I wish to convert all the lowercase letters of a user given string to '#'.But upon running the code,only the last lowercase char is getting converted

I wish to convert all the lowercase letters of a user given string to '#'. But upon running the code, only the last lowercase char is getting converted. Could someone suggest a solution to this ? (I am new in java)

import java.util.Scanner;

class replaceEx{
    public static void main(String args[])
    {
        Scanner s=new Scanner(System.in);
        String s1,s2=null;
        s1=s.nextLine();
        for(int i=0;i<s1.length();i++)
        {
            if(s1.charAt(i)>='a' && s1.charAt(i)<='z')
            s2=s1.replace(s1.charAt(i),'#');
        }
        System.out.println(s2);
    }
}

Input : ABCDabcd

Output: ABCDabc#

Solution

Here:

s2=s1.replace(s1.charAt(i),'#');

What happens is: s2 becomes s1, where that char at i gets replaced with '#'.

You assume that somehow s2 remembers previous assignments. It doesn't.

In other words: your code does call:

s2=s1.replace('a','#');
s2=s1.replace('b','#');
s2=s1.replace('c','#');
s2=s1.replace('d','#');

So, s2 becomes assigned ABCD#bcd, ABCDa#cd, ABCDab#d, ABCDabc#. And as said: only that last assignment sticks. Worse: replace() doesn't care about that index i that you used for charAt(). It simply replaces the first occurrence of the char to look for. In other words: if your word is "Aaba", you always keep replacing only that first 'a' with #.

In other words: your whole approach can't work, for multiple reasons.

There are multiple ways to solve this, for example to use replaceAll(). That method also allows to use regular expressions, so that you can say (in one call) "replace all lowercase chars with #" easily.

Another approach would be:

create a StringBuilder sb
iterate your s1 string
if a char is a lowercase letter: append # to sb
otherwise append the char
in the end, turn sb into a real string