Exercise1.sol

pragma solidity ^0.5.3;

contract SimpleStorage {
    
    function getValue() public pure returns(uint256){
        uint256 value = 1000;
        return value;
    }
    
}

Exercise2.sol

pragma solidity ^0.5.3;

contract SimpleStorage {
    uint256 value;
    
    constructor() public {
        value = 1000;    
    }
    
    function getValue() public view returns(uint256){
        return value;
    }
}

Exercise3.sol

pragma solidity ^0.5.3;

contract SimpleStorage {
    uint256 value;
    
    constructor() public {
        value = 1000;    
    }
    
    function getValue() public view returns(uint256){
        return value;
    }
    
    function setValue(uint256 _newValue) public {
        value = _newValue;
    }
}

Exercise4.sol

pragma solidity ^0.5.3;

contract SimpleStorage {
    uint256 value;
    address owner;
    
    constructor() public {
        value = 1000;    
        owner = msg.sender;
    }
    
    function getValue() public view returns(uint256){
        return value;
    }
    
    function setValue(uint256 _newValue) public {
        require(owner == msg.sender, 'Only owner can set value');
        value = _newValue;
    }
}

PromiseContract.sol

pragma solidity ^0.5.3;

contract PromiseContract {
    string promiseValue;
    address owner;
    address friend;
    
    constructor() public {
        owner = msg.sender;
        friend = 0x14723A09ACff6D2A60DcdF7aA4AFf308FDDC160C; // TODO Replace with the address of your friend
    }
    
    function whatIsThePromise() public view returns(string memory){
        return promiseValue;
    }
    
    function makePromise(string memory _promise) public {
        require(friend == msg.sender, 'Only a friend can make a promise');
        promiseValue = _promise;
    }
}