Default constructor in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a default constructor for the class.

Java

public class Car {
    String model;
    int year;

    public [1]() {
        model = "Unknown";
        year = 0;
    }
}

Drag options to blanks, or click blank then click option'

ADefaultConstructor

Bcar

CCar

DCar()

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase class name for constructor

Adding parentheses after the constructor name in the declaration

Using a different name than the class

In Java, the default constructor has the same name as the class and no parameters.

2fill in blank

medium

Complete the code to create an object using the default constructor.

Java

Car myCar = new [1]();

Drag options to blanks, or click blank then click option'

Acar

BCar

CMyCar

Dcar()

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase class name

Adding parentheses in the option name

Using a variable name instead of class name

To create an object, use the class name with 'new' and parentheses.

3fill in blank

hard

Fix the error in the constructor declaration.

Java

public class Book {
    String title;

    public [1]() {
        title = "Unknown";
    }
}

Drag options to blanks, or click blank then click option'

ABook

BBook()

Cbook

Dvoid Book

Attempts:

3 left

💡 Hint

Common Mistakes

Adding 'void' before constructor name

Using lowercase constructor name

Adding parentheses in the constructor name in declaration

Constructors do not have a return type and must have the same name as the class.

4fill in blank

hard

Fill both blanks to complete the default constructor and initialize the fields.

Java

public class Student {
    String name;
    int age;

    public [1]() {
        name = [2];
        age = 0;
    }
}

Drag options to blanks, or click blank then click option'

AStudent

B"Unknown"

C"Default"

Dstudent

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase constructor name

Assigning string without quotes

Using wrong string value

The constructor name must match the class name, and string values must be in quotes.

5fill in blank

hard

Fill all three blanks to complete the class with a default constructor and create an object.

Java

public class Laptop {
    String brand;
    int price;

    public [1]() {
        brand = [2];
        price = [3];
    }
}

Laptop myLaptop = new [4]();

Drag options to blanks, or click blank then click option'

ALaptop

B"Generic"

C1000

Dlaptop

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase class name

Missing quotes around string

Using string for price instead of number

Constructor name matches class name, string values use quotes, and price is an integer.

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