Unicode char - hex code

Input:
Tiếng Việt có dấu, đỡ đi em

Output:
Ti\u1ebfng Vi\u1ec7t c\u00f3 d\u1ea5u, \u0111\u1ee1 \u0111i em!


JavaScript function:
function dec2hex(i)
{
  var result = "0000";
  if      (i >= 0    && i <= 15)    { result = "\\u000" + i.toString(16); }
  else if (i >= 16   && i <= 255)   { result = "\\u00"  + i.toString(16); }
  else if (i >= 256  && i <= 4095)  { result = "\\u0" + i.toString(16); }
  else if (i >= 4096 && i <= 65535) { result = "\\u"  + i.toString(16); }
  return result;
}



x = prompt("String", "...");
if (x != null) {
l = x.length;
y = '';
for (i = 0; i < l; i++) {
  c = x.charCodeAt(i);
  if (c > 128)
  y = y + dec2hex(c);
  else
  y = y+x.charAt(i);
}
document.write(y);
document.close();

Java function:
...
isr = new InputStreamReader(is,"UTF8");
buffer = new StringBuffer();
int ch;
while ((ch = isr.read()) > -1) {
   if (ch < 128) {
      System.out.print((char) ch);
   } else {
      System.out.print("\\u" + str_pad(java.lang.Integer.toHexString(ch), 4, '0'));
   }
 
 buffer.append((char)ch);
}
...


public static String str_pad(String str, int size, char padChar) {
  StringBuffer padded = new StringBuffer(str);
  while (padded.length() < size) {
    padded.insert(0, padChar);
  }
  return padded.toString();
}