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I am working with some Java code. Basically I have a wrapper class that holds a string and implements some of the many useful python string methods in Java. My goal here is to implement the Python method .ljust and my hope is to be as efficient as possible. Currently, I am using a while loop which I imagine is terrible inefficient especially because it includes a +=. What's more, I am not sure which of the following alternatives is more efficient

String news = "";
news = this.toString();
int length = news.length();
while (length < size) {
        news += " ";
        length++;
}
return new PythonString(news);

In the above case, news.length() is only called once, but now there is an extra int and a new instruction length++.

The other option would be

String news = "";
news = this.toString();
while (news.length() < size) {
        news += " ";
}
return new PythonString(news);

In this case, news.length() is called every time.

I am interested in perspectives on which of these alternatives is most efficient as well as any other optimizations you could provide for this method, which is meant to pad a string with size spaces to the right so that all following characters line up at the same column.

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    \$\begingroup\$ you should actually measure the differences. There are several options to do this, as explained in the answers. \$\endgroup\$ – eis Dec 24 '15 at 11:29
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I am interested in perspectives on which of these alternatives is most efficient

Using StringBuilder in the loop instead of += will likely be far faster than either of your current approaches, especially as size - length gets big. I'm not sure how much more space it would take.

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    \$\begingroup\$ Space-wise, likely normally less as you won't need to constantly reallocate. \$\endgroup\$ – Veedrac Dec 25 '15 at 10:27
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The String class can do this already, see answer here: https://stackoverflow.com/a/391978/4217399. You can always look at the source of the String class to check how it's done, should be fairly optimized there, as it's a part of the JDK. It's actually offloaded to the java.util.Formatter class.

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  • \$\begingroup\$ I thought I looked through the documentation, that is why i implemented these, but thanks for the info \$\endgroup\$ – Tom Dec 24 '15 at 16:13
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What your code seems to be doing is to add spaces at the end of a string to make it at least size long.

You could write it in a way that better expresses your intent:

String news = this.toString(); //no need to allocate an empty string first
int padding = size - news.length();

return new PythonString(appendSpaces(news, padding));

And have a utility method that does the appending:

private static String appendSpaces(String s, int n) {
  if (n <= 0) return s;

  char[] spaces = new char[n];
  Arrays.fill(spaces, ' ');
  return s.concat(new String(spaces));
}

This will also be more efficient than your approach of concatenating the spaces one by one.

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  • \$\begingroup\$ This. No need to reinvent the wheel when all the cogs are already there! \$\endgroup\$ – Boris the Spider Dec 24 '15 at 12:55

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