String concatenation concat vs operator

Question

Assuming String a and b   a    b a   a concat b    Under the hood  are they the same thing   Here is concat decompiled as reference  I d like to be able to decompile the   operator as well to see what that does   public String concat String s         int i   s length        if  i    0            return this            else           char ac     new char count   i           getChars 0  count  ac  0           s getChars 0  i  ac  count           return new String 0  count   i  ac

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Most answers here are from 2008  It looks that things have changed over the time   My latest benchmarks made with JMH shows that on Java 8   is around two times faster than concat    My benchmark    Warmup iterations   5  time   200  timeUnit   TimeUnit MILLISECONDS   Measurement iterations   5  time   200  timeUnit   TimeUnit MILLISECONDS  public class StringConcatenation         org openjdk jmh annotations State Scope Thread      public static class State2           public String a    abc           public String b    xyz               org openjdk jmh annotations State Scope Thread      public static class State3           public String a    abc           public String b    xyz           public String c    123                org openjdk jmh annotations State Scope Thread      public static class State4           public String a    abc           public String b    xyz           public String c    123           public String d                      Benchmark     public void plus 2 State2 state  Blackhole blackhole            blackhole consume state a state b               Benchmark     public void plus 3 State3 state  Blackhole blackhole            blackhole consume state a state b state c               Benchmark     public void plus 4 State4 state  Blackhole blackhole            blackhole consume state a state b state c state d               Benchmark     public void stringbuilder 2 State2 state  Blackhole blackhole            blackhole consume new StringBuilder   append state a  append state b  toString                 Benchmark     public void stringbuilder 3 State3 state  Blackhole blackhole            blackhole consume new StringBuilder   append state a  append state b  append state c  toString                 Benchmark     public void stringbuilder 4 State4 state  Blackhole blackhole            blackhole consume new StringBuilder   append state a  append state b  append state c  append state d  toString                 Benchmark     public void concat 2 State2 state  Blackhole blackhole            blackhole consume state a concat state b                Benchmark     public void concat 3 State3 state  Blackhole blackhole            blackhole consume state a concat state b concat state c                  Benchmark     public void concat 4 State4 state  Blackhole blackhole            blackhole consume state a concat state b concat state c concat state d                Results   Benchmark                             Mode  Cnt         Score         Error  Units StringConcatenation concat 2         thrpt   50  24908871 258    1011269 986  ops s StringConcatenation concat 3         thrpt   50  14228193 918     466892 616  ops s StringConcatenation concat 4         thrpt   50   9845069 776     350532 591  ops s StringConcatenation plus 2           thrpt   50  38999662 292    8107397 316  ops s StringConcatenation plus 3           thrpt   50  34985722 222    5442660 250  ops s StringConcatenation plus 4           thrpt   50  31910376 337    2861001 162  ops s StringConcatenation stringbuilder 2  thrpt   50  40472888 230    9011210 632  ops s StringConcatenation stringbuilder 3  thrpt   50  33902151 616    5449026 680  ops s StringConcatenation stringbuilder 4  thrpt   50  29220479 267    3435315 681  ops s

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When using    the speed decreases as the string s length increases  but when using concat  the speed is more stable  and the best option is using the StringBuilder class which has stable speed in order to do that   I guess you can understand why  But the totally best way for creating long strings is using StringBuilder   and append    either speed will be unacceptable

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How about some simple testing  Used the code below   long start   System currentTimeMillis     String a    a    String b    b    for  int i   0  i  lt  10000000  i        ten million times      String c   a concat b      long end   System currentTimeMillis     System out println end - start      The  a   b  version executed in 2500ms  The a concat b  executed in 1200ms    Tested several times  The concat   version execution took half of the time on average   This result surprised me because the concat   method always creates a new string  it returns a  new String result    It s well known that   String a   new String  a      more than 20 times slower than String a    a    Why wasn t the compiler capable of optimize the string creation in  a   b  code  knowing the it always resulted in the same string  It could avoid a new string creation  If you don t believe the statement above  test for your self

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For the sake of completeness  I wanted to add that the definition of the     operator can be found in the JLS SE8 15 18 1      If only one operand expression is of type String  then string   conversion    5 1 11  is performed on the other operand to produce a   string at run time       The result of string concatenation is a reference to a String object   that is the concatenation of the two operand strings  The characters   of the left-hand operand precede the characters of the right-hand   operand in the newly created string       The String object is newly created    12 5  unless the expression is a   constant expression    15 28     About the implementation the JLS says the following      An implementation may choose to perform conversion and concatenation   in one step to avoid creating and then discarding an intermediate   String object  To increase the performance of repeated string   concatenation  a Java compiler may use the StringBuffer class or a   similar technique to reduce the number of intermediate String objects   that are created by evaluation of an expression       For primitive types  an implementation may also optimize away the   creation of a wrapper object by converting directly from a primitive   type to a string    So judging from the  a Java compiler may use the StringBuffer class or a similar technique to reduce   different compilers could produce different byte-code

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No  not quite   Firstly  there s a slight difference in semantics  If a is null  then a concat b  throws a NullPointerException but a  b will treat the original value of a as if it were null  Furthermore  the concat   method only accepts String values while the   operator will silently convert the argument to a String  using the toString   method for objects   So the concat   method is more strict in what it accepts   To look under the hood  write a simple class with a    b   public class Concat       String cat String a  String b            a    b          return a            Now disassemble with javap -c  included in the Sun JDK   You should see a listing including   java lang String cat java lang String  java lang String     Code     0    new      2    class java lang StringBuilder    3    dup    4    invokespecial    3    Method java lang StringBuilder   lt init gt     V    7    aload 1    8    invokevirtual    4    Method java lang StringBuilder append  Ljava lang String  Ljava lang StringBuilder     11   aload 2    12   invokevirtual    4    Method java lang StringBuilder append  Ljava lang String  Ljava lang StringBuilder     15   invokevirtual    5    Method java lang StringBuilder toString   Ljava lang     String     18   astore 1    19   aload 1    20   areturn   So  a    b is the equivalent of  a   new StringBuilder        append a       append b       toString      The concat method should be faster  However  with more strings the StringBuilder method wins  at least in terms of performance   The source code of String and StringBuilder  and its package-private base class  is available in src zip of the Sun JDK  You can see that you are building up a char array  resizing as necessary  and then throwing it away when you create the final String  In practice memory allocation is surprisingly fast   Update  As Pawel Adamski notes  performance has changed in more recent HotSpot  javac still produces exactly the same code  but the bytecode compiler cheats  Simple testing entirely fails because the entire body of code is thrown away  Summing System identityHashCode  not String hashCode  shows the StringBuffer code has a slight advantage  Subject to change when the next update is released  or if you use a different JVM  From  lukaseder  a list of HotSpot JVM intrinsics

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I ran a similar test as  marcio but with the following loop instead   String c   a  for  long i   0  i  lt  100000L  i          c   c concat b      make sure javac cannot skip the loop        using c    b for the alternative     Just for good measure  I threw in StringBuilder append   as well   Each test was run 10 times  with 100k reps for each run   Here are the results    StringBuilder wins hands down   The clock time result was 0 for most the runs  and the longest took 16ms  a    b takes about 40000ms  40s  for each run  concat only requires 10000ms  10s  per run    I haven t decompiled the class to see the internals or run it through profiler yet  but I suspect a    b spends much of the time creating new objects of StringBuilder and then converting them back to String

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I don t think so   a concat b  is implemented in String and I think the implementation didn t change much since early java machines  The   operation implementation depends on Java version and compiler  Currently   is implemented using StringBuffer to make the operation as fast as possible  Maybe in the future  this will change  In earlier versions of java   operation on Strings was much slower as it produced intermediate results   I guess that    is implemented using   and similarly optimized

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Basically  there are two important differences between   and the concat method    If you are using the concat method then you would only be able to concatenate strings while in case of the   operator  you can also concatenate the string with any data type   For Example   String s   10    Hello     In this case  the output should be 10Hello   String s    I   String s1   s concat  am   concat  good   concat  boy    System out println s1     In the above case you have to provide two strings mandatory  The second and main difference between   and concat is that   Case 1  Suppose I concat the same strings with concat operator in this way  String s  I   String s1 s concat  am   concat  good   concat  boy    System out println s1     In this case total number of objects created in the pool are 7 like this   I am good boy Iam Iamgood Iamgoodboy   Case 2   Now I am going to concatinate the same strings via   operator  String s  I   am   good   boy   System out println s     In the above case total number of objects created are only 5   Actually when we concatinate the strings via   operator then it maintains a StringBuffer class to perform the same task as follows -  StringBuffer sb   new StringBuffer  I    sb append  am    sb append  good    sb append  boy    System out println sb     In this way it will create only five objects    So guys these are the basic differences between   and the concat method  Enjoy

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Tom is correct in describing exactly what the   operator does   It creates a temporary StringBuilder  appends the parts  and finishes with toString     However  all of the answers so far are ignoring the effects of HotSpot runtime optimizations   Specifically  these temporary operations are recognized as a common pattern and are replaced with more efficient machine code at run-time    marcio  You ve created a micro-benchmark  with modern JVM s this is not a valid way to profile code   The reason run-time optimization matters is that many of these differences in code -- even including object-creation -- are completely different once HotSpot gets going   The only way to know for sure is profiling your code in situ   Finally  all of these methods are in fact incredibly fast   This might be a case of premature optimization   If you have code that concatenates strings a lot  the way to get maximum speed probably has nothing to do with which operators you choose and instead the algorithm you re using

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Niyaz is correct  but it s also worth noting that the special   operator can be converted into something more efficient by the Java compiler   Java has a StringBuilder class which represents a non-thread-safe  mutable String   When performing a bunch of String concatenations  the Java compiler silently converts  String a   b   c   d    into  String a   new StringBuilder b  append c  append d  toString      which for large strings is significantly more efficient   As far as I know  this does not happen when you use the concat method   However  the concat method is more efficient when concatenating an empty String onto an existing String   In this case  the JVM does not need to create a new String object and can simply return the existing one   See the concat documentation to confirm this   So if you re super-concerned about efficiency then you should use the concat method when concatenating possibly-empty Strings  and use   otherwise   However  the performance difference should be negligible and you probably shouldn t ever worry about this

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The   operator can work between a string and a string  char  integer  double or float data type value  It just converts the value to its string representation before concatenation   The concat operator can only be done on and with strings  It checks for data type compatibility and throws an error  if they don t match   Except this  the code you provided does the same stuff

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