1. String .trim()
trim() removes leading and trailing spaces
2. str.replace(" ",""); removes all spaces, including leading and trailing spaces, and the middle
eg: String str = " hell o ";
String str2 = str.replaceAll(" ","");
-- System.out.println(str2);
3, or replaceAll(" +".""); Remove all spaces
4. str = .replaceAll("\s*",""); can replace most whitespace characters, not limited to spaces
s can match any whitespace characters such as spaces, tabs, form feeds, etc. a