abstract.sol

// SPDX-License-Identifier: Unlicensed
pragma solidity ^0.8.0;

abstract contract HelloWorld {
   string public message;
    constructor(string memory _message){
       message = _message;
    }

    function getMessage() public virtual view returns (string memory){
        return message;
    }

    function setMessage(string memory _message) public virtual {}

    function defaultMessage() public  pure returns (string memory) 
    {
        return "Hello World";
    }
}

 contract UseHelloWorld is HelloWorld {

    uint public num;
    constructor(string memory _message, uint _num) HelloWorld(_message){
       num = _num;
    }

    function setNum(uint _num ) public {
        num = _num;
    }

    function getNum() public virtual view returns (uint256){
        return num;
    }

    function defaultNum() public pure returns (uint256){
         return 50;
     }

  function setMessage(string memory _message ) public override virtual {
        message = _message;
  }

 }