How can I clear or empty a StringBuilder

Question

I m using a StringBuilder in a loop and every x iterations I want to empty it and start with an empty StringBuilder  but I can t see any method similar to the  NET StringBuilder Clear in the documentation  just the delete method which seems overly complicated   So what is the best way to clean out a StringBuilder in Java

User · Answer

If performance is the main concern then the irony, in my opinion, is the Java constructs to format the text that goes into the buffer, will be far more time consuming on the CPU than the allocation/reallocation/garbage collection ... well, possibly not the GC (garbage collection) depending on how many builders you create and discard.

But simply appending a compound string ("Hello World of " + 6E9 + " earthlings.") to the buffer is likely to make the whole matter inconsequential.

And, really, if an instance of StringBuilder is involved, then the content is complex and/or longer than a simple String str = "Hi"; (never mind that Java probably uses a builder in the background anyway).

Personally, I try not to abuse the GC. So if it's something that's going to be used a lot in a rapid fire scenario - like, say, writing debug output messages - I just assume declare it elsewhere and zero it out for reuse.

class MyLogger {
    StringBuilder strBldr = new StringBuilder(256);

    public void logMsg( String stuff, SomeLogWriterClass log ) {

        // zero out strBldr's internal index count, not every
        // index in strBldr's internal buffer
        strBldr.setLength(0);

        // ... append status level
        strBldr.append("Info");

        // ... append ' ' followed by timestamp
        // assuming getTimestamp() returns a String
        strBldr.append(' ').append(getTimestamp());

        // ... append ':' followed by user message
        strBldr.append(':').append(msg);

        log.write(strBldr.toString());
    }
}

User · Answer

I ll vote for sb setLength 0   not only because it s one function call  but because it doesn t actually copy the array into another array like sb delete 0  builder length      It just fill the remaining characters to be 0 and set the length variable to the new length   You can take a look into their implementation to validate my point from here at setLength function and delete0 function

User · Answer

I think many of the answers here may be missing a quality method included in StringBuilder   delete int start   int  end   I know this is a late reply  however  this should be made known  and explained a bit more thoroughly    Let s say you have a StringBuilder table - which you wish to modify  dynamically  throughout your program  one I am working on right now does this   e g    StringBuilder table   new StringBuilder      If you are looping through the method and alter the content  use the content  then wish to discard the content to  clean up  the StringBuilder for the next iteration  you can delete it s contents  e g    table delete int start  int end      start and end being the indices of the chars you wish to remove  Don t know the length in chars and want to delete the whole thing   table delete 0  table length       NOW  for the kicker  StringBuilders  as mentioned previously  take a lot of overhead when altered  frequently  and can cause safety issues with regard to threading   therefore  use StringBuffer - same as StringBuilder  with a few exceptions  - if your StringBuilder is used for the purpose of interfacing with the user

User · Answer

You should use sb delete 0  sb length    or sb setLength 0  and NOT create a new StringBuilder     See this related post for performance  Is it better to reuse a StringBuilder in a loop

User · Answer

delete is not overly complicated    myStringBuilder delete 0  myStringBuilder length       You can also do    myStringBuilder setLength 0

User · Answer

If you look at the source code for a StringBuilder or StringBuffer the setLength   call just resets an index value for the character array  IMHO using the setLength method will always be faster than a new allocation  They should have named the method  clear  or  reset  so it would be clearer

User · Answer

There are basically two alternatives  using setLength 0  to reset the StringBuilder or creating a new one in each iteration  Both can have pros and cons depending on the usage   If you know the expected capacity of the StringBuilder beforehand  creating a new one each time should be just as fast as setting a new length  It will also help the garbage collector  since each StringBuilder will be relatively short-lived and the gc is optimized for that   When you don t know the capacity  reusing the same StringBuilder might be faster  Each time you exceed the capacity when appending  a new backing array has to be allocated and the previous content has to be copied  By reusing the same StringBuilder  it will reach the needed capacity after some iterations and there won t be any copying thereafter

User · Answer

Two ways that work    Use stringBuilderObj setLength 0   Allocate a new one with new StringBuilder   instead of clearing the buffer

User · Answer

StringBuilder s   new StringBuilder    s append  a    s append  a       System out print s   is return  aa  s delete 0  s length     System out print s length        is return 0   is the easy way

[java] How can I clear or empty a StringBuilder?

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