String StringBuffer and StringBuilder

Question

Please tell me a real time situation to compare String  StringBuffer  and StringBuilder

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Note that if you are using Java 5 or newer, you should use StringBuilder instead of StringBuffer. From the API documentation:

As of release JDK 5, this class has been supplemented with an equivalent class designed for use by a single thread, StringBuilder. The StringBuilder class should generally be used in preference to this one, as it supports all of the same operations but it is faster, as it performs no synchronization.

In practice, you will almost never use this from multiple threads at the same time, so the synchronization that StringBuffer does is almost always unnecessary overhead.

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String  The String class represents character strings  All string literals in Java program  such as  abc  are implemented as instances of this class   String objects are immutable once they are created we can t change   Strings are constants    If a String is created using constructor or method then those strings will be stored in Heap Memory as well as SringConstantPool  But before saving in pool it invokes intern   method to check object availability with same content in pool using equals method  If String-copy is available in the Pool then returns the reference  Otherwise  String object is added to the pool and returns the reference    The Java language provides special support for the string concatenation operator      and for conversion of other objects to strings  String concatenation is implemented through the StringBuilder or StringBuffer  class and its append method      String heapSCP   new String  Yash    heapSCP concat       heapSCP   heapSCP    M   heapSCP   heapSCP   777      For Example  String Source Code  public String concat String str        int otherLen   str length        if  otherLen    0            return this            int len   value length      char buf     Arrays copyOf value  len   otherLen       str getChars buf  len       return new String buf  true        String literals are stored in StringConstantPool     String onlyPool    Yash          StringBuilder and StringBuffer are mutable sequence of characters  That means one can change the value of these object s  StringBuffer has the same methods as the StringBuilder  but each method in StringBuffer is synchronized so it is thread safe    StringBuffer and StringBuilder data can only be created using new operator  So  they get stored in Heap memory  Instances of StringBuilder are not safe for use by multiple threads  If such synchronization is required then it is recommended that StringBuffer be used     StringBuffer threadSafe   new StringBuffer  Yash    threadSafe append   M    threadSafe toString     StringBuilder nonSync   new StringBuilder  Yash    nonSync append   M    nonSync toString       StringBuffer and StringBuilder are having a Special methods like   replace int start  int end  String str  and reverse        NOTE  StringBuffer and SringBuilder are mutable as they provides the implementation of Appendable Interface       When to use which one    If a you are not going to change the value every time then its better to Use String Class  As part of Generics if you want to Sort Comparable lt T gt  or compare a values then go for String Class     ClassCastException  java lang StringBuffer cannot be cast to java lang Comparable Set lt StringBuffer gt  set   new TreeSet lt StringBuffer gt     set add  threadSafe    System out println  Set      set      If you are going to modify the value every time the go for StringBuilder which is faster than StringBuffer  If multiple threads are modifying the value the go for StringBuffer

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Also  StringBuffer is thread-safe  which StringBuilder is not   So in a real-time situation when different threads are accessing it  StringBuilder could have an undeterministic result

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Personally  I don t think there is any real world use for StringBuffer  When would I ever want to communicate between multiple threads by manipulating a character sequence  That doesn t sound useful at all  but maybe I have yet to see the light

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Do you mean  for concatenation   Real world example   You want to create a new string out of many others    For instance to send a message   String  String s    Dear     user name     lt br gt        I saw your profile and got interested in you  lt br gt       I m      user age    yrs  old too    StringBuilder  String s   new StringBuilder   append   Dear    append  user name   append    lt br gt                 append   I saw your profile and got interested in you  lt br gt                append   I m      append  user age   append   yrs  old too              toString     Or   String s   new StringBuilder 100  appe      etc         The difference is a size of 100 will be allocated upfront as  fuzzy lollipop points out    StringBuffer   the syntax is exactly as with StringBuilder  the effects differ      About  StringBuffer vs  StringBuilder   The former is synchonized and later is not    So  if you invoke it several times in a single thread   which is 90  of the cases     StringBuilder will run much faster because it won t stop to see if it owns the thread lock    So  it is recommendable to use StringBuilder   unless of course you have more than one thread accessing to it at the same time  which is rare      String concatenation   using the   operator    may be optimized by the compiler to use StringBuilder underneath  so  it not longer something to worry about  in the elder days of Java  this was something that everyone says should be avoided at all cost  because every concatenation created a new String object  Modern compilers don t do this anymore  but still it is a good practice to use StringBuilder instead just in case you use an  old  compiler   edit  Just for who is curious  this is what the compiler does for this class   class StringConcatenation       int x      String literal    Value is    x      String builder   new StringBuilder   append  Value is   append x  toString        javap -c StringConcatenation  Compiled from  StringConcatenation java  class StringConcatenation extends java lang Object  int x   java lang String literal   java lang String builder   StringConcatenation      Code     0    aload 0    1    invokespecial    1    Method java lang Object   lt init gt     V    4    aload 0    5    new  2    class java lang StringBuilder    8    dup    9    invokespecial    3    Method java lang StringBuilder   lt init gt     V    12   ldc  4    String Value is    14   invokevirtual    5    Method java lang StringBuilder append  Ljava lang String  Ljava lang StringBuilder     17   aload 0    18   getfield     6    Field x I    21   invokevirtual    7    Method java lang StringBuilder append  I Ljava lang StringBuilder     24   invokevirtual    8    Method java lang StringBuilder toString   Ljava lang String     27   putfield     9    Field literal Ljava lang String     30   aload 0    31   new  2    class java lang StringBuilder    34   dup    35   invokespecial    3    Method java lang StringBuilder   lt init gt     V    38   ldc  4    String Value is    40   invokevirtual    5    Method java lang StringBuilder append  Ljava lang String  Ljava lang StringBuilder     43   aload 0    44   getfield     6    Field x I    47   invokevirtual    7    Method java lang StringBuilder append  I Ljava lang StringBuilder     50   invokevirtual    8    Method java lang StringBuilder toString   Ljava lang String     53   putfield     10    Field builder Ljava lang String     56   return      Lines numbered 5 - 27 are for the String named  literal   Lines numbered 31-53 are for the String named  builder   Ther s no difference  exactly the same code is executed for both strings

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In java  String is immutable  Being immutable we mean that once a String is created  we can not change its value   StringBuffer is mutable  Once a StringBuffer object is created  we just append the content to the value of object instead of creating a new object  StringBuilder is similar to StringBuffer but it is not thread-safe  Methods of StingBuilder are not synchronized but in comparison to other Strings  the Stringbuilder runs fastest   You can learn difference between String  StringBuilder and StringBuffer by implementing them

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Difference between String and the other two classes is that String is immutable and the other two are mutable classes   But why we have two classes for same purpose   Reason is that StringBuffer is Thread safe and StringBuilder is not  StringBuilder is a new class on StringBuffer Api and it was introduced in JDK5 and is always recommended if you are working in a Single threaded environment as it is much Faster  For complete Details you can read http   www codingeek com java stringbuilder-and-stringbuffer-a-way-to-create-mutable-strings-in-java

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----------------------------------------------------------------------------------                   String                    StringBuffer         StringBuilder ----------------------------------------------------------------------------------                  Storage Area   Constant String Pool         Heap                   Heap  Modifiable      No  immutable               Yes  mutable           Yes  mutable   Thread Safe         Yes                     Yes                     No  Performance       Fast                 Very slow                  Fast ----------------------------------------------------------------------------------

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Mutability Difference   String is immutable  if you try to alter their values  another object gets created  whereas StringBuffer and StringBuilder are mutable so they can change their values   Thread-Safety Difference   The difference between StringBuffer and StringBuilder is that StringBuffer is thread-safe  So when the application needs to be run only in a single thread then it is better to use StringBuilder  StringBuilder is more efficient than StringBuffer   Situations    If your string is not going to change use a String class because a String object is immutable  If your string can change  example  lots of logic and operations in the construction of the string  and will only be accessed from a single thread  using a StringBuilder is good enough  If your string can change  and will be accessed from multiple threads  use a StringBuffer because StringBuffer is synchronous so you have thread-safety

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You use String when an immutable structure is appropriate  obtaining a new character sequence from a String may carry an unacceptable performance penalty  either in CPU time or memory  obtaining substrings is CPU efficient because the data is not copied  but this means a potentially much larger amount of data may remain allocated   You use StringBuilder when you need to create a mutable character sequence  usually to concatenate several character sequences together  You use StringBuffer in the same circumstances you would use StringBuilder  but when changes to the underlying string must be synchronized  because several threads are reading modifyind the string buffer     See an example here

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The Basics   String is an immutable class  it can t be changed  StringBuilder is a mutable class that can be appended to  characters replaced or removed and ultimately converted to a String StringBuffer is the original synchronized version of StringBuilder  You should prefer StringBuilder in all cases where you have only a single thread accessing your object   The Details   Also note that StringBuilder Buffers aren t magic  they just use an Array as a backing object and that Array has to be re-allocated when ever it gets full  Be sure and create your StringBuilder Buffer objects large enough originally where they don t have to be constantly re-sized every time  append   gets called   The re-sizing can get very degenerate  It basically re-sizes the backing Array to 2 times its current size every time it needs to be expanded  This can result in large amounts of RAM getting allocated and not used when StringBuilder Buffer classes start to grow large   In Java String x    A     B   uses a StringBuilder behind the scenes  So for simple cases there is no benefit of declaring your own  But if you are building String objects that are large  say less than 4k  then declaring StringBuilder sb   StringBuilder 4096   is much more efficient than concatenation or using the default constructor which is only 16 characters  If your String is going to be less than 10k then initialize it with the constructor to 10k to be safe  But if it is initialize to 10k then you write 1 character more than 10k  it will get re-allocated and copied to a 20k array  So initializing high is better than to low   In the auto re-size case  at the 17th character the backing Array gets re-allocated and copied to 32 characters  at the 33th character this happens again and you get to re-allocated and copy the Array into 64 characters  You can see how this degenerates to lots of re-allocations and copies which is what you really are trying to avoid using StringBuilder Buffer in the first place   This is from the JDK 6 Source code for AbstractStringBuilder     void expandCapacity int minimumCapacity        int newCapacity    value length   1    2          if  newCapacity  lt  0                newCapacity   Integer MAX VALUE            else if  minimumCapacity  gt  newCapacity            newCapacity   minimumCapacity                value   Arrays copyOf value  newCapacity           A best practice is to initialize the StringBuilder Buffer a little bit larger than you think you are going to need if you don t know right off hand how big the String will be but you can guess  One allocation of slightly more memory than you need is going to be better than lots of re-allocations and copies   Also beware of initializing a StringBuilder Buffer with a String as that will only allocated the size of the String   16 characters  which in most cases will just start the degenerate re-allocation and copy cycle that you are trying to avoid  The following is straight from the Java 6 source code   public StringBuilder String str        super str length     16       append str           If you by chance do end up with an instance of StringBuilder Buffer that you didn t create and can t control the constructor that is called  there is a way to avoid the degenerate re-allocate and copy behavior  Call  ensureCapacity   with the size you want to ensure your resulting String will fit into   The Alternatives   Just as a note  if you are doing really heavy String building and manipulation  there is a much more performance oriented alternative called Ropes   Another alternative  is to create a StringList implemenation by sub-classing ArrayList lt String gt   and adding counters to track the number of characters on every  append   and other mutation operations of the list  then override  toString   to create a StringBuilder of the exact size you need and loop through the list and build the output  you can even make that StringBuilder an instance variable and  cache  the results of  toString   and only have to re-generate it when something changes   Also don t  forget about String format   when building fixed formatted output  which can be optimized by the compiler as they make it better

[java] String, StringBuffer, and StringBuilder

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