使用PrintWriter时出错

Question

I am trying to write a program which prompts the user to enter a velocity and time. After this I need to calculate distance = velocity * time . If the user enters a time less than zero and bigger than time, then I need to re-prompt the user.

Hour        Distance Travelled
===========================
1           40
2           80
3           120

For example if the user enters time as 5, the table should resemble the following:

Hour        Distance Travelled
===========================
1           40
2           80
3           120
4           160
5           200

I need a table like above, but I need to output the table to a text file.

Here is my code:

import java.io.*;
import java.util.Scanner;

public class Lab4 {
    public static void main(String[] args) throws IOException {
        double distance;
        double velocity;
        double time;

        System.out.println("enter the velocity and time");
        Scanner sn = new Scanner(System.in);
        velocity = sn.nextDouble();
        time = sn.nextDouble();

        do {
            System.out.print(" Time cannot be smaller than zero and larger than ten");
            System.out.print("Please enter again");
            time = sn.nextDouble();
        } while(time < 0 && time > 10);

        System.out.print("please enter the file name");
        String filename = sn.nextLine();

        PrintWriter outputFile = new PrintWriter(filename);
        distance = velocity*time;
        outputFile.println(distance);
    }
}

Question 1 why I am getting this error:

PrintWriter outputFile = new PrintWriter(filename);
^
bad source file: .\PrintWriter.java
file does not contain class PrintWriter
Please remove or make sure it appears in the correct subdirectory of the sourcepath.

Question 2: How can I draw that file ?

Answer 1

There are a number of issues with your code and aleb2000 has already mentioned a big one (take aleb2000 's advice) but we'll only touch on the one your question is ultimately about and that is the error you are receiving.

You're getting this error because your supplied output file name is actually a Scanner newline character and PrintWriter doesn't know what to do with that. It's not something it recognizes as a valid path and file name.

Why is your supplied file name incorrect? Well, it's actually quite simple, there is a little quirk with the Scanner Class when utilizing the nextInt(), nextDouble() methods, or any Scanner method for that matter that expects a numerical value to be supplied, and that is when you hit the enter button on your keyboard you are also supplying a newline character which remains stored within the Scanner buffer and is only released when the Scanner.newLine() method is used as you do when you ask for a file name to be supplied. The newline character is not issued with the numerical values you supply however, when you supply the file name it is pulled forward from the buffer and takes the place of what was actually typed for the file name. Does this make sense to you?

Fortunately there is an easy solution to this problem and that is to simply apply the Scanner.newLine() method to nothing (no variable) directly after your last numerical input request, for example:

time = sn.nextDouble(); sn.nextLine();

You will obviously want to do this within your do/while loop as well but in my opinion you should do away with the do/while and just use a while loop like this (can you see why?):

while(time < 0.0 || time > 10.0) {
    System.out.println("Time cannot be smaller than zero and larger than ten!");
    System.out.print("Please enter again: --> ");
    time = sn.nextDouble(); sn.nextLine();
} 

Oh...and don't for get to close both your PrintWriter Object and your Scanner Object once the task is complete.