Declaring variables inside or outside of a loop

Question

Why does the following work fine   String str  while  condition        str   calculateStr                  But this one is said to be dangerous incorrect   while  condition        String str   calculateStr                  Is it necessary to declare variables outside the loop

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I compared the byte code of those two  similar  examples   Let s look at 1  example   package inside   public class Test       public static void main String   args            while true               String str   String valueOf System currentTimeMillis                 System out println str                       after javac Test java  javap -c Test you ll get   public class inside Test extends java lang Object  public inside Test      Code     0    aload 0    1    invokespecial    1    Method java lang Object   lt init gt     V    4    return  public static void main java lang String       Code     0    invokestatic     2    Method java lang System currentTimeMillis   J    3    invokestatic     3    Method java lang String valueOf  J Ljava lang String     6    astore 1    7    getstatic        4    Field java lang System out Ljava io PrintStream     10   aload 1    11   invokevirtual    5    Method java io PrintStream println  Ljava lang String  V    14   goto    0      Let s look at 2  example   package outside   public class Test       public static void main String   args            String str          while true               str    String valueOf System currentTimeMillis                 System out println str                       after javac Test java  javap -c Test you ll get   public class outside Test extends java lang Object  public outside Test      Code     0    aload 0    1    invokespecial    1    Method java lang Object   lt init gt     V    4    return  public static void main java lang String       Code     0    invokestatic     2    Method java lang System currentTimeMillis   J    3    invokestatic     3    Method java lang String valueOf  J Ljava lang String     6    astore 1    7    getstatic        4    Field java lang System out Ljava io PrintStream     10   aload 1    11   invokevirtual    5    Method java io PrintStream println  Ljava lang String  V    14   goto    0      The observations shows that there is no difference among those two examples  It s the result of JVM specifications     But in the name of best coding practice it is recommended to declare the variable in the smallest possible scope  in this example it is inside the loop  as this is the only place where the variable is used

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Declaring inside the loop limits the scope of the respective variable  It all depends on the requirement of the project on the scope of the variable

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One solution to this problem could be to provide a variable scope encapsulating the while loop          all tmp loop variables here                  String str    while condition         str   calculateStr                        They would be automatically de-reference when the outer scope ends

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The scope of local variables should always be the smallest possible   In your example I presume str is not used outside of the while loop  otherwise you would not be asking the question  because declaring it inside the while loop would not be an option  since it would not compile   So  since str is not used outside the loop  the smallest possible scope for str is within the while loop   So  the answer is emphatically that str absolutely ought to be declared within the while loop  No ifs  no ands  no buts   The only case where this rule might be violated is if for some reason it is of vital importance that every clock cycle must be squeezed out of the code  in which case you might want to consider instantiating something in an outer scope and reusing it instead of re-instantiating it on every iteration of an inner scope  However  this does not apply to your example  due to the immutability of strings in java  a new instance of str will always be created in the beginning of your loop and it will have to be thrown away at the end of it  so there is no possibility to optimize there   EDIT   injecting my comment below in the answer   In any case  the right way to do things is to write all your code properly  establish a performance requirement for your product  measure your final product against this requirement  and if it does not satisfy it  then go optimize things  And what usually ends up happening is that you find ways to provide some nice and formal algorithmic optimizations in just a couple of places which make our program meet its performance requirements instead of having to go all over your entire code base and tweak and hack things in order to squeeze clock cycles here and there

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These two examples result in the same thing  However  the first provides you with using the str variable outside of the while loop  the second is not

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Inside  the less scope the variable is visible into the better

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Declaring String str outside of the while loop allows it to be referenced inside  amp  outside the while loop  Declaring String str inside of the while loop allows it to only be referenced inside that while loop

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if you want to use str outside looop also  declare it outside  otherwise  2nd version is fine

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As many people have pointed out   String str  while condition       str   calculateStr                  is NOT better than this   while condition       String str   calculateStr                  So don t declare variables outside their scopes if you are not reusing it

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Please skip to the updated answer     For those who care about performance take out the System out and limit the loop to 1 byte  Using double  test 1 2  and using String  3 4  the elapsed times in milliseconds is given below with Windows nbsp 7 Professional 64 bit and JDK-1 7 0 21  Bytecodes  also given below for test1 and test2  are not the same  I was too lazy to test with mutable  amp  relatively complex objects   double  Test1 took  2710 msecs  Test2 took  2790 msecs  String  just replace double with string in the tests   Test3 took  1200 msecs  Test4 took  3000 msecs  Compiling and getting bytecode  javac exe LocalTest1 java  javap exe -c LocalTest1  gt  LocalTest1 bc   public class LocalTest1        public static void main String   args  throws Exception           long start   System currentTimeMillis            double test          for  double i   0  i  lt  1000000000  i                  test   i                    long finish   System currentTimeMillis            System out println  Test1 Took       finish - start      msecs              public class LocalTest2        public static void main String   args  throws Exception           long start   System currentTimeMillis            for  double i   0  i  lt  1000000000  i                  double test   i                    long finish   System currentTimeMillis            System out println  Test1 Took       finish - start      msecs              Compiled from  LocalTest1 java  public class LocalTest1     public LocalTest1        Code         0  aload 0        1  invokespecial  1                     Method java lang Object   lt init gt     V        4  return    public static void main java lang String    throws java lang Exception      Code         0  invokestatic   2                     Method java lang System currentTimeMillis   J        3  lstore 1        4  dconst 0        5  dstore        5        7  dload         5        9  ldc2 w         3                     double 1 0E9d       12  dcmpg       13  ifge          28       16  dload         5       18  dstore 3       19  dload         5       21  dconst 1       22  dadd       23  dstore        5       25  goto          7       28  invokestatic   2                     Method java lang System currentTimeMillis   J       31  lstore        5       33  getstatic      5                     Field java lang System out Ljava io PrintStream        36  new            6                     class java lang StringBuilder       39  dup       40  invokespecial  7                     Method java lang StringBuilder   lt init gt     V       43  ldc            8                     String Test1 Took        45  invokevirtual  9                     Method java lang StringBuilder append  Ljava lang String  Ljava lang StringBuilder        48  lload         5       50  lload 1       51  lsub       52  invokevirtual  10                    Method java lang StringBuilder append  J Ljava lang StringBuilder        55  ldc            11                    String  msecs       57  invokevirtual  9                     Method java lang StringBuilder append  Ljava lang String  Ljava lang StringBuilder        60  invokevirtual  12                    Method java lang StringBuilder toString   Ljava lang String        63  invokevirtual  13                    Method java io PrintStream println  Ljava lang String  V       66  return     Compiled from  LocalTest2 java  public class LocalTest2     public LocalTest2        Code         0  aload 0        1  invokespecial  1                     Method java lang Object   lt init gt     V        4  return    public static void main java lang String    throws java lang Exception      Code         0  invokestatic   2                     Method java lang System currentTimeMillis   J        3  lstore 1        4  dconst 0        5  dstore 3        6  dload 3        7  ldc2 w         3                     double 1 0E9d       10  dcmpg       11  ifge          24       14  dload 3       15  dstore        5       17  dload 3       18  dconst 1       19  dadd       20  dstore 3       21  goto          6       24  invokestatic   2                     Method java lang System currentTimeMillis   J       27  lstore 3       28  getstatic      5                     Field java lang System out Ljava io PrintStream        31  new            6                     class java lang StringBuilder       34  dup       35  invokespecial  7                     Method java lang StringBuilder   lt init gt     V       38  ldc            8                     String Test1 Took        40  invokevirtual  9                     Method java lang StringBuilder append  Ljava lang String  Ljava lang StringBuilder        43  lload 3       44  lload 1       45  lsub       46  invokevirtual  10                    Method java lang StringBuilder append  J Ljava lang StringBuilder        49  ldc            11                    String  msecs       51  invokevirtual  9                     Method java lang StringBuilder append  Ljava lang String  Ljava lang StringBuilder        54  invokevirtual  12                    Method java lang StringBuilder toString   Ljava lang String        57  invokevirtual  13                    Method java io PrintStream println  Ljava lang String  V       60  return     UPDATED ANSWER  It s really not easy to compare performance with all the JVM optimizations  However  it is somewhat possible  Better test and detailed results in Google Caliper   Some details on blog Should you declare a variable inside a loop or before the loop  GitHub repository  https   github com gunduru jvdt Test Results for double case and 100M loop  and yes all JVM details   https   microbenchmarks appspot com runs b1cef8d1-0e2c-4120-be61-a99faff625b4      DeclaredBefore 1 759 209 ns DeclaredInside 2 242 308 ns   Partial Test Code for double Declaration  This is not identical to the code above  If you just code a dummy loop JVM skips it  so at least you need to assign and return something  This is also recommended in Caliper documentation    Param int size     Set automatically by framework  provided in the Main       Variable is declared inside the loop       param reps    return    public double timeDeclaredInside int reps           Dummy variable needed to workaround smart JVM        double dummy   0          Test loop        for  double i   0  i  lt   size  i                  Declaration and assignment            double test   i              Dummy assignment to fake JVM            if i    size                dummy   test                      return dummy           Variable is declared before the loop       param reps    return    public double timeDeclaredBefore int reps            Dummy variable needed to workaround smart JVM        double dummy   0          Actual test variable        double test   0          Test loop        for  double i   0  i  lt   size  i                  Assignment            test   i              Not actually needed here  but we need consistent performance results            if i    size                dummy   test                      return dummy         Summary  declaredBefore indicates better performance -really tiny- and it s against the smallest scope principle  JVM should actually do this for you

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I think the best resource to answer your question would be the following post   Difference between declaring variables before or in loop   According to my understanding this thing would be language dependent  IIRC Java optimises this  so there isn t any difference  but JavaScript  for example  will do the whole memory allocation each time in the loop In Java particularly I think the second would run faster when done profiling

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Warning for almost everybody in this question  Here is sample code where inside the loop it can easily be 200 times slower on my computer with Java 7  and the memory consumption is also slightly different   But it is about allocation and not only scope   public class Test       private final static int STUFF SIZE   512      private final static long LOOP   10000000l       private static class Foo               private long   bigStuff   new long STUFF SIZE            public Foo long value                        setValue value                      public void setValue long value                           Putting value in a random place              bigStuff  int   value   STUFF SIZE     value                     public long getValue                            Retrieving whatever value              return bigStuff STUFF SIZE   2                        public static long test1                 long total   0           for  long i   0  i  lt  LOOP  i                          Foo foo   new Foo i               total    foo getValue                       return total             public static long test2                 long total   0           Foo foo   new Foo 0           for  long i   0  i  lt  LOOP  i                          foo setValue i               total    foo getValue                       return total             public static void main String   args                long start           start   System currentTimeMillis            test1            System out println System currentTimeMillis   - start            start   System currentTimeMillis            test2            System out println System currentTimeMillis   - start             Conclusion  Depending of the size of the local variable  the difference can be huge  even with not so big variables   Just to say that sometimes  outside or inside the loop DOES matter

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According to Google Android Development guide  the variable scope should be limited  Please check this link   Limit Variable Scope

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Truly  the question stated above is an programming issue  How would you like to program your code  Where do you need the  STR  to be accessed  There is no use of declaring a variable which is used locally as a global variable  Basics of programming I believe

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If you don t need to use the str after the while loop  scope related  then the second condition i e       while condition           String str   calculateStr                          is better since if you define an object on the stack only if the condition is true  I e  use it if you need it

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Declaring objects in the smallest scope improve readability   Performance doesn t matter for today s compilers  in this scenario  From a maintenance perspective  2nd option is better  Declare and initialize variables in the same place  in the narrowest scope possible   As Donald Ervin Knuth told       We should forget about small efficiencies  say about 97  of the time    premature optimization is the root of all evil    i e  situation where a programmer lets performance considerations affect the design of a piece of code  This can result in a design that is not as clean as it could have been or code that is incorrect  because the code is complicated by the optimization and the programmer is distracted by optimizing

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Variables should be declared as close to where they are used as possible   It makes RAII  Resource Acquisition Is Initialization  easier   It keeps the scope of the variable tight  This lets the optimizer work better

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You have a risk of NullPointerException if your calculateStr   method returns null and then you try to call a method on str   More generally  avoid having variables with a null value  It stronger for class attributes  by the way

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The str variable will be available and reserved some space in memory even after while executed below code    String str      while condition           str   calculateStr                          The str variable will not be available and also the memory will be released which was allocated for str variable in below code   while condition       String str   calculateStr                  If we followed the second one surely this will reduce our system memory and increase performance

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I think the size of the object matters as well  In one of my projects  we had declared and initialized a large two dimensional array that was making the application throw an out-of-memory exception  We moved the declaration out of the loop instead and cleared the array at the start of every iteration

[java] Declaring variables inside or outside of a loop

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