Determine if a String is an Integer in Java

Question

I m trying to determine if a particular item in an Array of strings is an integer or not   I am  split      ing an infix expression in String form  and then trying to split the resultant array into two arrays  one for integers  one for operators  whilst discarding parentheses  and other miscellaneous items  What would be the best way to accomplish this    I thought I might be able to find a Integer isInteger String arg  method or something  but no such luck

User · Answer

You want to use the Integer parseInt String  method   try    int num   Integer parseInt str        is an integer    catch  NumberFormatException e         not an integer

User · Answer

Or you can enlist a little help from our good friends at Apache Commons   StringUtils isNumeric String str

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You can use Integer parseInt str  and catch the NumberFormatException if the string is not a valid integer  in the following fashion  as pointed out by all answers    static boolean isInt String s     try     int i   Integer parseInt s   return true      catch NumberFormatException er      return false        However  note here that if the evaluated integer overflows  the same exception will be thrown  Your purpose was to find out whether or not  it was a valid integer  So its safer to make your own method to check for validity   static boolean isInt String s      assuming integer is in decimal number system    for int a 0 a lt s length   a           if a  0  amp  amp  s charAt a      -   continue      if   Character isDigit s charAt a     return false      return true

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The most naive way would be to iterate over the String and make sure all the elements are valid digits for the given radix  This is about as efficient as it could possibly get  since you must look at each element at least once  I suppose we could micro-optimize it based on the radix  but for all intents and purposes this is as good as you can expect to get   public static boolean isInteger String s        return isInteger s 10      public static boolean isInteger String s  int radix        if s isEmpty    return false      for int i   0  i  lt  s length    i              if i    0  amp  amp  s charAt i      -                 if s length      1  return false              else continue                    if Character digit s charAt i  radix   lt  0  return false            return true      Alternatively  you can rely on the Java library to have this  It s not exception based  and will catch just about every error condition you can think of  It will be a little more expensive  you have to create a Scanner object  which in a critically-tight loop you don t want to do  But it generally shouldn t be too much more expensive  so for day-to-day operations it should be pretty reliable   public static boolean isInteger String s  int radix        Scanner sc   new Scanner s trim         if  sc hasNextInt radix   return false         we know it starts with a valid int  now make sure        there s nothing left      sc nextInt radix       return  sc hasNext        If best practices don t matter to you  or you want to troll the guy who does your code reviews  try this on for size   public static boolean isInteger String s        try            Integer parseInt s          catch NumberFormatException e             return false         catch NullPointerException e            return false               only got here if we didn t return false     return true

User · Answer

Or simply  mystring matches    d     though it would return true for numbers larger than an int

User · Answer

Using regular expression is better   str matches  -   d       -      -- gt  negative sign  could have none or one   d    -- gt  one or more digits   It is not good to use NumberFormatException here if you can use if-statement instead     If you don t want leading zero s  you can just use the regular expression as follow   str matches  -  0  1-9   d

User · Answer

As an alternative approach to trying to parse the string and catching NumberFormatException  you could use a regex  e g  if  Pattern compile  quot -  0-9   quot   matches str            its an integer    This is likely to be faster  especially if you precompile and reuse the regex  However  the problem with this approach is that Integer parseInt str  will also fail if str represents a number that is outside range of legal int values   While it is possible to craft a regex that only matches integers in the range Integer MIN INT to Integer MAX INT  it is not a pretty sight    And I am not going to try it      On the other hand     it may be acceptable to treat  quot not an integer quot  and  quot integer too large quot  separately for validation purposes

User · Answer

public boolean isInt String str       return  str lastIndexOf  -      0  amp  amp   str equals  -0      str substring 1  matches                 d      str matches    d

User · Answer

You can use Integer parseInt   or Integer valueOf   to get the integer from the string  and catch the exception if it is not a parsable int   You want to be sure to catch the NumberFormatException it can throw   It may be helpful to note that valueOf   will return an Integer object  not the primitive int

[java] Determine if a String is an Integer in Java

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