Object creation in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Java code creating objects?

Consider the following Java class and main method. What will be printed when this code runs?

Java

class Box {
    int width;
    int height;
    Box(int w, int h) {
        width = w;
        height = h;
    }
    int area() {
        return width * height;
    }
}

public class Main {
    public static void main(String[] args) {
        Box b1 = new Box(3, 4);
        Box b2 = new Box(5, 6);
        System.out.println(b1.area() + "," + b2.area());
    }
}

A 0,0

B 3,5

C 12,30

D 7,11

💡 Hint

Remember area is width multiplied by height for each Box object.

The Box constructor sets width and height. The area method returns width * height. For b1: 3*4=12, for b2: 5*6=30.

🧠 Conceptual

intermediate

Which statement correctly creates an object of class Car?

Given a class named Car with a constructor that takes a String model, which of the following lines correctly creates a Car object?

Java

class Car {
    String model;
    Car(String m) {
        model = m;
    }
}

A Car myCar = new Car("Tesla");

B Car myCar = Car("Tesla");

C Car myCar = new Car();

D Car myCar = new Car;

💡 Hint

Remember to use the new keyword and provide the constructor argument.

Option A correctly uses new with the constructor and passes the required String argument. Option A misses new, A misses parentheses and argument, D misses argument.

🔧 Debug

advanced

What error does this object creation code produce?

Examine the code below. What error will the compiler show?

Java

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

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

A Compile-time error: missing semicolon

B Runtime NullPointerException

C Prints: Unknown

D Compile-time error: constructor Person(String) not found

💡 Hint

Check if the constructor with a String parameter exists.

The Person class only has a no-argument constructor. Trying to call Person("Alice") causes a compile error because no such constructor exists.

📝 Syntax

advanced

Which option correctly creates an array of 3 new Dog objects?

Given class Dog with a no-argument constructor, which code correctly creates an array of 3 Dog objects?

Java

class Dog {
    Dog() {}
}

A Dog[] dogs = new Dog[3];

B Dog[] dogs = {new Dog(), new Dog(), new Dog()};

C Dog dogs = new Dog[3];

D Dog[] dogs = new Dog(3);

💡 Hint

Remember that new Dog[3] creates an array but does not create Dog objects inside.

Option B creates an array and fills it with 3 new Dog objects. Option B creates an array of 3 null Dog references. Option B is invalid type assignment. Option B tries to call Dog constructor with int.

🚀 Application

expert

What is the value of count after creating objects?

Consider this class with a static count variable. What is the value of count after main runs?

Java

class Item {
    static int count = 0;
    Item() {
        count++;
    }
}

public class Main {
    public static void main(String[] args) {
        Item i1 = new Item();
        Item i2 = new Item();
        Item i3 = new Item();
        System.out.println(Item.count);
    }
}

A 3

B 0

C 1

D Compilation error

💡 Hint

Static variables are shared across all objects and incremented in constructor.

Each new Item increments count by 1. After 3 objects, count is 3.