print the title and author of the myBook object using System.out.println.

Java

public class Book {
    String title;
    String author;

    public Book() {
        title = "Unknown Title";
        author = "Unknown Author";
    }
}

public class Main {
    public static void main(String[] args) {
        Book myBook = new Book();
        // Your code here
    }
}

Hint

Use System.out.println(myBook.title); and System.out.println(myBook.author); to print the values.

Practice

1. What is a default constructor in Java?

easy

A. A method that returns the default value of a class.

B. A constructor with no parameters that Java provides automatically if none is written.

C. A constructor that must always be written by the programmer.

D. A special method that runs only when a program ends.

Solution

1. Step 1: Understand what a constructor is

   A constructor is a special method used to create objects of a class.

2. Step 2: Identify the default constructor

   If no constructor is written, Java automatically provides a constructor with no parameters called the default constructor.

3. Final Answer:

   A constructor with no parameters that Java provides automatically if none is written. -> Option B

4. Quick Check:

   Default constructor = automatic no-parameter constructor [OK]

Hint: Default constructor has no parameters and is auto-created if missing [OK]

Common Mistakes:

• Thinking default constructor must be written manually
• Confusing default constructor with methods returning default values
• Believing default constructor runs at program end

2. Which of the following is the correct syntax for a default constructor in Java?

easy

A. public ClassName(void) { }

B. public void ClassName() { }

C. public ClassName() { }

D. void ClassName() { }

Solution

1. Step 1: Recall constructor syntax

   A constructor has the same name as the class and no return type.

2. Step 2: Check each option

   public ClassName() { } matches the class name and has no return type, so it's correct syntax.

3. Final Answer:

   public ClassName() { } -> Option C

4. Quick Check:

   Constructor syntax = class name + no return type [OK]

Hint: Constructor name = class name, no return type, parentheses empty [OK]

Common Mistakes:

• Adding void return type to constructor
• Using wrong parameter list syntax
• Using lowercase class name in constructor

3. What will be the output of this Java code?

class Car {
  String model;
}
public class Test {
  public static void main(String[] args) {
    Car c = new Car();
    System.out.println(c.model);
  }
}

medium

A. null

B. Empty string

C. Compilation error

D. Runtime exception

Solution

1. Step 1: Understand default constructor usage

   No constructor is defined, so Java provides a default constructor that sets no initial values.

2. Step 2: Check default value of uninitialized String

   Instance variable 'model' is a String and defaults to null if not set.

3. Final Answer:

   null -> Option A

4. Quick Check:

   Uninitialized String = null by default [OK]

Hint: Uninitialized object fields default to null in Java [OK]

Common Mistakes:

• Expecting empty string instead of null
• Thinking default constructor sets values automatically
• Assuming compilation or runtime error

4. Identify the error in this Java class if any:

public class Book {
  String title;
  public Book() {
    title = "Java Basics";
  }
  public Book(String title) {
    title = title;
  }
}

medium

A. The second constructor does not set the instance variable correctly.

B. The default constructor is missing.

C. The class has no constructors.

D. The constructors have wrong return types.

Solution

1. Step 1: Analyze the second constructor

   It assigns parameter 'title' to itself, not to the instance variable.

2. Step 2: Understand correct assignment

   Use 'this.title = title;' to assign parameter to instance variable.

3. Final Answer:

   The second constructor does not set the instance variable correctly. -> Option A

4. Quick Check:

   Use 'this' to assign constructor parameters to fields [OK]

Hint: Use 'this.' to assign parameters to instance variables [OK]

Common Mistakes:

• Assigning parameter to itself instead of instance variable
• Missing default constructor (actually present)
• Adding return types to constructors

5. You want to create a class Person that sets the name to "Unknown" by default if no name is given. Which constructor code correctly implements this using a default constructor?

hard

A. public class Person {
String name;
public Person() {
name = name;
}
}

B. public class Person {
String name = "Unknown";
public Person(String name) {
name = name;
}
}

C. public class Person {
String name;
public Person(String name) {
this.name = name;
}
}

D. public class Person {
String name;
public Person() {
name = "Unknown";
}
public Person(String name) {
this.name = name;
}
}

Solution

1. Step 1: Check default constructor sets default value

   public class Person { String name; public Person() { name = "Unknown"; } public Person(String name) { this.name = name; } }'s default constructor sets name to "Unknown" correctly.

2. Step 2: Verify parameterized constructor sets name properly

   public class Person { String name; public Person() { name = "Unknown"; } public Person(String name) { this.name = name; } } uses 'this.name = name;' to assign parameter to instance variable.

3. Final Answer:

   public class Person { String name; public Person() { name = "Unknown"; } public Person(String name) { this.name = name; } } -> Option D

4. Quick Check:

   Default constructor sets default value, parameterized sets given value [OK]

Hint: Default constructor sets default values; use 'this' for parameters [OK]

Common Mistakes:

• Assigning parameter to itself without 'this.'
• Not setting default value in default constructor
• Missing default constructor entirely