Interfaces in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an interface named IToken.

Blockchain / Solidity

interface [1] {
    function totalSupply() external view returns (uint256);
}

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

AToken

BInterfaceToken

CTokenInterface

DIToken

Attempts:

3 left

💡 Hint

Common Mistakes

Using a name without the 'I' prefix for the interface.

The interface name should start with 'I' to indicate it's an interface, so 'IToken' is correct.

2fill in blank

medium

Complete the code to declare a function in the interface that transfers tokens.

Blockchain / Solidity

function transfer(address to, uint256 amount) external [1];

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

Aview

Breturns (bool)

Cpure

Dpayable

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'view' or 'pure' for a function that changes state.

The transfer function returns a boolean indicating success, so 'returns (bool)' is correct.

3fill in blank

hard

Fix the error in the interface function declaration for balanceOf.

Blockchain / Solidity

function balanceOf(address owner) external [1] returns (uint256);

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

Aview

Bpayable

Cpure

Dexternal

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'payable' or 'pure' incorrectly in the function declaration.

The balanceOf function only reads data, so it should be marked as 'view'.

4fill in blank

hard

Fill both blanks to declare an interface with two functions: approve and allowance.

Blockchain / Solidity

interface IToken {
    function approve(address spender, uint256 amount) external [1];
    function allowance(address owner, address spender) external [2] returns (uint256);
}

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

Areturns (bool)

Bview

Cpure

Dpayable

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up 'view' and 'returns (bool)' in function declarations.

The approve function returns a boolean, so it uses 'returns (bool)'. The allowance function reads state, so it uses 'view'.

5fill in blank

hard

Fill all three blanks to declare an interface with functions: name, symbol, and decimals.

Blockchain / Solidity

interface IToken {
    function name() external [1] returns (string memory);
    function symbol() external [2] returns (string memory);
    function decimals() external [3] returns (uint8);
}

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

Aview

Bpure

Cpayable

Dreturns (string memory)

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'pure' or 'payable' incorrectly for these read-only functions.

All these functions only read data, so they should be marked as 'view'.

Practice

1.

What is the main purpose of an interface in blockchain smart contracts?

easy

A. To define a set of function signatures without implementation

B. To store data variables for the contract

C. To execute transactions automatically

D. To create user interfaces for blockchain apps

Solution

1. Step 1: Understand what an interface is

   An interface only declares function names and their inputs/outputs but does not provide code inside them.

2. Step 2: Identify the purpose in blockchain contracts

   Interfaces ensure different contracts agree on function names and types to interact properly.

3. Final Answer:

   To define a set of function signatures without implementation -> Option A

4. Quick Check:

   Interface = function signatures only [OK]

Hint: Interfaces declare functions without code to ensure consistency [OK]

Common Mistakes:

• Thinking interfaces store data variables
• Confusing interfaces with executable code
• Believing interfaces create UI elements

2.

Which of the following is the correct syntax to declare an interface named IPayment in Solidity?

?

easy

A. interface IPayment { function pay() external; }

B. contract IPayment { function pay() public {} }

C. interface IPayment { function pay() public {} }

D. contract IPayment { function pay() external; }

Solution

1. Step 1: Recall interface syntax in Solidity

   Interfaces use the keyword interface and declare functions without bodies, marked external.

2. Step 2: Check each option

   interface IPayment { function pay() external; } correctly declares an interface with an external function without implementation. Versions using contract are wrong. interface IPayment { function pay() public {} } has a function body which is not allowed in interfaces.

3. Final Answer:

   interface IPayment { function pay() external; } -> Option A

4. Quick Check:

   Interface + external function + no body = correct syntax [OK]

Hint: Interfaces use 'interface' keyword and external functions without bodies [OK]

Common Mistakes:

• Using 'contract' instead of 'interface'
• Adding function bodies inside interfaces
• Using 'public' instead of 'external' in interface functions

3.

Given the interface and contract below, what will be the output when calling getValue()?

interface IValue {
    function getValue() external view returns (uint);
}

contract Value is IValue {
    function getValue() external pure returns (uint) {
        return 42;
    }
}

medium

A. 42

B. 0

C. Runtime error

D. Compilation error

Solution

1. Step 1: Compare function signatures in interface and contract

   The interface declares getValue() as view, but the contract implements it as pure. These must match exactly.

2. Step 2: Understand Solidity rules for interface implementation

   Function modifiers like view and pure must match. Mismatch causes a compilation error.

3. Final Answer:

   Compilation error -> Option D

4. Quick Check:

   Modifier mismatch = compilation error [OK]

Hint: Function modifiers must match exactly in interface and implementation [OK]

Common Mistakes:

• Ignoring function modifier differences
• Assuming pure and view are interchangeable
• Expecting runtime error instead of compile error

4.

Identify the error in the following Solidity code implementing an interface:

interface ICalculator {
    function add(uint a, uint b) external returns (uint);
}

contract Calculator is ICalculator {
    function add(uint a, uint b) public returns (uint) {
        return a + b;
    }
}

medium

A. Contract should not use 'is' keyword

B. Function visibility mismatch between interface and contract

C. Interface cannot have functions with parameters

D. Missing function body in contract

Solution

1. Step 1: Check function visibility in interface and contract

   The interface function is declared as external, but the contract function is public. These must match.

2. Step 2: Understand Solidity interface implementation rules

   Function visibility must be the same; otherwise, it causes a compilation error.

3. Final Answer:

   Function visibility mismatch between interface and contract -> Option B

4. Quick Check:

   Visibility mismatch = compilation error [OK]

Hint: Match function visibility exactly between interface and contract [OK]

Common Mistakes:

• Using public instead of external in implementation
• Thinking interfaces cannot have parameters
• Ignoring 'is' keyword for inheritance

5.

You want to create a contract Token that implements two interfaces IERC20 and IERC721. Which of the following is the correct way to declare this contract?

hard

A. interface Token is IERC20, IERC721 { /* implementation */ }

B. contract Token implements IERC20, IERC721 { /* implementation */ }

C. contract Token is IERC20, IERC721 { /* implementation */ }

D. contract Token inherits IERC20, IERC721 { /* implementation */ }

Solution

1. Step 1: Recall Solidity syntax for multiple interface inheritance

   Solidity uses the keyword is to inherit from multiple interfaces or contracts.

2. Step 2: Evaluate each option

   contract Token is IERC20, IERC721 { /* implementation */ } correctly uses is with both interfaces. contract Token implements IERC20, IERC721 { /* implementation */ } uses implements which is invalid in Solidity. interface Token is IERC20, IERC721 { /* implementation */ } declares Token as an interface, not a contract. contract Token inherits IERC20, IERC721 { /* implementation */ } uses inherits which is not a Solidity keyword.

3. Final Answer:

   contract Token is IERC20, IERC721 { /* implementation */ } -> Option C

4. Quick Check:

   Use 'is' for multiple interface inheritance [OK]

Hint: Use 'is' keyword to inherit multiple interfaces in Solidity [OK]

Common Mistakes:

• Using 'implements' instead of 'is'
• Declaring contract as interface by mistake
• Using non-existent keywords like 'inherits'