Question

如何从文件中读取数字，字符串或字符。我发现了很多方法，但我不知道最佳和清晰的方法。我想进行诸如系统控制台之类的输入/输出操作。

我创建了2个文件input.in和output.out，我想从input.in读取一个数字并将其打印到output.out。

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

public class MainClass {

    public static void main(String[] args) throws IOException {
        File file = new File("input.in");
        Scanner scanner = new Scanner(file);
        String a = scanner.nextLine();
        System.out.println(a);
    }
}

我首先要测试是否可以从文件中读取内容，但是我知道了：

Exception in thread "main" java.util.NoSuchElementException: No line found
  at java.util.Scanner.nextLine(Unknown Source)
  at MainClass.main(MainClass.java:9)

在文件input.in中，我有：

文件

我/ o

Answer 1

就像评论中提到的RealSkeptic一样，我怀疑您的应用程序找不到您提供的文件。我建议您采用以下代码：

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

public class MainClass {
    public static void main(String[] args) throws Exception {
        String filePath = "c:\\absolute\\path\\to\\input.in";
        File file = new File(filePath);
        if(file.exists()) {
            System.out.println("Reading contents of file at: " + filePath)
            try (Scanner scanner = new Scanner(file)) {
                while(scanner.hasNext()) {
                    String a = scanner.nextLine();
                    System.out.println(a);
                }
            }
        }
        else {
            System.out.println("File not found at: " + filePath);
        }
    }
}

参考文献：

Answer 2

您可以使用FileInputStream和FileOutputStream来做到这一点。

希望这会达到您的目的

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

public class MainClass {

    public static void main(String[] args) throws IOException {
        FileInputStream inputStream = null;
        FileOutputStream outputStream = null;
        Scanner sc = null;
        try {
            inputStream = new FileInputStream("IN File");
            outputStream = new FileOutputStream("OUT File");
            sc = new Scanner(inputStream, "UTF-8");
            while (sc.hasNextLine()) {
                String line = sc.nextLine();
                System.out.println(line);
                byte[] strToBytes = line.getBytes();
                outputStream.write(strToBytes);
                outputStream.write(System.getProperty("line.separator").getBytes());
            }
            // note that Scanner suppresses exceptions
            if (sc.ioException() != null) {
                throw sc.ioException();
            }
        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } finally {
            if (inputStream != null) {
                inputStream.close();
            }
            if (sc != null) {
                sc.close();
            }
        }
    }
}

Answer 3

路径错误。对于那些也有我的问题的人，这里是输入/输出操作的代码：

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

public class MainClass {

    public static void main(String[] args) throws IOException {
        //the root path is your project name
        File in = new File("input.in");
        File out = new File("output.out");

        Scanner scannerInput = new Scanner(in);
        BufferedWriter writer = new BufferedWriter(new FileWriter(out));
        String string;
        while(scannerInput.hasNextLine()) {
            string = scannerInput.nextLine();
            writer.write(string);
            writer.newLine();
        }

        writer.flush();//very important
        scannerInput.close();
        writer.close();

    }
}

从文件读取和写入Java

3 个答案: