What best describes the functionality of the following method?

public int mystery$($String s$)$

$\{$

char $[]$ letters $=$ s$.$toCharArray$()$;

int $x=0$;

for $($int $i=0$; $i$ letters. length; $i++)\{$

if $($letters$[$i$]==\text{'}\text{'})\{$

letters $[i]=\text{'}\_\text{'}{?}^{\text{;}}$

$x++$;

$\}$

$\}$

return $x$;

$\}$