Should try catch go inside or outside a loop

Question

I have a loop that looks something like this   for  int i   0  i  lt  max  i          String myString            float myNum   Float parseFloat myString       myFloats i    myNum      This is the main content of a method whose sole purpose is to return the array of floats  I want this method to return null if there is an error  so I put the loop inside a try   catch block  like this   try       for  int i   0  i  lt  max  i              String myString                float myNum   Float parseFloat myString           myFloats i    myNum          catch  NumberFormatException ex        return null      But then I also thought of putting the try   catch block inside the loop  like this   for  int i   0  i  lt  max  i          String myString            try           float myNum   Float parseFloat myString         catch  NumberFormatException ex            return null            myFloats i    myNum      Is there any reason  performance or otherwise  to prefer one over the other     Edit  The consensus seems to be that it is cleaner to put the loop inside the try catch  possibly inside its own method  However  there is still debate on which is faster  Can someone test this and come back with a unified answer

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I s like to add my own 0 02c about two competing considerations when looking at the general problem of where to position exception handling    The  wider  the responsibility of the try-catch block  i e  outside the loop in your case  means that when changing the code at some later point  you may mistakenly add a line which is handled by your existing catch block  possibly unintentionally  In your case  this is less likely because you are explicitly catching a NumberFormatException The  narrower  the responsibility of the try-catch block  the more difficult refactoring becomes  Particularly when  as in your case  you are executing a  non-local  instruction from within the catch block  the return null statement

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Performance  as Jeffrey said in his reply  in Java it doesn t make much difference   Generally  for readability of the code  your choice of where to catch the exception depends upon whether you want the loop to keep processing or not     In your example you returned upon catching an exception   In that case  I d put the try catch around the loop   If you simply want to catch a bad value but carry on processing  put it inside     The third way  You could always write your own static ParseFloat method and have the exception handling dealt with in that method rather than your loop   Making the exception handling isolated to the loop itself   class Parsing       public static Float MyParseFloat string inputValue                try                       return Float parseFloat inputValue                     catch   NumberFormatException e                         return null                                your code     for int i   0  i  lt  max  i                   String myString                Float myNum   Parsing MyParseFloat myString           if   myNum    null   return          myFloats i     float  myNum

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All right  after Jeffrey L Whitledge said that there was no performance difference  as of 1997   I went and tested it  I ran this small benchmark   public class Main        private static final int NUM TESTS   100      private static int ITERATIONS   1000000         time counters     private static long inTime   0L      private static long aroundTime   0L       public static void main String   args            for  int i   0  i  lt  NUM TESTS  i                  test                ITERATIONS    1     so the tests don t always return the same number                   System out println  Inside loop       inTime 1000000 0      ms             System out println  Around loop       aroundTime 1000000 0      ms               public static void test             aroundTime    testAround            inTime    testIn              public static long testIn             long start   System nanoTime            Integer i   tryInLoop            long ret   System nanoTime   - start          System out println i      don t optimize it away         return ret            public static long testAround             long start   System nanoTime            Integer i   tryAroundLoop            long ret   System nanoTime   - start          System out println i      don t optimize it away         return ret            public static Integer tryInLoop             int count   0          for  int i   0  i  lt  ITERATIONS  i                  try                   count   Integer parseInt Integer toString count     1                catch  NumberFormatException ex                    return null                                  return count            public static Integer tryAroundLoop             int count   0          try               for  int i   0  i  lt  ITERATIONS  i                      count   Integer parseInt Integer toString count     1                            return count            catch  NumberFormatException ex                return null                      I checked the resulting bytecode using javap to make sure that nothing got inlined   The results showed that  assuming insignificant JIT optimizations  Jeffrey is correct  there is absolutely no performance difference on Java 6  Sun client VM  I did not have access to other versions   The total time difference is on the order of a few milliseconds over the entire test   Therefore  the only consideration is what looks cleanest  I find that the second way is ugly  so I will stick to either the first way or Ray Hayes s way

User · Answer

While performance might be the same and what  looks  better is very subjective  there is still a pretty big difference in functionality  Take the following example   Integer j   0      try           while  true                  j               if  j    20    throw new Exception                  if  j 4    0    System out println j                 if  j    40    break                    catch  Exception e            System out println  in catch block            The while loop is inside the try catch block  the variable  j  is incremented until it hits 40  printed out when j mod 4 is zero and an exception is thrown when j hits 20   Before any details  here the other example   Integer i   0      while  true            try                 i               if  i    20    throw new Exception                  if  i 4    0    System out println i                 if  i    40    break               catch  Exception e    System out println  in catch block              Same logic as above  only difference is that the try catch block is now inside the while loop    Here comes the output  while in try catch    4 8 12  16 in catch block   And the other output  try catch in while    4 8 12 16 in catch block 24 28 32 36 40   There you have quite a significant difference    while in try catch breaks out of the loop  try catch in while keeps the loop active

User · Answer

In your examples there is no functional difference   I find your first example more readable

User · Answer

You should prefer the outer version over the inner version  This is just a specific version of the rule  move anything outside the loop that you can move outside the loop  Depending on the IL compiler and JIT compiler your two versions may or may not end up with different performance characteristics   On another note you should probably look at float TryParse or Convert ToFloat

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If you want to catch Exception for each iteration  or check at what iteration Exception is thrown and catch every Exceptions in an itertaion  place try   catch inside the loop  This will not break the loop if Exception occurs and you can catch every Exception in each iteration throughout the loop   If you want to break the loop and examine the Exception whenever thrown  use try   catch out of the loop  This will break the loop and execute statements after catch  if any    It all depends on your need  I prefer using try   catch inside the loop while deploying as  if Exception occurs  the results aren t ambiguous and loop will not break and execute completely

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If it s inside  then you ll gain the overhead of the try catch structure N times  as opposed to just the once on the outside     Every time a Try Catch structure is called it adds overhead to the execution of the method  Just the little bit of memory  amp  processor ticks needed to deal with the structure   If you re running a loop 100 times  and for hypothetical sake  let s say the cost is 1 tick per try catch call  then having the Try Catch inside the loop costs you 100 ticks  as opposed to only 1 tick if it s outside of the loop

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PERFORMANCE   There is absolutely no performance difference in where the try catch structures are placed  Internally  they are implemented as a code-range table in a structure that is created when the method is called  While the method is executing  the try catch structures are completely out of the picture unless a throw occurs  then the location of the error is compared against the table   Here s a reference  http   www javaworld com javaworld jw-01-1997 jw-01-hood html  The table is described about half-way down

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Another aspect not mentioned in the above is the fact that every try-catch has some impact on the stack  which can have implications for recursive methods   If method  outer    calls method  inner     which may call itself recursively   try to locate the try-catch in method  outer    if possible   A simple  stack crash  example we use in a performance class fails at about 6 400 frames when the try-catch is in the inner method  and at about 11 600 when it is in the outer method   In the real world  this can be an issue if you re using the Composite pattern and have large  complex nested structures

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As long as you are aware of what you need to accomplish in the loop you could put the try catch outside the loop  But it is important to understand that the loop will then end as soon as the exception occurs and that may not always be what you want  This is actually a very common error in Java based software  People need to process a number of items  such as emptying a queue  and falsely rely on an outer try catch statement handling all possible exceptions  They could also be handling only a specific exception inside the loop and not expect any other exception to occur  Then if an exception occurs that is not handled inside the loop then the loop will be  preemted   it ends possibly prematurely and the outer catch statement handles the exception   If the loop had as its role in life to empty a queue then that loop very likely could end before that queue was really emptied   Very common fault

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The whole point of exceptions is to encourage the first style  letting the error handling be consolidated and handled once  not immediately at every possible error site

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setting up a special stack frame for the try catch adds additional overhead  but the JVM may be able to detect the fact that you re returning and optimize this away   depending on the number of iterations  performance difference will likely be negligible   However i agree with the others that having it outside the loop make the loop body look cleaner   If there s a chance that you ll ever want to continue on with the processing rather than exit if there an invalid number  then you would want the code to be inside the loop

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I ll put my  0 02 in   Sometimes you wind up needing to add a  finally  later on in your code  because who ever writes their code perfectly the first time     In those cases  suddenly it makes more sense to have the try catch outside the loop   For example   try       for int i   0  i  lt  max  i              String myString                float myNum   Float parseFloat myString           dbConnection update  MY FLOATS   INDEX  i  VALUE  myNum           catch  NumberFormatException ex        return null    finally       dbConnection release        Always release DB connection  even if transaction fails      Because if you get an error  or not  you only want to release your database connection  or pick your favorite type of other resource     once

User · Answer

If you put the try catch inside the loop  you ll keep looping after an exception   If you put it outside the loop you ll stop as soon as an exception is thrown

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My perspective would be try catch blocks are necessary to insure proper exception handling  but creating such blocks has performance implications  Since  Loops contain intensive repetitive computations  it is not recommended to put try catch blocks inside loops  Additionally  it seems where this condition occurs  it is often  Exception  or  RuntimeException  which is caught  RuntimeException being caught in code should be avoided  Again  if if you work in a big company it s essential to log that exception properly  or stop runtime exception to happen  Whole point of this description is PLEASE AVOID USING TRY-CATCH BLOCKS IN LOOPS

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I agree with all the performance and readability posts   However  there are cases where it really does matter   A couple other people mentioned this  but it might be easier to see with examples   Consider this slightly modified example   public static void main String   args        String   myNumberStrings   new String     1 2345    asdf    2 3456        ArrayList asNumbers   parseAll myNumberStrings      public static ArrayList parseAll String   numberStrings       ArrayList myFloats   new ArrayList         for int i   0  i  lt  numberStrings length  i             myFloats add new Float numberStrings i               return myFloats      If you want the parseAll   method to return null if there are any errors  like the original example   you d put the try catch on the outside like this   public static ArrayList parseAll1 String   numberStrings       ArrayList myFloats   new ArrayList        try          for int i   0  i  lt  numberStrings length  i                 myFloats add new Float numberStrings i                     catch  NumberFormatException nfe             fail on any error         return null            return myFloats      In reality  you should probably return an error here instead of null  and generally I don t like having multiple returns  but you get the idea   On the other hand  if you want it to just ignore the problems  and parse whatever Strings it can  you d put the try catch on the inside of the loop like this   public static ArrayList parseAll2 String   numberStrings       ArrayList myFloats   new ArrayList         for int i   0  i  lt  numberStrings length  i             try              myFloats add new Float numberStrings i               catch  NumberFormatException nfe                 don t add just this one                      return myFloats

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That depends on the failure handling  If you just want to skip the error elements  try inside   for int i   0  i  lt  max  i          String myString            try           float myNum   Float parseFloat myString           myFloats i    myNum        catch  NumberFormatException ex            --i            In any other case i would prefer the try outside  The code is more readable  it is more clean  Maybe it would be better to throw an IllegalArgumentException in the error case instead if returning null

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I ll put my  0 02 in   Sometimes you wind up needing to add a  finally  later on in your code  because who ever writes their code perfectly the first time     In those cases  suddenly it makes more sense to have the try catch outside the loop   For example   try       for int i   0  i  lt  max  i              String myString                float myNum   Float parseFloat myString           dbConnection update  MY FLOATS   INDEX  i  VALUE  myNum           catch  NumberFormatException ex        return null    finally       dbConnection release        Always release DB connection  even if transaction fails      Because if you get an error  or not  you only want to release your database connection  or pick your favorite type of other resource     once

User · Answer

That depends on the failure handling  If you just want to skip the error elements  try inside   for int i   0  i  lt  max  i          String myString            try           float myNum   Float parseFloat myString           myFloats i    myNum        catch  NumberFormatException ex            --i            In any other case i would prefer the try outside  The code is more readable  it is more clean  Maybe it would be better to throw an IllegalArgumentException in the error case instead if returning null

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As already mentioned  the performance is the same   However  user experience isn t necessarily identical   In the first case  you ll fail fast  i e  after the first error   however if you put the try catch block inside the loop  you can capture all the errors that would be created for a given call to the method   When parsing an array of values from strings where you expect some formatting errors  there are definitely cases where you d like to be able to present all the errors to the user so that they don t need to try and fix them one by one

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put it inside   You can keep processing  if you want  or you can throw a helpful exception that tells the client the value of myString and the index of the array containing the bad value  I think NumberFormatException will already tell you the bad value but the principle is to place all the helpful data in the exceptions that you throw   Think about what would be interesting to you in the debugger at this point in the program     Consider    try         parse   catch  NumberFormatException nfe      throw new RuntimeException  Could not parse as a Float       myString                                     found at index      i  nfe        In the time of need you will really appreciate an exception like this with as much information in it as possible

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If its an all-or-nothing fail  then the first format makes sense   If you want to be able to process return all the non-failing elements  you need to use the second form   Those would be my basic criteria for choosing between the methods   Personally  if it is all-or-nothing  I wouldn t use the second form

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I s like to add my own 0 02c about two competing considerations when looking at the general problem of where to position exception handling    The  wider  the responsibility of the try-catch block  i e  outside the loop in your case  means that when changing the code at some later point  you may mistakenly add a line which is handled by your existing catch block  possibly unintentionally  In your case  this is less likely because you are explicitly catching a NumberFormatException The  narrower  the responsibility of the try-catch block  the more difficult refactoring becomes  Particularly when  as in your case  you are executing a  non-local  instruction from within the catch block  the return null statement

[java] Should try...catch go inside or outside a loop?

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