Question

Trying to read double data from the file that has different contents. For example if it is a double then the message should be “Double number is 23.5”. and if it is not double number the message should be “Sixty three is not a double number”. The file contents are

97.9

100.1

Three

Sixty three

77

12.4

3002.4

76

Cool

34.6

This is it

…………

The code i wrote opens the file and scans next line But does not seem to properly work.

class ReadDouble 
 {

 Scanner scan = new Scanner(System.in);


try

{

    File textFile = new File ("doubleData.txt");
    Scanner scanFile = new Scanner (textFile);
    String str = scan.nextLine();

    while(scanFile.hasNextLine())
    {

        double num = Double.parseDouble(str);
        if(str == num)
        {
            System.out.println("Double number is" + str);
        }

    }//end while


}//end try

catch (NumberFormatException nfe)
{
   System.out.println(str + "Is not a Double number");
}

}

}//end class

Answer 1

First, you should call String str = scan.nextLine(); within the loop otherwise you only ever read the first line. Also, your try / catch block should be wrapped around double num = Double.parseDouble(str); within the while loop otherwise you will not make another call to scan.nextLine() after you encounter your first non-double.

Finally, you shouldn’t do if(str == num) as this will always be false. If Double.parseDouble(str) does not throw an exception, it contains the double found on that line.

Here is a solution that reads from standard in:

import java.util.Scanner;

public class ReadDouble {

    public static void main(String[] args){
        Scanner scan = new Scanner (System.in);

        while(scan.hasNextLine()){
        String str = scan.nextLine();

        try {
            num = Double.parseDouble(str);
            System.out.println("Double number is " + num);
        } catch (NumberFormatException nfe) {
            System.out.println(str + " is not a Double number");
        }       
    }
}
}

Another option is to use Scanner to see if the next element is a double if it is read it using nextDouble() otherwise read using nextLine().