Check if a String is numeric in Java

Question

Check a String if it is a number before we parse the string.

Answer 1

1. With Apache Commons Lang 3.4 and below: NumberUtils.isNumber or StringUtils.isNumeric.
2. While using Apache Commons Lang 3.5 and above: NumberUtils.isCreatable or StringUtils.isNumeric.
3. You can also use StringUtils.isNumericSpace which returns true for empty strings and ignores internal spaces in the string

Answer 2

This is the fastest way i know to check if String is Number or not:

public static boolean isNumber(String str){
  int i=0, len=str.length();
  boolean a=false,b=false,c=false, d=false;
  if(i<len && (str.charAt(i)=='+' || str.charAt(i)=='-')) i++;
  while( i<len && isDigit(str.charAt(i)) ){ i++; a=true; }
  if(i<len && (str.charAt(i)=='.')) i++;
  while( i<len && isDigit(str.charAt(i)) ){ i++; b=true; }
  if(i<len && (str.charAt(i)=='e' || str.charAt(i)=='E') && (a || b)){ i++; c=true; }
  if(i<len && (str.charAt(i)=='+' || str.charAt(i)=='-') && c) i++;
  while( i<len && isDigit(str.charAt(i)) ){ i++; d=true;}
  return i==len && (a||b) && (!c || (c && d));
}
static boolean isDigit(char c){
  return c=='0' || c=='1' || c=='2' || c=='3' || c=='4' || c=='5' || c=='6' || c=='7' || c=='8' || c=='9';
}

Answer 3

Java 8 Lambda Expression is used:

String someString = "123123";
boolean isNumeric = someString.chars().allMatch( Character::isDigit );