Classes and objects in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a class named Car.

Java

public class [1] {
    // class body
}

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

Acar

BCar

CVehicle

DcarClass

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase names for class names.

Using a different name than the one intended.

The class name should be Car to match the declaration.

2fill in blank

medium

Complete the code to create an object of class Car named myCar.

Java

Car [1] = new Car();

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

ACar

BnewCar

CcarObject

DmyCar

Attempts:

3 left

💡 Hint

Common Mistakes

Using the class name as the variable name.

Using a name not mentioned in the instruction.

The variable name for the object should be myCar as specified.

3fill in blank

hard

Fix the error in the constructor declaration of class Car.

Java

public class Car {
    public [1]() {
        System.out.println("Car created");
    }
}

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

ACar

Bcar

Cvoid Car

DCar()

Attempts:

3 left

💡 Hint

Common Mistakes

Adding a return type to the constructor.

Using lowercase or different names for the constructor.

Constructors have no return type and must have the same name as the class, here Car.

4fill in blank

hard

Fill both blanks to define a method that returns the car's model name.

Java

public class Car {
    private String model;

    public [1] [2]() {
        return model;
    }
}

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

AString

Bint

CgetModel

DsetModel

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'int' as return type for a String variable.

Naming the method 'setModel' instead of 'getModel'.

The method should return a String and be named getModel to follow naming conventions.

5fill in blank

hard

Fill all three blanks to create a constructor that sets the model name.

Java

public class Car {
    private String model;

    public [1]([2] model) {
        this.[3] = model;
    }
}

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

ACar

BString

Cmodel

DcarModel

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong constructor name.

Using wrong parameter type.

Assigning to a wrong field name.

The constructor name is Car, it takes a String parameter named model, and assigns it to the field this.model.

Practice

1. What is the main purpose of a class in Java?

easy

A. To serve as a blueprint for creating objects

B. To execute the program's main method

C. To store data permanently on disk

D. To perform input and output operations

Solution

1. Step 1: Understand the role of a class

   A class defines the structure and behavior that objects created from it will have.

2. Step 2: Identify the correct purpose

   Classes are not for running programs or storing data on disk; they are blueprints for objects.

3. Final Answer:

   To serve as a blueprint for creating objects -> Option A

4. Quick Check:

   Class = blueprint for objects [OK]

Hint: Classes define objects' structure and behavior [OK]

Common Mistakes:

• Confusing classes with methods
• Thinking classes store data permanently
• Believing classes execute the program

2. 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. Car myCar = new Car();

D. new Car myCar = Car();

Solution

1. Step 1: Recall object creation syntax

   In Java, objects are created using the new keyword followed by the class constructor with parentheses.

2. Step 2: Check each option

   Car myCar = new Car(); uses correct syntax: Car myCar = new Car();. Others miss parentheses or have wrong order.

3. Final Answer:

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

4. Quick Check:

   Use new ClassName() to create objects [OK]

Hint: Use 'new ClassName()' to create objects [OK]

Common Mistakes:

• Omitting parentheses after class name
• Placing 'new' keyword incorrectly
• Missing semicolon at the end

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

class Dog {
  String name;
  void bark() {
    System.out.println(name + " says Woof!");
  }
}

public class Main {
  public static void main(String[] args) {
    Dog dog1 = new Dog();
    dog1.name = "Buddy";
    dog1.bark();
  }
}

medium

A. null says Woof!

B. Woof!

C. Compilation error

D. Buddy says Woof!

Solution

1. Step 1: Understand object property assignment

   The object dog1 has its name set to "Buddy" before calling bark().

2. Step 2: Analyze the bark method output

   The method prints the name followed by " says Woof!" so it prints "Buddy says Woof!".

3. Final Answer:

   Buddy says Woof! -> Option D

4. Quick Check:

   Object property used in method = Buddy says Woof! [OK]

Hint: Check object fields before method calls for output [OK]

Common Mistakes:

• Assuming default null value prints
• Forgetting to assign the name
• Thinking method prints only 'Woof!'

4. Identify the error in the following code snippet:

class Person {
  String name;
  void setName(String name) {
    name = name;
  }
}

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

medium

A. Missing semicolon after setName method

B. The method setName does not assign the parameter to the instance variable

C. Cannot print p.name directly

D. Class Person should be public

Solution

1. Step 1: Analyze the setName method

   The assignment name = name; assigns the parameter to itself, not to the instance variable.

2. Step 2: Understand instance variable shadowing

   To assign correctly, use this.name = name; to refer to the instance variable.

3. Final Answer:

   The method setName does not assign the parameter to the instance variable -> Option B

4. Quick Check:

   Use 'this' to assign instance variables [OK]

Hint: Use 'this.variable' to refer to instance variables [OK]

Common Mistakes:

• Confusing parameter and instance variable names
• Forgetting 'this' keyword
• Assuming assignment works without 'this'

5. Given the class below, how can you create a method that returns a new Rectangle object with double the width and height of the current one?

class Rectangle {
  int width;
  int height;

  Rectangle(int w, int h) {
    width = w;
    height = h;
  }

  // Your method here
}

hard

A. Rectangle doubleSize() { return new Rectangle(width * 2, height * 2); }

B. void doubleSize() { width *= 2; height *= 2; }

C. Rectangle doubleSize() { width *= 2; height *= 2; return this; }

D. Rectangle doubleSize() { return new Rectangle(width + 2, height + 2); }

Solution

1. Step 1: Understand the requirement

   The method should return a new Rectangle object with width and height doubled, without changing the current object.

2. Step 2: Evaluate each option

   Rectangle doubleSize() { return new Rectangle(width * 2, height * 2); } creates and returns a new Rectangle with doubled dimensions. void doubleSize() { width *= 2; height *= 2; } changes current object and returns void. Rectangle doubleSize() { width *= 2; height *= 2; return this; } changes current object and returns it. Rectangle doubleSize() { return new Rectangle(width + 2, height + 2); } adds 2 instead of doubling.

3. Final Answer:

   Rectangle doubleSize() { return new Rectangle(width * 2, height * 2); } -> Option A

4. Quick Check:

   Return new object with doubled size = Rectangle doubleSize() { return new Rectangle(width * 2, height * 2); } [OK]

Hint: Return new object; don't modify current one [OK]

Common Mistakes:

• Modifying current object instead of returning new
• Adding instead of multiplying dimensions
• Returning void instead of new object