Interfaces in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a Solidity Interface Call

Consider the following Solidity interface and contract. What will be the output when getValue() is called on MyContract?

Blockchain / Solidity

pragma solidity ^0.8.0;

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

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

A Compilation error due to missing implementation

B 42

C 0

D Runtime error due to interface call

💡 Hint

The contract implements the interface function and returns a fixed number.

The contract MyContract implements the interface IValue and overrides getValue() to return 42. Calling getValue() returns 42.

🧠 Conceptual

intermediate

Purpose of Interfaces in Smart Contracts

What is the main purpose of using interfaces in blockchain smart contracts?

A To define a contract's external functions without implementation, enabling interaction with other contracts

B To store data permanently on the blockchain

C To execute complex computations off-chain

D To create user interfaces for decentralized applications

💡 Hint

Think about how contracts communicate with each other.

Interfaces declare function signatures without implementation. They allow contracts to interact by specifying how to call external contracts without needing their full code.

🔧 Debug

advanced

Identify the Error in Interface Implementation

What error will this Solidity contract produce?

Blockchain / Solidity

pragma solidity ^0.8.0;

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

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

A No error, contract compiles and runs correctly

B Runtime error: Function add not found

C Compiler error: Missing 'override' keyword in function add

D Compiler error: Interface functions cannot be pure

💡 Hint

Check the function signature and keywords for interface implementation.

The contract function implementing the interface must include the 'override' keyword. Missing it causes a compiler error.

📝 Syntax

advanced

Correct Interface Syntax in Solidity

Which option shows the correct syntax for declaring an interface with a function transfer that takes an address and uint and returns a bool?

A interface IToken { function transfer(address to, uint amount) external returns (bool); }

B interface IToken { function transfer(address to, uint amount) public returns (bool) {} }

C interface IToken { function transfer(address to, uint amount) external {} }

D interface IToken { function transfer(address to, uint amount) external returns bool; }

💡 Hint

Interfaces declare function signatures without bodies and use 'external' visibility.

Option A correctly declares the function signature with 'external' visibility and return type, without a function body.

🚀 Application

expert

Number of Functions in a Contract Implementing Multiple Interfaces

Given these interfaces and contract, how many functions must MultiContract implement?

Blockchain / Solidity

pragma solidity ^0.8.0;

interface IA {
    function foo() external;
    function bar() external;
}

interface IB {
    function bar() external;
    function baz() external;
}

contract MultiContract is IA, IB {
    // Implementations here
}

A 5

B 4

C 2

D 3

💡 Hint

Count unique function signatures from both interfaces.

Functions foo, bar, and baz must be implemented. 'bar' appears in both interfaces but only needs one implementation. Total is 3.