java中替换问号

String srt="abc?abc";
str.replaceAll("?", "#");
会提示这样的错误
Dangling meta character '?' near index 0
?
^
at java.util.regex.Pattern.error(Unknown Source)
at java.util.regex.Pattern.sequence(Unknown Source)
at java.util.regex.Pattern.expr(Unknown Source)
at java.util.regex.Pattern.compile(Unknown Source)
at java.util.regex.Pattern.<init>(Unknown Source)
at java.util.regex.Pattern.compile(Unknown Source)
at java.lang.String.replaceAll(Unknown Source)
str.replaceAll("[?]", "#");
这样写就可以了