java use StringBuilder to insert at the beginning

Question

I could only do this with String  for example    String str     for int i 0 i lt 100 i         str i str      Is there a way to achieve this with StringBuilder  Thanks

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Difference Between String  StringBuilder And StringBuffer Classes String String is immutable   once created can not be changed  object  The object created as a String is stored in the Constant String Pool  Every immutable object in Java is thread-safe  which implies String is also thread-safe  String can not be used by two threads simultaneously  String once assigned can not be changed  StringBuffer StringBuffer is mutable means one can change the value of the object  The object created through StringBuffer is stored in the heap  StringBuffer has the same methods as the StringBuilder   but each method in StringBuffer is synchronized that is StringBuffer is thread safe   Due to this  it does not allow two threads to simultaneously access the same method  Each method can be accessed by one thread at a time  But being thread-safe has disadvantages too as the performance of the StringBuffer hits due to thread-safe property  Thus StringBuilder is faster than the StringBuffer when calling the same methods of each class  String Buffer can be converted to the string by using toString   method       StringBuffer demo1   new StringBuffer  Hello         The above object stored in heap and its value can be changed       Above statement is right as it modifies the value which is allowed in the StringBuffer StringBuilder StringBuilder is the same as the StringBuffer  that is it stores the object in heap and it can also be modified  The main difference between the StringBuffer and StringBuilder is that StringBuilder is also not thread-safe  StringBuilder is fast as it is not thread-safe       The above object is stored in the heap and its value can be modified      Above statement is right as it modifies the value which is allowed in the StringBuilder

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How about  StringBuilder builder   new StringBuilder    for int i 99 i gt  0 i--       builder append Integer toString i      builder toString     OR StringBuilder builder   new StringBuilder    for int i 0 i lt 100 i       builder insert 0  Integer toString i      builder toString     But with this  you are making the operation O N 2  instead of O N   Snippet from java docs   Inserts the string representation of the Object argument into this character sequence  The overall effect is exactly as if the second argument were converted to a string by the method String valueOf Object   and the characters of that string were then inserted into this character sequence at the indicated offset

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Maybe I m missing something but you want to wind up with a String that looks like this   999897969594   543210   correct   StringBuilder sb   new StringBuilder    for int i 99 i gt  0 i--       sb append String valueOf i

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You can use the insert method with the offset  as offset set to  0  means you are appending to the front of your StringBuilder   StringBuilder sb   new StringBuilder    for int i 0 i lt 100 i         sb insert 0 i       NOTE  as the insert method accept all types of primitives  you can use for int  long  char   etc

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As an alternative solution you can use a LIFO structure  like a stack  to store all the strings and when you are done just take them all out and put them into the StringBuilder  It naturally reverses the order of the items  strings  placed in it   Stack lt String gt  textStack   new Stack lt String gt        push the strings to the stack while  isReadingTextDone          String text   readText        textStack push text        pop the strings and add to the text builder String builder   new StringBuilder     while   textStack empty            builder append textStack pop          get the final string String finalText    builder toString

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This thread is quite old  but you could also think about a recursive solution passing the StringBuilder to fill  This allows to prevent any reverse processing etc  Just need to design your iteration with a recursion and carefully decide for an exit condition   public class Test        public static void main String   args            StringBuilder sb   new StringBuilder            doRecursive sb  100  0           System out println sb toString                public static void doRecursive StringBuilder sb  int limit  int index            if  index  lt  limit                doRecursive sb  limit  index   1               sb append Integer toString index

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StringBuilder sb   new StringBuilder    for int i 0 i lt 100 i         sb insert 0  Integer toString i        Warning  It defeats the purpose of StringBuilder  but it does what you asked     Better technique  although still not ideal     Reverse each string you want to insert  Append each string to a StringBuilder  Reverse the entire StringBuilder when you re done    This will turn an O n    solution into O n

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you can use strbuilder insert 0 i

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I had a similar requirement when I stumbled on this post  I wanted a fast way to build a String that can grow from both sides ie  add new letters on the front as well as back arbitrarily  I know this is an old post  but it inspired me to try out a few ways to create strings and I thought I d share my findings  I am also using some Java 8 constructs in this  which could have optimised the speed in cases 4 and 5    https   gist github com SidWagz e41e836dec65ff24f78afdf8669e6420  The Gist above has the detailed code that anyone can run  I took few ways of growing strings in this  1  Append to StringBuilder  2  Insert to front of StringBuilder as as shown by  Mehrdad  3  Partially insert from front as well as end of the StringBuilder  4  Using a list to append from end  5  Using a Deque to append from the front      Case 2     StringBuilder build3   new StringBuilder    IntStream range 0  MAX STR                       sequential                        forEach i - gt                            if  i 2    0  build3 append Integer toString i    else build3 insert 0  Integer toString i                            String build3Out   build3 toString        Case 5 Deque lt String gt  deque   new ArrayDeque lt  gt     IntStream range 0  MAX STR                   sequential                    forEach i - gt                        if  i 2    0  deque addLast Integer toString i    else deque addFirst Integer toString i                         String dequeOut   deque stream   collect Collectors joining         I ll focus on the front append only cases ie  case 2 and case 5  The implementation of StringBuilder internally decides how the internal buffer grows  which apart from moving all buffer left to right in case of front appending limits the speed  While time taken when inserting directly to the front of the StringBuilder grows to really high values  as shown by  Mehrdad  if the need is to only have strings of length less than 90k characters  which is still a lot   the front insert will build a String in the same time as it would take to build a String of the same length by appending at the end  What I am saying is that time time penalty indeed kicks and is huge  but only when you have to build really huge strings  One could use a deque and join the strings at the end as shown in my example  But StringBuilder is a bit more intuitive to read and code  and the penalty would not matter for smaller strings   Actually the performance for case 2 is much faster than case 1  which I don t seem to understand  I assume the growth for the internal buffer in StringBuilder would be the same in case of front append and back append  I even set the minimum heap to a very large amount to avoid delay in heap growth  if that would have played a role  Maybe someone who has a better understanding can comment below

[java] java: use StringBuilder to insert at the beginning

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