When to use StringBuilder in Java

Question

It is supposed to be generally preferable to use a StringBuilder for string concatenation in Java  Is this always the case   What I mean is this  Is the overhead of creating a StringBuilder object  calling the append   method and finally toString   already smaller then concatenating existing strings with the   operator for two strings  or is it only advisable for more  than two  strings   If there is such a threshold  what does it depend on  perhaps the string length  but in which way    And finally  would you trade the readability and conciseness of the   concatenation for the performance of the StringBuilder in smaller cases like two  three or four strings   Explicit use of StringBuilder for regular concatenations is being mentioned as obsolete at obsolete Java optimization tips as well as at Java urban myths

User · Answer

The Microsoft certification material addresses this same question  In the  NET world  the overhead for the StringBuilder object makes a simple concatenation of 2 String objects more efficient  I would assume a similar answer for Java strings

User · Answer

If you use String concatenation in a loop  something like this   String s       for  int i   0  i  lt  100  i          s           i      then you should use a StringBuilder  not StringBuffer  instead of a String  because it is much faster and consumes less memory   If you have a single statement   String s    1       2       3       4           then you can use Strings  because the compiler will use StringBuilder automatically

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The problem with String concatenation is that it leads to copying of the String object with all the associated cost  StringBuilder is not threadsafe and is therefore faster than StringBuffer  which used to be the preferred choice before Java 5  As a rule of thumb  you should not do String concatenation in a loop  which will be called often  I guess doing a few concatenations here and there will not hurt you as long as you are not talking about hundreds and this of course depends on your performance requirements  If you are doing real time stuff  you should be very careful

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For two strings concat is faster  in other cases StringBuilder is a better choice  see my explanation in concatenation operator     vs concat

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The   operator uses public String concat String str  internally  This method copies the characters of the two strings  so it has memory requirements and runtime complexity proportional to the length of the two strings  StringBuilder works more efficent   However I have read here that the concatination code using the   operater is changed to StringBuilder on post Java 4 compilers  So this might not be an issue at all   Though I would really check this statement if I depend on it in my code

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Some compilers may not replace any string concatenations with StringBuilder equivalents   Be sure to consider which compilers your source will use before relying on compile time optimizations

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Have a look at  http   www javaspecialists eu archive Issue068 html and http   www javaspecialists eu archive Issue105 html  Do the same tests in your environment and check if newer JDK or your Java implementation do some type of string operation better with String or better with StringBuilder

User · Answer

As a general rule  always use the more readable code and only refactor if performance is an issue  In this specific case  most recent JDK s will actually optimize the code into the StringBuilder version in any case    You usually only really need to do it manually if you are doing string concatenation in a loop or in some complex code that the compiler can t easily optimize

User · Answer

Ralph s answer is fabulous  I would rather use StringBuilder class to build decorate the String because the usage of it is more look like Builder pattern   public String decorateTheString String orgStr               StringBuilder builder   new StringBuilder                builder append orgStr               builder deleteCharAt orgStr length  -1               builder insert 0 builder hashCode                 return builder toString        It can be use as a helper builder to build the String  not the String itself

[java] When to use StringBuilder in Java

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