Java/J2EE interview questions: - Is using StringBuffer always yields better performance than concatenation using +?

No, String Buffer need not yield a good performance over String concatenation using "+".

The type of concatenation plays a major role in deciding performance. If the concatenation can be completely resolved at coimpile time then String concatenation using + will yield better results
On the other hand if concatenation is done at run time using StringBuffer is more advisable, since StringBuffer objects are resolved at run time

Following are the two programs one showing compile time resolution and other run time and thus showing where StringBuffer will be useful.

Compile time resolution: 


This program takes String literals and performs concatenation using + as well as StringBuffer. Since the String literals could be resolved at compile time itself, this indicates a case of compile time resolution.


Code is self explanatory with proper comments given where ever required.
On executing the above code we get following result

Result:

Concatenation using + takes 0 millisecs
Concatenation using StringBuffer takes 63 millisecs
Performance using concatenation using + better than StringBuffer concatenation.

Conclusion: Since the above case if of compile time resolution performance using + concatenation yields a better result.

Run Time resolution:


Just a small modification to compile time code shown above can give us run time case for further analysis.

We change the concatenation used above to String conResult =""; conResult = conResult +"Welcome to the World of Java" +"Performance" +"@"+"questpond";

a similar change is done in StringBuffer . code snippet after change will look like





Result :

Concatenation using + takes 47 millisecs
Concatenation using StringBuffer takes 32 millisecs
Performance using concatenation using StringBuffer better than + concatenation.

Conclusion : the above case clearly shows StringBuffer giving more performance as compared to + concatenation at run time.

Overall final conclusion: Prefer StringBuffer whenever run time resolution concatenation is involved
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