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.

Practice

1. Which of the following is the correct way to create an object of class Car in Java?

easy

A. Car myCar = new Car();

B. Car myCar = Car();

C. new Car myCar();

D. Car myCar = new car();

Solution

1. Step 1: Understand object creation syntax

   In Java, objects are created using the new keyword followed by the class name and parentheses.

2. Step 2: Check each option

   Car myCar = new Car(); uses new Car() correctly with proper capitalization and assignment. Others have syntax errors or wrong capitalization.

3. Final Answer:

   Car myCar = new Car(); -> Option A

4. Quick Check:

   Use new ClassName() to create objects [OK]

Hint: Remember: new + ClassName() creates an object [OK]

Common Mistakes:

• Omitting the new keyword
• Using wrong capitalization for class name
• Missing parentheses after class name

2. Which of the following lines will cause a syntax error when creating an object of class Book?

easy

A. Book b=new Book();

B. Book b = new Book();

C. Book b = new Book;

D. Book b = new Book( );

Solution

1. Step 1: Recall syntax for object creation

   In Java, when creating an object, parentheses must follow the class name even if the constructor has no parameters.

2. Step 2: Identify the incorrect option

   Book b = new Book; misses the parentheses after new Book, causing a syntax error. Others are correct.

3. Final Answer:

   Book b = new Book; -> Option C

4. Quick Check:

   Always use parentheses after class name in new [OK]

Hint: Always include () after class name when using new [OK]

Common Mistakes:

• Forgetting parentheses after class name
• Confusing object creation with method calls
• Using semicolon inside parentheses

3. What will be the output of the following code?

class Dog {
    String name;
    Dog(String n) {
        name = n;
    }
}

public class Main {
    public static void main(String[] args) {
        Dog d = new Dog("Buddy");
        System.out.println(d.name);
    }
}

medium

A. Buddy

B. null

C. Dog@someHashCode

D. Compilation error

Solution

1. Step 1: Understand constructor assignment

   The constructor sets the name field to the string passed, which is "Buddy".

2. Step 2: Check output of print statement

   Printing d.name outputs the string "Buddy" stored in the object.

3. Final Answer:

   Buddy -> Option A

4. Quick Check:

   Constructor sets field, printing field shows assigned value [OK]

Hint: Constructor sets values; print field to see stored data [OK]

Common Mistakes:

• Expecting default null instead of assigned value
• Confusing object reference print with field print
• Missing constructor parameters

4. Identify the error in the following code snippet:

class Person {
    String name;
    Person(String n) {
        name = n;
    }
}

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

medium

A. Missing semicolon after System.out.println

B. Constructor name does not match class name

C. Variable p is not declared

D. Missing new keyword when creating object

Solution

1. Step 1: Check object creation syntax

   The code tries to create an object with Person("Alice") but misses the new keyword.

2. Step 2: Confirm other parts are correct

   Constructor name matches class name, variable is declared, and semicolon is present.

3. Final Answer:

   Missing new keyword when creating object -> Option D

4. Quick Check:

   Use new keyword to create objects [OK]

Hint: Always use new before class name to create objects [OK]

Common Mistakes:

• Omitting new keyword
• Confusing method call with object creation
• Incorrect constructor naming

5. Given the class below, which code correctly creates two Student objects with names "John" and "Jane" and prints their names?

class Student {
    String name;
    Student(String n) {
        name = n;
    }
}

hard

A. Student s1 = new Student("John");
Student s2 = new Student();
System.out.println(s1.name + ", " + s2.name);

B. Student s1 = new Student("John");
Student s2 = new Student("Jane");
System.out.println(s1.name + ", " + s2.name);

C. Student s1 = new Student();
Student s2 = new Student();
System.out.println(s1.name + ", " + s2.name);

D. Student s1 = Student("John");
Student s2 = Student("Jane");
System.out.println(s1.name + ", " + s2.name);

Solution

1. Step 1: Check constructor usage

   The constructor requires a String parameter. Student s1 = new Student("John"); Student s2 = new Student("Jane"); System.out.println(s1.name + ", " + s2.name); correctly passes names "John" and "Jane".

2. Step 2: Verify object creation and printing

   Student s1 = new Student("John"); Student s2 = new Student("Jane"); System.out.println(s1.name + ", " + s2.name); creates both objects properly and prints their names separated by a comma.

3. Final Answer:

   Student s1 = new Student("John"); Student s2 = new Student("Jane"); System.out.println(s1.name + ", " + s2.name); -> Option B

4. Quick Check:

   Use new with constructor parameters to create objects [OK]

Hint: Pass required parameters in new ClassName(params) [OK]

Common Mistakes:

• Calling constructor without new keyword
• Using default constructor when none exists
• Not passing required parameters