Parameterized constructor in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a parameterized constructor for the class.

Java

public class Car {
    String model;
    int year;

    public [1](String model, int year) {
        this.model = model;
        this.year = year;
    }
}

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

ACar

Bcar

CCarConstructor

DConstructor

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different name than the class for the constructor.

Using lowercase instead of uppercase for the constructor name.

In Java, the constructor name must exactly match the class name, which is 'Car' here.

2fill in blank

medium

Complete the code to create an object of the class using the parameterized constructor.

Java

Car myCar = new [1]("Toyota", 2020);

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

AVehicle

BCar

Ccar

DMyCar

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase 'car' instead of 'Car'.

Using a different class name.

To create an object, use the class name 'Car' with the new keyword.

3fill in blank

hard

Fix the error in the constructor declaration.

Java

public class Book {
    String title;
    int pages;

    public [1](String title, int pages) {
        this.title = title;
        this.pages = pages;
    }
}

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

ABook

Bbook

CsetBook

DBookConstructor

Attempts:

3 left

💡 Hint

Common Mistakes

Adding a return type to the constructor.

Using a different name than the class for the constructor.

Constructors should not have a return type like 'void' and must be named exactly as the class.

4fill in blank

hard

Fill both blanks to complete the parameterized constructor and assign values.

Java

public class Student {
    String name;
    int age;

    public [1](String name, int age) {
        this.[2] = name;
        this.age = age;
    }
}

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

AStudent

Bstudent

Cname

Dage

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase 'student' as constructor name.

Assigning to a wrong field name.

Constructor name must match class name 'Student'. Use 'name' to assign the parameter to the field.

5fill in blank

hard

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

Java

public class Laptop {
    String brand;
    int ram;

    public [1](String brand, int ram) {
        this.brand = [2];
        this.ram = ram;
    }

    public static void main(String[] args) {
        Laptop myLaptop = new [3]("Dell", 16);
    }
}

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

ALaptop

Bbrand

Dlaptop

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase 'laptop' instead of 'Laptop'.

Assigning wrong variable to the field.

Constructor name and object creation use class name 'Laptop'. Use parameter 'brand' to assign to field.

Practice

1.

What is the main purpose of a parameterized constructor in Java?

easy

A. To initialize an object with specific values when it is created

B. To create multiple objects without any initial values

C. To delete an object from memory

D. To define a method that returns a value

Solution

1. Step 1: Understand constructor purpose

   A constructor is used to initialize objects when they are created.

2. Step 2: Identify parameterized constructor role

   A parameterized constructor takes arguments to set initial values for the object's fields.

3. Final Answer:

   To initialize an object with specific values when it is created -> Option A

4. Quick Check:

   Parameterized constructor = initialize with values [OK]

Hint: Constructors with parameters set initial object values [OK]

Common Mistakes:

• Confusing constructors with methods
• Thinking constructors delete objects
• Believing constructors don't take parameters

2.

Which of the following is the correct syntax for a parameterized constructor in Java?

public class Car {
    String model;
    int year;

    // Constructor here
}

easy

A. void Car(String model, int year) { this.model = model; this.year = year; }

B. public Car(String model, int year) { this.model = model; this.year = year; }

C. public Car() { model = ""; year = 0; }

D. public void Car(String model, int year) { model = model; year = year; }

Solution

1. Step 1: Check constructor syntax

   A constructor has no return type and matches the class name exactly.

2. Step 2: Verify parameter usage

   public Car(String model, int year) { this.model = model; this.year = year; } correctly uses parameters and assigns them to fields with this.

3. Final Answer:

   public Car(String model, int year) { this.model = model; this.year = year; } -> Option B

4. Quick Check:

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

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

Common Mistakes:

• Adding return type to constructor
• Not using 'this' to assign fields
• Using void or other return types

3.

What will be the output of the following Java code?

class Book {
    String title;
    int pages;

    Book(String title, int pages) {
        this.title = title;
        this.pages = pages;
    }

    void display() {
        System.out.println(title + ": " + pages + " pages");
    }
}

public class Main {
    public static void main(String[] args) {
        Book b = new Book("Java Basics", 250);
        b.display();
    }
}

medium

A. Java Basics: 250 pages

B. Java Basics pages

C. 250 pages

D. Compilation error

Solution

1. Step 1: Understand constructor initialization

   The constructor sets title to "Java Basics" and pages to 250.

2. Step 2: Analyze display method output

   The display() method prints the title, colon, pages, and "pages" text.

3. Final Answer:

   Java Basics: 250 pages -> Option A

4. Quick Check:

   Constructor sets fields, display prints them [OK]

Hint: Constructor sets fields, display prints combined string [OK]

Common Mistakes:

• Ignoring 'this' keyword effect
• Expecting only title or pages printed
• Assuming syntax error without reason

4.

Identify the error in this Java class with a parameterized constructor:

public class Student {
    String name;
    int age;

    public Student(String n, int a) {
        name = n;
        age = a;
    }

    public Student() {
        name = "Unknown";
        age = 0;
    }

    public void Student(String name, int age) {
        this.name = name;
        this.age = age;
    }
}

medium

A. No default constructor defined

B. Missing return type in parameterized constructor

C. The method named Student with void return type is not a constructor

D. Fields name and age are not declared

Solution

1. Step 1: Check constructor definitions

   Constructors have no return type and match class name exactly.

2. Step 2: Identify method named Student with void

   The method public void Student(String name, int age) is not a constructor but a method, which is confusing.

3. Final Answer:

   The method named Student with void return type is not a constructor -> Option C

4. Quick Check:

   Constructors have no return type [OK]

Hint: Constructors never have a return type, not even void [OK]

Common Mistakes:

• Writing constructors with void return type
• Confusing methods with constructors
• Missing default constructor when needed

5.

Given the class below, what will be the output when creating two Employee objects with different parameters and printing their details?

class Employee {
    String name;
    double salary;

    public Employee(String name, double salary) {
        this.name = name;
        this.salary = salary;
    }

    public void printInfo() {
        System.out.println(name + " earns $" + salary);
    }
}

public class Test {
    public static void main(String[] args) {
        Employee e1 = new Employee("Alice", 50000);
        Employee e2 = new Employee("Bob", 60000);
        e1.printInfo();
        e2.printInfo();
    }
}

hard

A. Runtime error due to uninitialized fields

B. Alice earns $60000.0
Bob earns $50000.0

C. Compilation error due to missing default constructor

D. Alice earns $50000.0
Bob earns $60000.0

Solution

1. Step 1: Analyze object creation with parameters

   Two Employee objects are created with names and salaries passed to the parameterized constructor.

2. Step 2: Check printInfo output

   Each object's printInfo() prints the name and salary correctly.

3. Final Answer:

   Alice earns $50000.0 Bob earns $60000.0 -> Option D

4. Quick Check:

   Parameterized constructor sets fields, printInfo shows them [OK]

Hint: Each object holds its own values set by constructor [OK]

Common Mistakes:

• Mixing up values between objects
• Expecting errors without reason
• Assuming default constructor is needed here