String replaceAll not replacing i++;

just use replace() instead of replaceAll()

String preCode = "helloi++;world";
String newCode = preCode.replace("i++;", "");

or if you want replaceAll(), apply following regex

String preCode = "helloi++;world";
String newCode = preCode.replaceAll("i\\+\\+;", "");

Note : in the case of replace() the first argument is a character sequence, but in the case of replaceAll the first argument is regex

try this one

 public class Practice {
 public static void main(String...args) {
 String preCode = "Helloi++;world";
 String newCode = preCode.replace(String.valueOf("i++;"),"");
 System.out.println(newCode);
}  
}

The problem is the string that you are using to replace , that is cnsidered as regex pattern to skip the meaning you will have to use escape sequence like below.

String newCode = preCode.replaceAll("i\\+\\+;", "");