public class Ineficient
{
	public static void main (String[] args)
	{
		String str = "";
		for (int i = 0; i < 20000; i++) str += i;
	}
}

Promig de tres execucions: 41.342s.

public class Eficient
{
	public static void main (String[] args)
	{
		StringBuffer strb = new StringBuffer ();
		for (int i = 0; i < 20000; i++) strb.append (i);
	}
}

Promig de tres execucions: 0.173s.

#!/usr/bin/env ruby
str = ""
20000.times { |i| str += "#{i}" }

Promig de tres execucions: 2.104s.

#!/usr/bin/env ruby
str = ""
20000.times { |i| str.concat("#{i}") }

Promig de tres execucions: 0.054s.