I\'m wondering if I can use string.replace() to replace all alphabets in a string?
String sentence = \"hello world! 722\"
String str = sentence
Java's String#replaceAll takes a regex string as argument. Tha being said, [a-ZA-Z] matches any char from a to z (lowercase) and A to Z (uppercase) and that seems to be what you need.
String sentence = "hello world! 722";
String str = sentence.replaceAll("[a-zA-Z]", "@");
System.out.println(str); // "@@@@@ @@@@@! 722"
See demo here.