These few lines of code are giving me a "java.lang.ArrayIndexOutOfBoundsException" exception, could someone please take a look and point out why (the exception is caused in the second arraycopy() call):

byte [] newContentBytes = EntityUtils.toByteArray((serverResponse.getEntity()));
  newContent = new String(newContentBytes);
  System.out.println( newContent);
  byte [] headerBytes = headers.getBytes();
  byte[] res = new byte[newContentBytes.length + headerBytes.length];
  //headerBytes.
  System.arraycopy(headerBytes, 0, res, 0, headerBytes.length);
  System.out.println( "length: " + newContentBytes.length);
  System.arraycopy(newContentBytes, 0, res, newContentBytes.length , newContentBytes.length);

The problem is in allocating res size, for example if I write new byte[newContentBytes.length + headerBytes.length+ 2000] instead the exception doesn't occur, so what should the accurate size be?