Here is a short code to reproduce,

-------------Start of Code-------------
gtee@bela: /net/sqenfs-1/export1/comp/jsn/users/evergreen/wzhu/testcodes $ cat Test_US_ASCII_IsLegalReplace.java
import java.nio.charset.Charset;
import java.nio.charset.CharsetEncoder;

public class Test_US_ASCII_IsLegalReplace
{
  public static void main (String[]args)
  {
    CharsetEncoder enc = Charset.forName("US-ASCII").newEncoder();
    byte[] newsubs = { 'A', 'S'};
    boolean result = enc.isLegalReplacement (newsubs);
    System.out.println("isLegalReplacement: " + result);
  }
}
-------------End of Code-------------


gtee@bela: /net/sqenfs-1/export1/comp/jsn/users/evergreen/wzhu/testcodes $ /net/koori.sfbay/onestop/jdk/7/promoted/all/b52/binaries/linux-i586/bin/java Test_US_ASCII_IsLegalReplace
isLegalReplacement: true
gtee@bela: /net/sqenfs-1/export1/comp/jsn/users/evergreen/wzhu/testcodes $ /net/koori.sfbay/onestop/jdk/7/promoted/all/b53/binaries/linux-i586/bin/java Test_US_ASCII_IsLegalReplace
isLegalReplacement: false