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

Hope this helps!

Check out the Java Course and learn more about Java strings.

Thanks!

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 );