为何需链式使用FileReader与BufferedReader等读取器？FileScanner能否单独用？read()返回值如何转字符？

Great questions—let's unpack each one clearly, since these are common points of confusion when starting out with Java file I/O.

1. Why chain FileReader with BufferedReader/Scanner? #

First off, it's not technically required to chain them—but it's almost always recommended for two big reasons:

• Performance: FileReader's read() method reads one character at a time directly from the file. That means a lot of slow I/O calls to the disk. BufferedReader adds a buffer (default 8KB) that reads chunks of data at once, drastically reducing disk access and speeding things up.
• Convenience: Scanner and BufferedReader give you built-in methods to handle common tasks without writing extra code. For example, BufferedReader has readLine() to grab an entire line, and Scanner can parse integers/doubles directly with nextInt() or nextDouble(). FileReader only gives you single-character or bulk-array reads, so you'd have to manually build strings or parse values yourself.

2. Can I use FileReader alone for practice? #

Absolutely! If you're just learning the basics and don't care about performance or fancy features, using FileReader by itself is totally fine. It's a great way to understand the low-level mechanics of reading characters from a file.

Here's a quick example of using FileReader alone to read a small text file:

import java.io.FileReader;
import java.io.IOException;

public class SimpleFileRead {
    public static void main(String[] args) {
        try (FileReader reader = new FileReader("example.txt")) {
            int charCode;
            while ((charCode = reader.read()) != -1) {
                System.out.print((char) charCode);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

This works perfectly for small files or practice scenarios—no need to overcomplicate it with extra readers if you don't need their features.

3. How to convert the int returned by FileReader's read() to a char? #

The read() method returns an int for two key reasons:

• It holds the Unicode code point of the character (ranging from 0 to 65535).
• It uses the value -1 to signal that you've reached the end of the file (since char is an unsigned type, it can't represent a negative value to indicate EOF).

To convert it to a char, you just need to:

1. First check if the returned int is not -1 (if it is, stop reading—you're done).
2. Cast the int to a char directly.

Using the example from above:

int charCode = reader.read();
if (charCode != -1) {
    char actualChar = (char) charCode;
    // Do something with actualChar, like print it
}

This cast is safe because when charCode isn't -1, it's guaranteed to be a valid Unicode code point that fits into a char (for basic multilingual plane characters, which are the vast majority you'll work with in practice).

内容的提问来源于stack exchange，提问作者Kağan Uğur