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From: Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid>
Newsgroups: comp.lang.java.programmer
Subject: Re: StringBuilder.append and CharSequence
Date: Thu, 26 Sep 2013 23:11:58 +0200
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On 2013-09-26 19:37, Andreas Leitgeb allegedly wrote:
> markspace <markspace@nospam.nospam> wrote:
>> On 9/25/2013 1:02 PM, markspace wrote:
>>> On 9/25/2013 7:19 AM, Andreas Leitgeb wrote:
>>>> Is it safe to use "stringBuilder.append(this);" in a CharSequence
>>>> implementation's .toString(), or could this lead to a recursive
>>>> loop, if some version of a Java class library implements the
>>>> append() for CharSequence by using it's toString() ?
>>> [...]
>> Actually, I mis-read your problem statement, I think.  Having 
>> implemented a couple of CharSequence classes, the only rational approach 
>> would be to copy the chars out of a CharSequence one at a time with a 
>> loop and charAt().  toString() must create an immutable String and so 
>> must create a new buffer, so that's an extra buffer copy, which seems 
>> undesirable no matter who's implementing the append() method.
> 
> That's my point. My outset is, that eventually I need to append a number
> of copies of some particular char to a StringBuilder, and my approach is:
> 
>    static class ChSeq implements CharSequence {
>       int m_len; char m_ch;
>       public ChSeq(int len, char ch) { m_len = len; m_ch = ch; }
>       public char charAt(int pos) { return m_ch; }// OutOfBounds? meh!
>       public int length() { return m_len; }
>       public String toString() { // just 'cause it's required...
>         return new StringBuilder(this).toString(); }
>       public CharSequence subSequence(int fr,int to) {
>         return ChSeq(to-fr,m_ch); }
>    } // not yet tested!
>    // then later:
>    sb.append( new ChSeq(n,ch) );
> 
> This would look like a reasonable OO approach to the problem, but
> if StringBuilder.append actually used my .toString(), then it would
> of course crash&burn.
> 
> If instead I had to implement the toString() by doing the loop myself
> and hammer the char n times to the temporary StringBuilder, then I'd
> instead just write a static method that would take sb,n,ch and feed n
> copies of ch to sb.
> 

I'd make the toString() as follows to be on the safe side:
  char[] cs = new char[ m_len ];
  Arrays.fill( cs, m_ch );
  return new String(cs);

If that hits performance, then you have got a conflict between your goal
of being OO and that of performance, and you'll have to scrap one.

-- 
DF.