How to avoid Number Format Exception in java

Question

In my day to day web application development there are many instances where we need to take some number inputs from the user    Then pass on this number input to may be service or DAO layer of the application    At some stage since its a number  integer or float   we need to convert it into Integer as shown in the following code snippet   String cost   request getParameter  cost     if  cost   null  amp  amp      equals cost         Integer intCost   Integer parseInt cost       List lt Book gt  books   bookService   findBooksCheaperThan intCost         Here in the above case I have to check if the input is not null or if there is no input  blank  or sometimes there is a possibility of a non number inputs e g  blah  test etc   What is the best possible way of handling such situations

User · Answer

As always  the Jakarta Commons have at least part of the answer    NumberUtils isNumber    This can be used to check most whether a given String is a number  You still have to choose what to do in case your String isnt a number

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Try to convert Prize into decimal format     import java math BigDecimal  import java math RoundingMode   public class Bigdecimal       public static boolean isEmpty  String st            return st    null    st length    lt  1             public static BigDecimal bigDecimalFormat String Preis                     MathContext   mi   new MathContext 2           BigDecimal bd   new BigDecimal 0 00                             bd   new BigDecimal Preis                 return bd setScale 2  RoundingMode HALF UP                  public static void main String   args            String cost    12 12           if   isEmpty cost                 try                  BigDecimal intCost   bigDecimalFormat cost                  System out println intCost                  List lt Book gt  books   bookService findBooksCheaperThan intCost                   catch  NumberFormatException e                   System out println  This is not a number                   System out println e getMessage

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I suggest to do 2 things    validate the input on client side before passing it to the Servlet catch the exception and show an error message within the user frontend as Tobiask mentioned  This case should normally not happen  but never trust your clients   -

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That depends on your environment  JSF for example would take the burden of manually checking and converting Strings  lt -  Numbers from you  Bean Validation is another option   What you can do immediately in the snippet you provide    Extract method getAsInt String param   in it  Use String isEmpty    Since Java 6    surround with try   catch   What you should definitely think about if you happen to write a lot of code like this   public void myBusinessMethod  ValidNumber String numberInput                    This would be interceptor-based    Last but not least  Save the work and try to switch to a framework which gives you support for these common tasks

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Documentation for the method from the Apache Commons Lang  from here    Checks whether the String a valid Java number  Valid numbers include hexadecimal marked with the 0x qualifier  scientific notation and numbers marked with a type qualifier  e g  123L   Null and empty String will return false  Parameters   str  - the  String  to check  Returns   true  if the string is a correctly formatted number   isNumber from java org apache commons lang3 math NumberUtils  public static boolean isNumber final String str        if  StringUtils isEmpty str             return false            final char   chars   str toCharArray        int sz   chars length      boolean hasExp   false      boolean hasDecPoint   false      boolean allowSigns   false      boolean foundDigit   false         deal with any possible sign up front     final int start    chars 0      -     1   0      if  sz  gt  start   1  amp  amp  chars start      0   amp  amp  chars start   1      x             int i   start   2          if  i    sz                return false     str     quot 0x quot                       checking hex  it can t be anything else          for    i  lt  chars length  i                  if   chars i   lt   0     chars i   gt   9                    amp  amp   chars i   lt   a     chars i   gt   f                    amp  amp   chars i   lt   A     chars i   gt   F                      return false                                  return true            sz--     don t want to loop to the last char  check it afterwords              for type qualifiers     int i   start         loop to the next to last char or to the last char if we need another digit to        make a valid number  e g  chars 0  5     quot 1234E quot       while  i  lt  sz     i  lt  sz   1  amp  amp  allowSigns  amp  amp   foundDigit             if  chars i   gt    0   amp  amp  chars i   lt    9                 foundDigit   true              allowSigns   false             else if  chars i                        if  hasDecPoint    hasExp                       two decimal points or dec in exponent                    return false                            hasDecPoint   true            else if  chars i      e     chars i      E                    we ve already taken care of hex              if  hasExp                       two E s                 return false                            if   foundDigit                    return false                            hasExp   true              allowSigns   true            else if  chars i            chars i      -                 if   allowSigns                    return false                            allowSigns   false              foundDigit   false     we need a digit after the E           else               return false                    i              if  i  lt  chars length            if  chars i   gt    0   amp  amp  chars i   lt    9                    no type qualifier  OK             return true                    if  chars i      e     chars i      E                    can t have an E at the last byte             return false                    if  chars i                        if  hasDecPoint    hasExp                       two decimal points or dec in exponent                 return false                               single trailing decimal point after non-exponent is ok             return foundDigit                    if   allowSigns              amp  amp   chars i      d                     chars i      D                     chars i      f                     chars i      F                  return foundDigit                    if  chars i      l                 chars i      L                    not allowing L with an exponent or decimal point             return foundDigit  amp  amp   hasExp  amp  amp   hasDecPoint                       last character is illegal         return false               allowSigns is true iff the val ends in  E         found digit it to make sure weird stuff like     and  1E-  doesn t pass     return  allowSigns  amp  amp  foundDigit      code is under version 2 of the Apache License

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To Determine if a string is Int or Float and to represent in longer format   Integer   String  cost Long MAX VALUE       if  isNumeric  cost         returns false for non numeric             BigInteger bi    new BigInteger cost         public static boolean isNumeric String str        NumberFormat formatter   NumberFormat getInstance       ParsePosition pos   new ParsePosition 0      formatter parse str  pos      return str length      pos getIndex

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I don t know about the runtime disadvantages about the following but you could run a regexp match on your string to make sure it is a number before trying to parse it  thus  cost matches  -   d       d      for a float  and   cost matches  -   d      for an integer  EDIT  please notices  Voo s comment about max int

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Just catch your exception and do proper exception handling      if  cost   null  amp  amp      equals cost             try              Integer intCost   Integer parseInt cost              List lt Book gt  books   bookService   findBooksCheaperThan intCost               catch  NumberFormatException e               System out println  This is not a number               System out println e getMessage

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public class Main       public static void main String   args             String number           while true                try                  number   JOptionPane showInputDialog null                    if  Main isNumber number                        break                catch NumberFormatException e                   System out println e getMessage                                       System out println  Your number is     number               public static boolean isNumber Object o           boolean isNumber   true           for  byte b   o toString   getBytes                  char c    char b              if  Character isDigit c                   isNumber   false                     return isNumber

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Exceptions in recent versions of Java aren t expensive enough to make their avoidance important  Use the try catch block people have suggested  if you catch the exception early in the process  i e   right after the user has entered it  then you re not going to have the problem later in the process  because it ll be the right type anyway    Exceptions used to be a lot more expensive than they are now  don t optimize for performance until you know the exceptions are actually causing a problem  and they won t  here

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You can avoid the unpleasant looking try catch or regex by using the Scanner class   String input    123   Scanner sc   new Scanner input   if  sc hasNextInt        System out println  an int      sc nextInt     else         handle the bad input

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one posibility  catch the exception and show an error message within the user frontend    edit  add an listener to the field within the gui and check the user inputs there too  with this solution the exception case should be very rare

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In Java there s sadly no way you can avoid using the parseInt function and just catching the exception  Well you could theoretically write your own parser that checks if it s a number  but then you don t need parseInt at all anymore   The regex method is problematic because nothing stops somebody from including a number   INTEGER MAX VALUE which will pass the regex test but still fail

[java] How to avoid Number Format Exception in java?

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