MyContract.sol

// SPDX-License-Identifier: MIT
//
// https://cryptomarketpool.com/interface-in-solidity-smart-contracts/

pragma solidity ^0.8.0;

interface IERC20 {
    function totalSupply() external view returns (uint256);

    function balanceOf(address account) external view returns (uint256);

    function allowance(address owner, address spender) external view returns (uint256);

    function transfer(address recipient, uint256 amount) external returns (bool);

    function approve(address spender, uint256 amount) external returns (bool);

    function transferFrom(
        address sender,
        address recipient,
        uint256 amount
    ) external returns (bool);

    event Transfer(address indexed from, address indexed to, uint256 value);
    event Approval(address indexed owner, address indexed spender, uint256 value);
}

contract MyContract is IERC20 {
    string public constant name = "ERC20Basic";
    string public constant symbol = "ERC";
    uint8 public constant decimals = 18;

    mapping(address => uint256) balances;
    mapping(address => mapping(address => uint256)) allowed;
    uint256 totalSupply_ = 10 ether;

    constructor() {
        balances[msg.sender] = totalSupply_;
    }

    function totalSupply() public view override returns (uint256) {
        return totalSupply_;
    }

    function balanceOf(address tokenOwner) public view override returns (uint256) {
        return balances[tokenOwner];
    }

    function transfer(address receiver, uint256 numTokens) public override returns (bool) {
        require(numTokens <= balances[msg.sender]);
        balances[msg.sender] = balances[msg.sender] - numTokens;
        balances[receiver] = balances[receiver] + numTokens;
        emit Transfer(msg.sender, receiver, numTokens);
        return true;
    }

    function approve(address delegate, uint256 numTokens) public override returns (bool) {
        allowed[msg.sender][delegate] = numTokens;
        emit Approval(msg.sender, delegate, numTokens);
        return true;
    }

    function allowance(address owner, address delegate) public view override returns (uint256) {
        return allowed[owner][delegate];
    }

    function transferFrom(
        address owner,
        address buyer,
        uint256 numTokens
    ) public override returns (bool) {
        require(numTokens <= balances[owner]);
        require(numTokens <= allowed[owner][msg.sender]);

        balances[owner] = balances[owner] - numTokens;
        allowed[owner][msg.sender] = allowed[owner][msg.sender] + numTokens;
        balances[buyer] = balances[buyer] + numTokens;
        emit Transfer(owner, buyer, numTokens);
        return true;
    }
}