Android – how to replace part of a string by another string?

It is working, but it wont modify the caller object, but returning a new String.

So you just need to assign it to a new String variable, or to itself:

string = string.replace("to", "xyz");

or

String newString = string.replace("to", "xyz");

API Docs

public String replace (CharSequence target, CharSequence replacement) 

Since: API Level 1

Copies this string replacing
occurrences of the specified target
sequence with another sequence. The
string is processed from the beginning
to the end.

Parameters

• target the sequence to replace.
• replacement the replacement
  sequence.

Returns the resulting string.
Throws NullPointerException if target or replacement is null.