You could either get the prefix and suffix subsequences  and put the
new char in the middle as such:

index i = 1;
String abc = "abc";
String adc = abc.substring(0, i) + "d" + abc.substring(i+1,
abc.length);

This is untested though and may throw IndexOutOfBounds if the index is
= length.

Or you could put this string in a StringBuffer and use its
setCharAt(int index, char ch) method.

index i = 1;
String abc = "abc";
StringBuffer abcBuffer = new StringBuffer(abc);
abcBuffer.setCharAt(i, 'd');
String adc = abcBuffer.toString();

There may be other ways to solve this, but these seemed like fairly
simple ways to do it, the second being simpler/safer it seems.

Scott

You can't, because String is designed to be immutable.
You should use StringBuffer (which is like a mutable String). It has a
setCharAt(...) method.

There are methods/constructors to convert a StringBuffer to String and v.v.

A String is immutable.
Use StringBuffer to create a new String.

Are you talking about the  indexOf("stringtoFind")   method of the String
object?
its closest to the opposite of charAt(n) I casn think of.
HTH
Hal

No can do.  You can construct a new String like this:

package cljp;

import java.io.PrintWriter;

public class Main {

    private static PrintWriter out = new PrintWriter(System.out, true);

    private static String replaceCharAt(String s, int index, char c) {
        StringBuilder b = new StringBuilder(s);
        b.setCharAt(index, c);
        return b.toString();
    }

    public static void main(String[] args) {
        String s = "abc";

        out.println(s); // abc
        s = replaceCharAt(s, 1, 'Q');
        out.println(s); // aQc
    }
}