Parameterized constructor in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of parameterized constructor with multiple objects

What is the output of the following Java program that uses a parameterized constructor to initialize objects?

Java

public class Car {
    String model;
    int year;

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

    public void display() {
        System.out.println(model + " " + year);
    }

    public static void main(String[] args) {
        Car car1 = new Car("Toyota", 2020);
        Car car2 = new Car("Honda", 2018);
        car1.display();
        car2.display();
    }
}

A

Toyota 2020
Honda 2018

B

Toyota 2018
Honda 2020

C

null 2020
null 2018

D Compilation error due to missing default constructor

💡 Hint

Look at how the constructor assigns values to the instance variables.

The parameterized constructor sets the model and year for each Car object. The display method prints these values. So car1 prints 'Toyota 2020' and car2 prints 'Honda 2018'.

❓ Predict Output

intermediate

Value of instance variable after parameterized constructor call

What will be the value of the instance variable 'price' after creating the object with the parameterized constructor?

Java

public class Book {
    double price;

    public Book(double price) {
        this.price = price;
    }

    public static void main(String[] args) {
        Book b = new Book(29.99);
        System.out.println(b.price);
    }
}

A Compilation error due to missing default constructor

B 0.0

C null

D 29.99

💡 Hint

The constructor sets the price variable directly.

The parameterized constructor assigns the value 29.99 to the instance variable price. So printing b.price outputs 29.99.

🔧 Debug

advanced

Identify the error in parameterized constructor usage

What error will this code produce when compiled and run?

Java

public class Student {
    String name;
    int age;

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

    public void display() {
        System.out.println(name + " " + age);
    }

    public static void main(String[] args) {
        Student s = new Student("Alice", 20);
        s.display();
    }
}

A Output: Alice 20

B Runtime NullPointerException

C Output: null 0

D Compilation error: variable name is not defined

💡 Hint

Check how the constructor assigns values to instance variables.

The constructor parameters shadow the instance variables. The assignments 'name = name;' and 'age = age;' assign the parameters to themselves, not to the instance variables. So instance variables remain default null and 0.

📝 Syntax

advanced

Which option causes a compilation error in parameterized constructor?

Which of the following constructor definitions will cause a compilation error?

A public Person(String name, int age) { name = name; age = age; }

B public Person(String name, int age) { this.name = name; this.age = age }

C public Person(String name, int age) { this.name = name; age = age; }

D public Person(String name, int age) { this.name = name; this.age = age; }

💡 Hint

Check for missing semicolons and correct syntax inside the constructor.

Option B is missing a semicolon after 'this.age = age', causing a compilation error.

🚀 Application

expert

Number of objects created and their state after parameterized constructor calls

Consider the following Java code. How many objects are created and what is the output when the main method runs?

Java

public class Employee {
    String name;
    int id;

    public Employee(String name, int id) {
        this.name = name;
        this.id = id;
    }

    public static void main(String[] args) {
        Employee e1 = new Employee("John", 101);
        Employee e2 = new Employee("Jane", 102);
        Employee e3 = e1;
        e3.name = "Mike";
        System.out.println(e1.name + " " + e1.id);
        System.out.println(e2.name + " " + e2.id);
    }
}

A

2 objects created; Output:
Mike 101
Jane 102

B

3 objects created; Output:
John 101
Jane 102

C

2 objects created; Output:
John 101
Jane 102

D

3 objects created; Output:
Mike 101
Jane 102

💡 Hint

Remember that e3 is a reference to e1, not a new object.

Only two Employee objects are created by the constructor calls. e3 points to the same object as e1. Changing e3.name changes e1.name. So output shows 'Mike 101' and 'Jane 102'.