Text files: write and read with Scanner · ⁨文本文件:用 Scanner 写和读⁩ — Java for AP CS A

English

Reading and writing text files

A text file stores text that stays on disk after the program ends.
To read a file in Java we use a Scanner connected to a File.
To write a file we use a PrintWriter. In this lesson each program writes a file, then reads it back.
Bonus lesson: the AP CSA exam does not test file input/output. Learn this for real programs — you will not be examined on it.

中文

读写文本文件

文本文件(text file)把文字存到磁盘上,程序结束后它还在。
在 Java 里读文件,我们用一个连到 File 的 Scanner。
写文件我们用 PrintWriter。在这一课里,每个程序先写一个文件,再把它读回来。
附加内容: AP CSA 考试不考文件输入输出。学它是为了写真实的程序 —— 考试不会考。

Explore · ⁨探索⁩

Reading and writing a file · ⁨读写一个文件⁩

A file is used in three steps: open · ⁨开放的⁩ it, read or write lines, then close · ⁨关闭⁩ it. · ⁨用文件分三步:打开它,读或写每一行,然后关闭它。⁩

English

The imports you need

File tools live in two places, so we import both at the top.
import java.io.*; — for File, PrintWriter, and IOException.
import java.util.Scanner; — for the Scanner reader.

中文

你需要的 import

文件工具放在两个地方,所以我们在顶部把两个都导入。
import java.io.*; —— 用于 File、PrintWriter 和 IOException。
import java.util.Scanner; —— 用于读取器 Scanner。

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

English

Files can fail, so we say `throws IOException`

Reading or writing a file can go wrong (the file may be missing).
Java makes you handle this. The simplest way is to add throws IOException to main.
This says "if a file error happens, let the program stop". That is fine for these tasks.

中文

文件可能出错,所以我们写 `throws IOException`

读或写文件可能会出问题(文件可能不存在)。
Java 要求你处理这种情况。最简单的办法是给 main 加上 throws IOException。
这句话的意思是"如果发生文件错误,就让程序停下来"。对这些任务来说这样就够了。

English

Writing a file with `PrintWriter`

new PrintWriter("data.txt") makes (or empties) a file called data.txt.
Use writer.println(...) to write one line, just like printing to the screen.
Always call writer.close() when you finish, so the text is saved.

中文

用 `PrintWriter` 写文件

new PrintWriter("data.txt") 会创建(或清空)一个叫 data.txt 的文件。
用 writer.println(...) 写一行,就像往屏幕上打印一样。
写完后一定要调用 writer.close(),这样文字才会被保存。

import java.io.*;

public class WriteDemo {
    public static void main(String[] args) throws IOException {
        PrintWriter writer = new PrintWriter("data.txt");
        writer.println(10);
        writer.println(20);
        writer.close();           // save the file
        System.out.println("wrote data.txt");
    }
}

English

Reading numbers with `Scanner`

new Scanner(new File("data.txt")) opens the file for reading.
in.hasNextInt() is true while there is another whole number to read.
in.nextInt() reads the next whole number. Call in.close() when done.

中文

用 `Scanner` 读数字

new Scanner(new File("data.txt")) 打开文件来读取。
只要还有下一个整数,in.hasNextInt() 就是 true。
in.nextInt() 读取下一个整数。读完后调用 in.close()。

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

public class ReadNumbers {
    public static void main(String[] args) throws IOException {
        Scanner in = new Scanner(new File("data.txt"));
        int total = 0;
        while (in.hasNextInt()) {     // more numbers?
            total = total + in.nextInt();
        }
        in.close();
        System.out.println(total);
    }
}

English

Reading whole lines

Sometimes a line is text, not a number. Then read lines instead.
in.hasNextLine() is true while there is another line.
in.nextLine() reads the whole next line as a String.

中文

整行读取

有时一行是文字,而不是数字。那就改成读整行。
只要还有下一行,in.hasNextLine() 就是 true。
in.nextLine() 把下一整行作为一个 String 读进来。

English

hasNext() (no Int or Line) checks for the next word, and next() reads one word.
The pattern is always: check with a hasNext... method, then read with the matching method.

中文

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

public class ReadLines {
    public static void main(String[] args) throws IOException {
        Scanner in = new Scanner(new File("data.txt"));
        while (in.hasNextLine()) {
            String line = in.nextLine();
            System.out.println("read: " + line);
        }
        in.close();
    }
}

hasNext()(后面没有 Int 或 Line)检查下一个单词,next() 读取一个单词。
套路始终是:先用 hasNext... 方法检查,再用对应的方法读取。

English

Write then read in one program

In each task your program first writes a file, then reads the same file back.
This means you can run it again and again and get the same result.

中文

在一个程序里先写后读

在每个任务里,你的程序先写一个文件,然后把同一个文件读回来。
这样你可以一遍又一遍地运行它,得到相同的结果。

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

public class RoundTrip {
    public static void main(String[] args) throws IOException {
        PrintWriter writer = new PrintWriter("data.txt");
        writer.println(3);
        writer.println(4);
        writer.close();

        Scanner in = new Scanner(new File("data.txt"));
        int total = 0;
        while (in.hasNextInt()) {
            total = total + in.nextInt();
        }
        in.close();
        System.out.println(total);   // 7
    }
}

English

Common mistakes

nextInt() leaves the newline, so a following nextLine() reads an empty line.
Check hasNext() before reading to avoid running off the end.

中文

常见错误

nextInt() 会留下换行符,导致紧接的 nextLine() 读到空行。
读之前检查 hasNext(),避免读过末尾。

English

Now you try

Each task pre-fills a class. Write the file first, then read it back, as the prompt asks.
Remember the throws IOException on main, and close() both the writer and the Scanner.
Press Run to compile and run, then Check answer.

中文

现在轮到你

每个任务都预填好了一个类。按提示要求,先写文件,再把它读回来。
记得给 main 加上 throws IOException,并且把 writer 和 Scanner 都 close()。
按运行来编译并运行,然后按检查答案。

First write the numbers 10, 20, 5, 7 to · ⁨ 到 ⁩data.txt (one per line) with a PrintWriter. Then open it with new Scanner(new File("data.txt")) and use hasNextInt/nextInt to add them up. Print only the total. Expected output: 42. · ⁨先用 PrintWriter 把数字 10、20、5、7 写到 data.txt(每行一个)。然后用 new Scanner(new File("data.txt")) 打开它,用 hasNextInt/nextInt 把它们加起来。只打印总和。期望输出:42。⁩

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

public class SumFile {
    public static void main(String[] args) throws IOException {
        // 1) Write 10, 20, 5, 7 to data.txt (one number per line).
        PrintWriter writer = new PrintWriter("data.txt");
        // TODO: writer.println(...) for each number, then writer.close();
        writer.close();

        // 2) Open data.txt and add the numbers with hasNextInt / nextInt.
        int total = 0;
        // TODO: Scanner in = new Scanner(new File("data.txt")); ... in.close();

        System.out.println(total);
    }
}

Click Run to see the output here. · ⁨点击“运行”查看此处输出。⁩

First write the lines apple, banana, cherry to · ⁨ 到 ⁩words.txt with a PrintWriter. Then open it with a Scanner and use hasNextLine/nextLine to count the lines. Print only the count. Expected output: 3. · ⁨先用 PrintWriter 把 apple、banana、cherry 三行写到 words.txt。然后用 Scanner 打开它,用 hasNextLine/nextLine 数出有多少行。只打印行数。期望输出:3。⁩

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

public class CountLines {
    public static void main(String[] args) throws IOException {
        // 1) Write apple, banana, cherry to words.txt (one per line).
        PrintWriter writer = new PrintWriter("words.txt");
        // TODO: writer.println(...) for each word, then writer.close();
        writer.close();

        // 2) Open words.txt and count lines with hasNextLine / nextLine.
        int count = 0;
        // TODO: Scanner in = new Scanner(new File("words.txt")); ... in.close();

        System.out.println(count);
    }
}

Click Run to see the output here. · ⁨点击“运行”查看此处输出。⁩

Text files: write and read with Scanner · ⁨文本文件:用 Scanner 写和读⁩

Reading and writing text files

读写文本文件

Reading and writing a file · ⁨读写一个文件⁩

The imports you need

你需要的 import

Files can fail, so we say `throws IOException`

文件可能出错,所以我们写 `throws IOException`

Writing a file with `PrintWriter`

用 `PrintWriter` 写文件

Reading numbers with `Scanner`

用 `Scanner` 读数字

Reading whole lines

整行读取

Write then read in one program

在一个程序里先写后读

Common mistakes

常见错误

Now you try

现在轮到你

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Feedback & help · ⁨反馈与帮助⁩

Text files: write and read with Scanner · ⁨文本文件:用 Scanner 写和读⁩

Reading and writing text files

读写文本文件

Reading and writing a file · ⁨读写一个文件⁩

The imports you need

你需要的 import

Files can fail, so we say throws IOException

文件可能出错,所以我们写 throws IOException

Writing a file with PrintWriter

用 PrintWriter 写文件

Reading numbers with Scanner

用 Scanner 读数字

Reading whole lines

整行读取

Write then read in one program

在一个程序里先写后读

Common mistakes

常见错误

Now you try

现在轮到你

Files can fail, so we say `throws IOException`

文件可能出错,所以我们写 `throws IOException`

Writing a file with `PrintWriter`

用 `PrintWriter` 写文件

Reading numbers with `Scanner`

用 `Scanner` 读数字