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.
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