contains method Null safety
- covariant String singleCharacterString
singleCharacterString occurs in this
sequence of characters.
Returns true only if
a string containing a single character
and that character is one of the characters
in this character sequence, and false otherwise.
This behavior is inherited from
which is why it is not
Use containsAll for a method which acts like
String.contains for characters.
@override bool contains(covariant String singleCharacterString);