time difference in various string concat methods

[Rajdeep Biswas]

Hi, I have a program wherein I run a loop a given number of times (iterationCount variable) concatenating a string to another string using 4 techniques: +, concat, StringBuffer and StringBuilder, and print out the time on screen for each of the 4 methods. Each method is allocated a different string to concatenate so as to avoid the constant pool interference between methods. Now

1. for iterationCount = 5000, the StringBuilder method's time is always 0ms. StringBuffer method's time is sometimes 0, sometimes ~16 ms.
2. for iterationCount = 50000, the StringBuffer method's time is always 0ms. StringBuilder method's time is sometimes 0, sometimes ~16 ms.

Q1. Why should one method in either case yield 0 always(5000→StringBuilder , 50000→StringBuffer )?
Q2. Why, upon changing the iterationCount from 5000 to 50000 or vice-versa, the 0 ms elapse time method is changed?

The output on my "intel Core i5 XP" machine is as follows:

Output

47ms over 5000 iterations for "+"
15ms over 5000 iterations for String concat method
16ms over 5000 iterations for StringBuffer append method
0ms over 5000 iterations for StringBuilder append method

47ms over 5000 iterations for "+"
31ms over 5000 iterations for String concat method
0ms over 5000 iterations for StringBuffer append method
0ms over 5000 iterations for StringBuilder append method

47ms over 5000 iterations for "+"
16ms over 5000 iterations for String concat method
0ms over 5000 iterations for StringBuffer append method
0ms over 5000 iterations for StringBuilder append method

47ms over 5000 iterations for "+"
15ms over 5000 iterations for String concat method
16ms over 5000 iterations for StringBuffer append method
0ms over 5000 iterations for StringBuilder append method

7515ms over 50000 iterations for "+"
2938ms over 50000 iterations for String concat method
0ms over 50000 iterations for StringBuffer append method
0ms over 50000 iterations for StringBuilder append method

7640ms over 50000 iterations for "+"
2985ms over 50000 iterations for String concat method
0ms over 50000 iterations for StringBuffer append method
0ms over 50000 iterations for StringBuilder append method

7547ms over 50000 iterations for "+"
2969ms over 50000 iterations for String concat method
0ms over 50000 iterations for StringBuffer append method
15ms over 50000 iterations for StringBuilder append method

7765ms over 50000 iterations for "+"
3000ms over 50000 iterations for String concat method
0ms over 50000 iterations for StringBuffer append method
16ms over 50000 iterations for StringBuilder append method

[Seetharaman Venkatasamy]

How are you calculating run time ? I presume using System.currentTimeMillis ?
Use System.nanoTime
<edit>
the key point is that, if you use + or concat then each loop one new object is created (StringBuilder) . hence bit time required.
</edit>

[Rajdeep Biswas]

Thank you for replying Seetharaman. I understand that. My doubts are earmarked as Q1 and Q2.
Edit: With nanoTime(), I could get the time properly and see the time difference. It was < 0 ms for some which are posted as 0 here.