how to determine on Java if a string is a double or not

Question

I want to know if a string is a double or not

for example

   value1 = "236.685"   
   value2 = "it is 3"  

I want a code which will return

value1 is a double
value2 is not a double

Answer 1

You can check that by trying to parse the string with Double.parseDouble() and catching a NumberFormatException. If it doesn't throw this exception, it's a valid double. This code:

private static boolean isDouble(String string)
{
    try
    {
        Double.parseDouble(string);
    }
    catch (NumberFormatException e)
    {
        return false;
    }
    return true;
}

public static void main(String[] args)
{
    String maybeDouble0 = "123.4";
    String maybeDouble1 = "not a double 345";
    String maybeDouble2 = "45";
    String maybeDouble3 = "321.";
    String maybeDouble4 = ".753";
    String maybeDouble5 = "just text";
    System.out.println(maybeDouble0 + ": " + isDouble(maybeDouble0));
    System.out.println(maybeDouble1 + ": " + isDouble(maybeDouble1));
    System.out.println(maybeDouble2 + ": " + isDouble(maybeDouble2));
    System.out.println(maybeDouble3 + ": " + isDouble(maybeDouble3));
    System.out.println(maybeDouble4 + ": " + isDouble(maybeDouble4));
    System.out.println(maybeDouble5 + ": " + isDouble(maybeDouble5));
}

will result in this output:

123.4: true
not a double 345: false
45: true
321.: true
.753: true
just text: false

---

I prefer this method over a RegEx pattern because it's asking Java directly, instead of assuming to know Java's internal double representations.

Answer 2

Use a try catch statement for Double.parseDouble()

try{
    double num = Double.parseDouble(value);
    System.out.println("is a double");
catch (Exception e){
    System.out.println("not a double");
}

Answer 3

You can use a combination of parseInt and parseDouble

The reason you need both is because parseDouble will still parse ints. You can probably combine them, I just wrote two separate functions, one checks ints and the other checks double.

If isInt is false and isDouble is true then it is a double

If both are true, then it is an int(i think that is not a double from your example). If both are false then its not a number.

The last case is never possible.

   public static boolean isInt( String str ){ 
       try{
           Integer.parseInt( str );
           System.out.println("true");
           return true;
       }catch( Exception e ){
           System.out.println("false");
           return false;
      }  
  }


    public static boolean isDouble( String str ){
      try{
          Double.parseDouble( str );
          System.out.println("true");
         return true;
      }catch( Exception e ){
          System.out.println("false");
          return false;
      }
    }