Why design patterns improve quality in Blockchain / Solidity - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a reusable contract template.

Blockchain / Solidity

contract [1] {
    // common functions here
}

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

ATemplate

BMyContract

CMain

DToken

Attempts:

3 left

💡 Hint

Common Mistakes

Using specific contract names instead of a generic template.

Using a generic name like Template helps create reusable contract patterns.

2fill in blank

medium

Complete the code to implement a singleton pattern in a smart contract.

Blockchain / Solidity

contract Singleton {
    address private staticInstance;

    function getInstance() public view returns (address) {
        return [1];
    }
}

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

Amsg.sender

BstaticInstance

Cowner

Daddress(this)

Attempts:

3 left

💡 Hint

Common Mistakes

Returning the caller's address instead of the stored instance.

The singleton pattern returns the single stored instance address.

3fill in blank

hard

Fix the error in the code to properly use the factory pattern for contract creation.

Blockchain / Solidity

contract Factory {
    function create() public returns (address) {
        [1] newContract = new [1]();
        return address(newContract);
    }
}

contract Product {}

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

AContract

BFactory

CInstance

DProduct

Attempts:

3 left

💡 Hint

Common Mistakes

Using the factory contract name as the new contract type.

The factory creates new Product contracts, so the type must be Product.

4fill in blank

hard

Fill both blanks to implement an observer pattern in Solidity.

Blockchain / Solidity

contract Subject {
    address[] public observers;

    function [1](address observer) public {
        observers.[2](observer);
    }
}

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

AregisterObserver

Badd

Cpush

Dsubscribe

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect function or array method names.

The function to add observers is commonly named registerObserver, and push adds to the array.

5fill in blank

hard

Fill all three blanks to complete a decorator pattern example in Solidity.

Blockchain / Solidity

contract Base {
    function getValue() public pure virtual returns (uint) {
        return [1];
    }
}

contract Decorator is Base {
    Base base;
    constructor(Base _base) {
        base = _base;
    }
    function getValue() public view override returns (uint) {
        return base.getValue() [2] [3];
    }
}

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

A10

B+

C5

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using multiplication instead of addition.

Returning wrong base value.

The base returns 10, and the decorator adds 5 using the plus operator.

Practice

1. Why do design patterns improve the quality of blockchain code?

easy

A. They provide tested solutions that reduce errors and improve security.

B. They make the code run faster by optimizing blockchain transactions.

C. They automatically fix bugs in the blockchain code.

D. They replace the need for developers to write any code.

Solution

1. Step 1: Understand the role of design patterns

   Design patterns offer proven ways to solve common coding problems, which helps reduce errors and improve security.

2. Step 2: Compare other options

   Options A, B, and C describe unrealistic or incorrect benefits like automatic bug fixing or no coding needed.

3. Final Answer:

   They provide tested solutions that reduce errors and improve security. -> Option A

4. Quick Check:

   Tested solutions improve quality = D [OK]

Hint: Design patterns give proven solutions to avoid errors [OK]

Common Mistakes:

• Thinking design patterns speed up code execution
• Believing patterns fix bugs automatically
• Assuming patterns remove the need to code

2. Which of the following is the correct way to describe a design pattern in blockchain development?

easy

A. A tool that compiles blockchain smart contracts.

B. A reusable solution template for common blockchain coding problems.

C. A blockchain transaction that stores code automatically.

D. A database that holds blockchain user data.

Solution

1. Step 1: Define design pattern in blockchain

   A design pattern is a reusable solution template for common coding problems, helping developers write better code.

2. Step 2: Eliminate incorrect options

   Options A, B, and D describe unrelated blockchain concepts like transactions, compilers, or databases.

3. Final Answer:

   A reusable solution template for common blockchain coding problems. -> Option B

4. Quick Check:

   Design pattern = reusable solution template [OK]

Hint: Design patterns are templates, not blockchain data or tools [OK]

Common Mistakes:

• Confusing design patterns with blockchain transactions
• Mixing design patterns with compilers or databases

3. Consider this blockchain smart contract code snippet using a design pattern:

class SingletonContract:
    _instance = None

    def __new__(cls):
        if cls._instance is None:
            cls._instance = super().__new__(cls)
        return cls._instance

contract1 = SingletonContract()
contract2 = SingletonContract()
print(contract1 is contract2)

What will be the output?

medium

A. Error

B. False

C. None

D. True

Solution

1. Step 1: Understand the Singleton pattern in the code

   The Singleton pattern ensures only one instance of the class exists. Here, both contract1 and contract2 refer to the same instance.

2. Step 2: Evaluate the print statement

   Since contract1 and contract2 are the same object, 'contract1 is contract2' returns True.

3. Final Answer:

   True -> Option D

4. Quick Check:

   Singleton pattern means one instance = True [OK]

Hint: Singleton means one instance; identical objects print True [OK]

Common Mistakes:

• Assuming two objects are different instances
• Confusing 'is' with equality '=='
• Expecting an error due to class variable

4. This blockchain code tries to implement a Factory pattern but has an error:

class ContractFactory:
    def create_contract(type):
        if type == 'A':
            return ContractA()
        elif type == 'B':
            return ContractB()

factory = ContractFactory()
contract = factory.create_contract('A')

What is the error in this code?

medium

A. Factory pattern cannot create different contract types.

B. ContractFactory should inherit from ContractA.

C. Missing self parameter in create_contract method.

D. The create_contract method should be static.

Solution

1. Step 1: Check method definition in class

   In Python, instance methods must have 'self' as the first parameter. Here, 'create_contract' lacks 'self'.

2. Step 2: Understand impact of missing 'self'

   Without 'self', calling 'factory.create_contract' will raise a TypeError because the instance is not passed automatically.

3. Final Answer:

   Missing self parameter in create_contract method. -> Option C

4. Quick Check:

   Instance methods need self parameter = A [OK]

Hint: Instance methods always need self as first parameter [OK]

Common Mistakes:

• Thinking inheritance is required for Factory pattern
• Believing Factory pattern can't create multiple types
• Assuming static method is mandatory

5. You want to improve a blockchain smart contract's code quality by making it reusable and easier to maintain. Which design pattern should you apply and why? Options: A) Singleton - to ensure only one contract instance exists. B) Factory - to create different contract types from a single interface. C) Observer - to notify multiple contracts about state changes. D) Decorator - to add new features to contracts without changing their code.

hard

A. Factory - creates various contract types, improving reusability and maintenance.

B. Singleton - limits to one instance, not focused on reusability.

C. Observer - manages notifications, not primarily for reusability.

D. Decorator - adds features but can complicate maintenance.

Solution

1. Step 1: Identify goal - reusability and easier maintenance

   The Factory pattern helps by creating different contract types through a single interface, making code reusable and easier to maintain.

2. Step 2: Compare other patterns

   Singleton restricts instances, Observer handles notifications, and Decorator adds features but may increase complexity.

3. Final Answer:

   Factory - creates different contract types from a single interface. -> Option A

4. Quick Check:

   Reusability and maintenance = Factory pattern B [OK]

Hint: Factory pattern creates reusable contract types easily [OK]

Common Mistakes:

• Choosing Singleton for reusability
• Confusing Observer with reusability pattern
• Ignoring maintenance complexity with Decorator