Classes and objects in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of method call on object

What is the output of this Java program?

Java

public class Car {
    String brand;
    int year;

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

    public void displayInfo() {
        System.out.println(brand + " " + year);
    }

    public static void main(String[] args) {
        Car myCar = new Car("Toyota", 2015);
        myCar.displayInfo();
    }
}

A 2015 Toyota

B Toyota 2015

C Car@15db9742

D Compilation error

💡 Hint

Look at what the displayInfo method prints.

The displayInfo method prints the brand followed by the year separated by a space. So it prints "Toyota 2015".

🧠 Conceptual

intermediate

Understanding object reference assignment

What will be the output of this Java code snippet?

Java

class Box {
    int size;

    Box(int size) {
        this.size = size;
    }
}

public class Test {
    public static void main(String[] args) {
        Box b1 = new Box(5);
        Box b2 = b1;
        b2.size = 10;
        System.out.println(b1.size);
    }
}

A 10

B 5

C Compilation error

D Runtime error

💡 Hint

Remember that b1 and b2 refer to the same object.

Both b1 and b2 point to the same Box object. Changing size via b2 changes the same object that b1 points to, so b1.size is 10.

🔧 Debug

advanced

Identify the error in constructor usage

What error will this Java code produce when compiled?

Java

public class Person {
    String name;
    int age;

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

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

    public static void main(String[] args) {
        Person p = new Person("Alice", 30);
        System.out.println(p.name + " " + p.age);
    }
}

A Compilation error: no constructor Person(String, int)

B Runtime error: NullPointerException

C Output: Alice 0

D Output: Unknown 30

💡 Hint

Check the constructors defined and the one used in main.

The class defines constructors with zero and one parameter, but main tries to call a constructor with two parameters which does not exist, causing a compilation error.

📝 Syntax

advanced

Identify the syntax error in class definition

Which option correctly fixes the syntax error in this Java class?

Java

public class Animal {
    String type
    int age;

    public Animal(String type, int age) {
        this.type = type;
        this.age = age;
    }
}

A Remove the constructor

B Change int age to Integer age

C Add a constructor return type: public void Animal(String type, int age)

D Add a semicolon after String type: String type;

💡 Hint

Look carefully at the variable declarations.

The field declaration 'String type' is missing a semicolon, causing a syntax error. Adding the semicolon fixes it.

🚀 Application

expert

Determine the number of objects created

How many objects are created when this Java program runs?

Java

public class Node {
    int value;
    Node next;

    public Node(int value) {
        this.value = value;
        this.next = null;
    }

    public static void main(String[] args) {
        Node first = new Node(1);
        Node second = new Node(2);
        first.next = second;
        Node third = second;
    }
}

A 1

B 3

C 2

D 0

💡 Hint

Count how many times 'new' is used.

Only two objects are created with 'new Node(1)' and 'new Node(2)'. The variable 'third' just references the existing second object.