public boolean isNumber(String s) {    s = s.trim();        boolean pointSeen = false;    boolean eSeen = false;    boolean numberSeen = false;    boolean numberAfterE = true;    for(int i=0; i<s.length(); i++) {        if(‘0‘ <= s.charAt(i) && s.charAt(i) <= ‘9‘) {            numberSeen = true;            numberAfterE = true;        } else if(s.charAt(i) == ‘.‘) {            if(eSeen || pointSeen) {                return false;            }            pointSeen = true;        } else if(s.charAt(i) == ‘e‘) {            if(eSeen || !numberSeen) {                return false;            }            numberAfterE = false;            eSeen = true;        } else if(s.charAt(i) == ‘-‘ || s.charAt(i) == ‘+‘) {            if(i != 0 && s.charAt(i-1) != ‘e‘) {                return false;            }        } else {            return false;        }    }        return numberSeen && numberAfterE;}We start with trimming.    If we see [0-9] we reset the number flags.    We can only see . if we didn‘t see e or ..    We can only see e if we didn‘t see e but we did see a number. We reset numberAfterE flag.    We can only see + and - in the beginning and after an e    any other character break the validation.At the and it is only valid if there was at least 1 number and if we did see an e then a number after it as well.