为什么Scanner的hasNext（）方法最初为非空文本文件返回false？

Question

I'm trying to write some text to a file. I have a while loop that is supposed to just take some text and write the exact same text back to the file.

I discovered that the while loop is never entered because Scanner thinks there's no more text to read. But there is.

import java.util.Scanner;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.PrintWriter;

public class WriteToFile {

    public static void main(String[] args) throws FileNotFoundException {

        String whatToWrite = "";

        File theFile = new File("C:\\test.txt");

        Scanner readinput = new Scanner(theFile);

        PrintWriter output = new PrintWriter(theFile);

        while (readinput.hasNext()) { //why is this false initially?

            String whatToRead = readinput.next();

            whatToWrite = whatToRead;

            output.print(whatToWrite);
        }

        readinput.close();
        output.close();

    }

}

The text file just contains random words. Dog, cat, etc.

When I run the code, text.txt becomes empty.

There was a similar question: https://stackoverflow.com/questions/8495850/scanner-hasnext-returns-false which pointed to encoding issues. I use Windows 7 and US language. Can I find out how the text file is encoded somehow?

Update:

Indeed, as Ph.Voronov commented, the PrintWriter line erases the file contents! user2115021 is right, if you use PrintWriter you should not work on one file. Unfortunately, for the assignment I had to solve, I had to work with a single file. Here's what I did:

import java.util.ArrayList;
import java.util.Scanner;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.PrintWriter;

public class WriteToFile {

    public static void main(String[] args) throws FileNotFoundException {

        ArrayList<String> theWords = new ArrayList<String>();

        File theFile = new File("C:\\test.txt");

        Scanner readinput = new Scanner(theFile);

        while (readinput.hasNext()) {

            theWords.add(readinput.next());

        }

        readinput.close();

        PrintWriter output = new PrintWriter(theFile); //we already got all of
            //the file content, so it's safe to erase it now

        for (int a = 0; a < theWords.size(); a++) {
            output.print(theWords.get(a));
            if (a != theWords.size() - 1) {
                output.print(" ");
            }
        }

        output.close();

    }

}

Answer 1

PrintWriter output = new PrintWriter(theFile);

它会删除您的文件。

Answer 2

您正在尝试使用SCANNER读取文件并使用PRINTWRITER写入另一个文件，但是两者都在同一个文件上。PRINTWRITER清除文件的内容以写入内容。两个类都需要在不同的文件上工作。