How to change characters mapping?

How to change characters mapping?

Any one have solution of it, to change the mapping of unknown characters in 
java?

Pl visit following link for more details.

<http://java.sun.com/products/jdk/1.1/intl/html/intlspec.doc7.html>http://ja 
va.sun.com/products/jdk/1.1/intl/html/intlspec.doc7.<http://java.sun.com/pro 
ducts/jdk/1.1/intl/html/intlspec.doc7.html>html

Unknown Characters
There is seldom a one to one mapping between two character encodings. When 
a converter encounters a character in the source encoding that cannot be 
represented in the target encoding, an UnknownCharacterException is thrown. 
This behavior can be turned off however by calling 
setSubstitutionMode(true). In substitution mode, a converter will replace 
unknown characters with a substitution character. When converting to 
Unicode the default substitution character is \ufffd, the Unicode 
Replacement Character. When converting to an external character encoding 
the default is `?'. A different substitution character can be used by 
calling setSubstitutionBytes() and setSubstitutionChars().

Pl help me.

Thanks
Skashyap