Class diagrams in LLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a class named Vehicle.

LLD

class [1]:
    pass

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

AVehicle

Bvehicle

CCar

DvehicleClass

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase names for class names.

Using unrelated class names.

The class name should be Vehicle with a capital V to follow naming conventions.

2fill in blank

medium

Complete the code to add an attribute named speed to the Vehicle class.

LLD

class Vehicle:
    def __init__(self, [1]):
        self.speed = speed

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

Aspeed_value

Bvelocity

Cspeed

Dspeedy

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different parameter name than the attribute name.

Forgetting to add the parameter in __init__.

The constructor parameter should be named speed to match the attribute assignment.

3fill in blank

hard

Fix the error in the method that returns the speed of the vehicle.

LLD

class Vehicle:
    def __init__(self, speed):
        self.speed = speed

    def get_speed(self):
        return [1].speed

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

Aself

Bspeed

CVehicle

Dthis

Attempts:

3 left

💡 Hint

Common Mistakes

Using the class name instead of self.

Using undefined variables.

The method should return self.speed to access the instance attribute.

4fill in blank

hard

Fill both blanks to define a subclass Car that inherits from Vehicle and adds a new attribute brand.

LLD

class [1]([2]):
    def __init__(self, speed, brand):
        super().__init__(speed)
        self.brand = brand

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

ACar

BVehicle

CBike

DTransport

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong class names for subclass or parent.

Forgetting to inherit from the parent class.

The subclass is named Car and it inherits from Vehicle.

5fill in blank

hard

Fill all three blanks to complete the class diagram code with an association: Garage has many Cars.

LLD

class Garage:
    def __init__(self):
        self.[1] = []

    def add_car(self, [2]):
        self.[3].append(car)

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

Acars

Bcar

Dvehicles

Attempts:

3 left

💡 Hint

Common Mistakes

Using inconsistent names for the list attribute.

Using wrong parameter names.

The Garage class has a list named cars to hold multiple Car objects. The method parameter is car, and the list cars is appended.

Practice

1. What does a class diagram primarily represent in system design?

easy

A. The structure of classes and their relationships

B. The flow of data between servers

C. The user interface layout

D. The sequence of method calls

Solution

1. Step 1: Understand the purpose of class diagrams

   Class diagrams show the static structure of a system by representing classes and their relationships.

2. Step 2: Differentiate from other diagrams

   Flow of data, UI layout, and method sequences are shown in other diagram types like sequence or activity diagrams.

3. Final Answer:

   The structure of classes and their relationships -> Option A

4. Quick Check:

   Class diagram = structure and relationships [OK]

Hint: Class diagrams show classes and links, not flow or UI [OK]

Common Mistakes:

• Confusing class diagrams with sequence diagrams
• Thinking class diagrams show UI design
• Assuming class diagrams show runtime behavior

2. Which of the following is the correct way to represent inheritance in a class diagram?

easy

A. A dashed line with a hollow circle pointing to the child class

B. A dashed line with an open arrow pointing to the child class

C. A solid line with a filled diamond pointing to the parent class

D. A solid line with a hollow triangle arrow pointing to the parent class

Solution

1. Step 1: Recall UML notation for inheritance

   Inheritance is shown by a solid line with a hollow triangle arrow pointing to the parent class.

2. Step 2: Identify incorrect options

   Filled diamonds represent composition, dashed lines with open arrows represent dependencies, and hollow circles are not standard UML symbols.

3. Final Answer:

   A solid line with a hollow triangle arrow pointing to the parent class -> Option D

4. Quick Check:

   Inheritance = solid line + hollow triangle [OK]

Hint: Inheritance uses solid line + hollow triangle arrow [OK]

Common Mistakes:

• Mixing composition and inheritance symbols
• Using dashed lines for inheritance
• Pointing arrow to child instead of parent

3. Given this class diagram snippet:
Class A - attr1: int + method1()
What does the '-' before attr1 and '+' before method1() mean?

medium

A. '-' means protected attribute; '+' means private method

B. '-' means private attribute; '+' means public method

C. '-' means public attribute; '+' means protected method

D. '-' means static attribute; '+' means abstract method

Solution

1. Step 1: Understand UML visibility symbols

   In UML, '-' means private visibility, '+' means public visibility.

2. Step 2: Apply to attributes and methods

   So, attr1 is private attribute, method1() is public method.

3. Final Answer:

   '-' means private attribute; '+' means public method -> Option B

4. Quick Check:

   Visibility: '-' private, '+' public [OK]

Hint: '-' = private, '+' = public in UML [OK]

Common Mistakes:

• Confusing visibility symbols with data types
• Assuming '-' means protected
• Mixing static/abstract with visibility

4. Identify the error in this class diagram description:
Class Car - engine: Engine - wheels: List<Wheel> -- engine "1" --- "4" wheels
What is wrong with the association notation?

medium

A. The association should use a hollow diamond for aggregation

B. The multiplicity numbers are reversed; wheels should be "4" and engine "1"

C. The association line uses double dashes instead of a single solid line

D. The class names should be connected with a dashed line

Solution

1. Step 1: Check association line style

   Associations use a single solid line, not double dashes.

2. Step 2: Verify multiplicity and line type

   Multiplicity "1" to "4" is correct; aggregation uses hollow diamond but not required here.

3. Final Answer:

   The association line uses double dashes instead of a single solid line -> Option C

4. Quick Check:

   Association = single solid line [OK]

Hint: Association lines are single solid lines, not double dashes [OK]

Common Mistakes:

• Using double dashes for associations
• Confusing aggregation with association
• Reversing multiplicity numbers

5. You need to design a class diagram for a library system where a Library has many Book objects, and each Book can have multiple Authors. Which of the following best represents this relationship?

hard

A. Library "1" --- "*" Book; Book "*" --- "*" Author with an association class for author details

B. Library "*" --- "1" Book; Book "1" --- "*" Author with inheritance

C. Library "1" --- "1" Book; Book "1" --- "1" Author with composition

D. Library "*" --- "*" Book; Book "*" --- "1" Author with aggregation

Solution

1. Step 1: Analyze multiplicity between Library and Book

   A library has many books, so multiplicity is "1" to "*".

2. Step 2: Analyze multiplicity between Book and Author

   A book can have multiple authors, and authors can write multiple books, so "*" to "*" is correct, often modeled with an association class for author details.

3. Step 3: Check other options for correctness

   Other options have incorrect multiplicities or wrong relationships like inheritance or composition.

4. Final Answer:

   Library "1" --- "*" Book; Book "*" --- "*" Author with an association class for author details -> Option A

5. Quick Check:

   Library to Book = 1 to many; Book to Author = many to many [OK]

Hint: Many-to-many needs association class; library has many books [OK]

Common Mistakes:

• Using inheritance instead of association
• Incorrect multiplicity numbers
• Confusing composition with aggregation