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FileInputStream和FileOutputStream如何在Java中工作？

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我在Java Tutorials Docs上阅读java中的所有输入/输出流.教程编写者使用这个例子：

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;

public class CopyBytes {
    public static void main(String[] args) throws IOException {

        FileInputStream in = null;
        FileOutputStream out = null;

        try {
            in = new FileInputStream("xanadu.txt");
            out = new FileOutputStream("outagain.txt");
            int c;

            while ((c = in.read()) != -1) {
                out.write(c);
            }
        } finally {
            if (in != null) {
                in.close();
            }
            if (out != null) {
                out.close();
            }
        }
    }
}

xanadu.txt文件数据：

In Xanadu did Kubla Khan
A stately pleasure-dome decree:
Where Alph, the sacred river, ran
Through caverns measureless to man
Down to a sunless sea.

输出到outagain.txt文件：

In Xanadu did Kubla Khan
A stately pleasure-dome decree:
Where Alph, the sacred river, ran
Through caverns measureless to man
Down to a sunless sea.

>为什么即使我们正在阅读字符,作者也会使用int c？
>为什么在条件下使用-1？
> out.write(c)如何;方法将int转换为再次字符？

解决方法:

1: Now I want to ask why writer use int c? even we are reading characters.

FileInputStream.read()将一个字节的数据作为int返回.这是有效的,因为一个字节可以表示为int而不会损失精度.请参阅this answer以了解为什么返回int而不是byte.

2: The second why use -1 in while condition?

到达文件末尾时,返回-1.

3: How out.write(c); method convert int to again characters? that provide same output in outagain.txt file

FileOutputStream.write()将byte参数作为int.由于int跨越多于一个字节的值,因此忽略给定int的24个高位,使其成为字节兼容的值：Java中的int是always 32 bits.通过删除24个高位,你’重新下降到8位值,即一个字节.

我建议你仔细阅读每种方法的Javadocs.作为参考,他们回答了您的所有问题：

read：

Reads the next byte of data from the input stream. The value byte is returned as an int in the range 0 to 255. If no byte is available because the end of the stream has been reached, the value -1 is returned. This method blocks until input data is available, the end of the stream is detected, or an exception is thrown.

write：

Writes the specified byte to this output stream. The general contract for write is that one byte is written to the output stream. The byte to be written is the eight low-order bits of the argument b. The 24 high-order bits of b are ignored.

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