How to check if a String is numeric in Java

在解析它之前，你会如何检查一个String是否是一个数字？

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With Apache Commons Lang 3.5 and above: NumberUtils.isCreatable or StringUtils.isNumeric .

With Apache Commons Lang 3.4 and below: NumberUtils.isNumber or StringUtils.isNumeric .

You can also use StringUtils.isNumericSpace which returns true for empty strings and ignores internal spaces in the string. (The linked javadocs contain detailed examples for each method.)

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This is generally done with a simple user-defined function (ie 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 will fail if you're using non-Arabic digits (ie numerals other than 0 through to 9). 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();
}

---

if you are on android, then you should use:

android.text.TextUtils.isDigitsOnly(CharSequence str)

documentation can be found here

keep it simple. mostly everybody can "re-program" (the same thing).

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