HOW TO CHECK IF A STRING IS A NUMERIC TYPE IN JAVA

Jessica James

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Points:114
Posted by  Jessica James
Java  String 
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How would you check if a String was a number before parsing it?
  1. Mayank Tripathi

    Post:397

    Points:3117
    Re: How to check if a String is a numeric type in Java

    This is generally done with a simple user-defined function (i.e. Roll-your-own "isNumeric" function).

    Something like:
    public static boolean isNumeric(String str)  
    {  
      try  
      {  
        double d = Double.parseDouble(str);  
      }  
      catch(NumberFormatException nfe)  
      {  
        return false;  
      }  
      return true;  
    }
    However, if you're calling this function a lot, and you expect many of the checks to fail due to not being a number then performance of this mechanism will not be great, since you're relying upon exceptions being thrown for each failure, which is a fairly expensive operation.

    An alternative approach may be to use a regular expression to check for validity of being a number:

    public static boolean isNumeric(String str)
    {
      return str.matches("-?\\d+(\\.\\d+)?");  //match a number with optional '-' and decimal.
    }
    Be careful with the above RegEx mechanism, though, as it'll fail if your using non-latin (i.e. 0 to 9) digits. For example, arabic digits. This is because the "\d" part of the RegEx will only match [0-9] and effectively isn't internationally numerically aware. (Thanks to OregonGhost for pointing this out!)

    Or even another alternative is to use Java's built-in java.text.NumberFormat object to see if, after parsing the string the parser position is at the end of the string. If it is, we can assume the entire string is numeric:

    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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