使用Java从文本文件中读取/求和数字

Question

I feel like I've made a really simple mistake, but I can't figure out what it is. I know the program is reading the correct file, but every time I run the program it simply returns 0.0000 as the sum. What have I done wrong?

Answer 1

When constructing a Scanner object that will read from a file, you must create a File object and pass it to the Scanner class constructor, it has the following definition:

public Scanner(File source)
        throws FileNotFoundException

You aren't actually creating a Scanner object on the file, you are creating a Scanner object that is reading the filename as its input, which of course cannot be interpreted as a double.

Change this line:

Scanner input = new Scanner(filename);

To this:

Scanner input = new Scanner( new File(filename) );

Answer 2

You're using the wrong Scanner Constructor, that is your's is reading from the String filename . I tested this,

// Why void? Just return the sum
public double readFile(String filename) {     
  Scanner input = null;
  double sum = 0;
  try {
    input = new Scanner(new File(filename));
    while (input.hasNextDouble()) {
      sum += input.nextDouble();
    }
    // output results
    System.out.printf("The total sum of the "
        + "doubles in the input file is %f\n", sum);
  } catch (FileNotFoundException e) {
    e.printStackTrace();
  } finally {
    input.close();
  }

I got the output

The total sum of the doubles in the input file is -3.651000