public function get(string $index) : string
    {
        if (!$this->isValid($index)) {
            throw new \InvalidArgumentException('The specified value does not exist');
        }
        return $this->map[$index];
    }
public function get(string $index) : string
    {
        if (!$this->isValid($index)) {
            throw new \InvalidArgumentException('The specified value does not exist');
        }
        return $this->map[$index];
    }
public function get ( string $ index ) : string { if ( ! $ this -> isValid ( $ index ) ) { throw new \ InvalidArgumentException ( 'The specified value does not exist' ) ; } return $ this -> map [ $ index ] ; }
Retrieve a value

@param string $index
@return string
Retrieve a value