Why object-oriented programming is used in Java - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a class named Car.

Java

public class [1] {}

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

AObject

Bcar

CCar

DVehicle

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase for class names

Using unrelated names

The class name should start with a capital letter and match the intended object, here 'Car'.

2fill in blank

medium

Complete the code to create an object of class Car.

Java

Car myCar = new [1]();

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

ACar

BVehicle

Ccar

DObject

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase class names

Using wrong class names

To create an object, use the class name with 'new'. Here, 'Car' matches the class.

3fill in blank

hard

Fix the error in the method to return the car's speed.

Java

public int getSpeed() {
    return [1];
}

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

Aspeed

Bthis.speed

CSpeed

DgetSpeed

Attempts:

3 left

💡 Hint

Common Mistakes

Using method name instead of variable

Wrong case for variable name

Use 'this.speed' to refer to the instance variable speed inside the method.

4fill in blank

hard

Fill both blanks to define a constructor that sets the car's speed.

Java

public [1](int [2]) {
    this.speed = speed;
}

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

ACar

Bspeed

CCarSpeed

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Constructor name different from class

Parameter name mismatch

The constructor name must match the class name 'Car'. The parameter name is 'value' to assign to the instance variable 'speed'.

5fill in blank

hard

Fill all three blanks to create a method that increases the car's speed by a given amount.

Java

public void [1](int [2]) {
    this.speed [3] amount;
}

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

Aaccelerate

Bamount

C+=

DspeedUp

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong method name

Using '=' instead of '+='

Parameter name mismatch

The method name is 'accelerate', parameter is 'amount', and '+=' adds the amount to speed.

Practice

1. Why do programmers use object-oriented programming (OOP) in Java?

easy

A. To organize code by grouping data and actions into objects

B. To write code only once without any changes

C. To make programs run faster by skipping data

D. To avoid using any variables in the program

Solution

1. Step 1: Understand what OOP does

   OOP groups related data and actions into objects, making code organized.

2. Step 2: Compare options with OOP purpose

   Only To organize code by grouping data and actions into objects correctly describes grouping data and actions into objects.

3. Final Answer:

   To organize code by grouping data and actions into objects -> Option A

4. Quick Check:

   OOP groups data and actions = D [OK]

Hint: OOP groups data and actions into objects [OK]

Common Mistakes:

• Thinking OOP only makes code faster
• Believing OOP avoids variables
• Confusing code reuse with skipping changes

2. Which of the following is the correct way to define a simple class in Java?

easy

A. class Car int speed; void drive() {}

B. Car class { int speed; void drive() {} }

C. class Car { int speed; drive() void {} }

D. class Car { int speed; void drive() {} }

Solution

1. Step 1: Check Java class syntax

   Java classes start with 'class ClassName { ... }' and methods have return type before name.

2. Step 2: Validate each option

   class Car { int speed; void drive() {} } uses correct syntax: class keyword, braces, field with type, method with return type and braces.

3. Final Answer:

   class Car { int speed; void drive() {} } -> Option D

4. Quick Check:

   Correct class syntax = C [OK]

Hint: Class syntax: class Name { fields and methods } [OK]

Common Mistakes:

• Omitting 'class' keyword
• Missing braces {} around class body
• Incorrect method declaration order

3. What will be the output of this Java code?

class Dog {
  String name;
  void bark() {
    System.out.println(name + " barks");
  }
}
public class Test {
  public static void main(String[] args) {
    Dog d = new Dog();
    d.name = "Max";
    d.bark();
  }
}

medium

A. Maxbarks

B. barks Max

C. Max barks

D. Compilation error

Solution

1. Step 1: Understand the Dog class and method

   Dog has a name field and bark() prints name + " barks" with a space.

2. Step 2: Trace main method execution

   Creates Dog object d, sets d.name = "Max", calls d.bark() which prints "Max barks".

3. Final Answer:

   Max barks -> Option C

4. Quick Check:

   Prints name + " barks" = A [OK]

Hint: Prints field + string exactly as coded [OK]

Common Mistakes:

• Forgetting space between name and 'barks'
• Mixing order of printed words
• Assuming compilation error without reason

4. Find the error in this Java code that uses OOP:

class Cat {
  String name;
  void meow() {
    System.out.println(name + " meows");
  }
}
public class Test {
  public static void main(String[] args) {
    Cat c;
    c.name = "Luna";
    c.meow();
  }
}

medium

A. Method meow() is missing return type

B. Variable 'c' is not initialized before use

C. Class Cat is missing a constructor

D. String name should be static

Solution

1. Step 1: Check object creation

   Variable 'c' is declared but not assigned a new Cat object before use.

2. Step 2: Understand consequences

   Using c.name or c.meow() without initializing 'c' causes a runtime error (NullPointerException).

3. Final Answer:

   Variable 'c' is not initialized before use -> Option B

4. Quick Check:

   Uninitialized object causes error = A [OK]

Hint: Always create objects with 'new' before use [OK]

Common Mistakes:

• Thinking method lacks return type (void is valid)
• Assuming constructor is mandatory
• Believing fields must be static

5. You want to reuse code for different types of vehicles in Java. Which OOP feature helps you write a base class Vehicle and create specific classes like Car and Bike that share common code but also have their own details?

hard

A. Inheritance to extend Vehicle class for Car and Bike

B. Encapsulation to hide Vehicle data from Car and Bike

C. Polymorphism to create unrelated classes Car and Bike

D. Abstraction to write all code only in Vehicle class

Solution

1. Step 1: Understand code reuse in OOP

   Inheritance allows new classes to reuse code from a base class and add their own features.

2. Step 2: Match feature to scenario

   Vehicle is base class; Car and Bike extend it to share common code and add details.

3. Final Answer:

   Inheritance to extend Vehicle class for Car and Bike -> Option A

4. Quick Check:

   Code reuse via inheritance = B [OK]

Hint: Use inheritance to share and extend code [OK]

Common Mistakes:

• Confusing encapsulation with code reuse
• Thinking polymorphism creates unrelated classes
• Believing abstraction means no subclass code