Java 字符流

1. 字符编码

• ISO-8859-1 收录除 ASCII 外，还包括西欧、希腊语、泰语、阿拉伯语、希伯来语对应的文字符号
• UTF-8 针对 Unicode 码表的可变长度字符编码
• GB2312 简体中文
• GBK 简体中文扩充
• BIG5 台湾，繁体中文

当编码方式和解码方式不一致时，会出现乱码

1.1 各种编码占用字节数

编码	英文(Byte)	中文(Byte)
ISO-8859-1	1	1
UTF-8	1	3
UTF-16	4	4
GB2312	1	2
GBK	1	2
BIG5	1	2

2. 字符流

java.io.FileInputStream#read() 是一个字节一个字节读取的，直接读取中文时会按单个字节读取，改用字符流

2.1 抽象父类 Reader / Writer

Reader 字符输入流
Writer 字符输出流

public abstract class Reader implements Readable, Closeable {
    // Reads a single character
    public int read() throws IOException {}
}

2.2 文件字符流 FileReader/FileWriter

    Reader reader = new FileReader("src/main/resources/demo.txt");
    int length = 0;
    // 读取一个字符
    while ((length = reader.read()) != -1) {
        System.out.println((char) length);
    }

增加缓冲区

    Reader reader = new FileReader("src/main/resources/demo.txt");
    // 字符数组
    char[] buf = new char[1024];
    int length = 0;
    while ((length = reader.read(buf)) != -1) {
        System.out.println(new String(buf, 0, length));
    }
    reader.close();

    Writer writer = new FileWriter("src/main/resources/out.txt");
    for (int i = 0; i < 10; i++) {
        writer.write("中文测试\r\n");
        writer.flush();
    }
    writer.close();

2.3 字符流复制文件

FileReader 和 FileWriter 复制文本文件，不能复制图片或二进制文件（要用字节流）
原因是字符流在读取时是有编码的，图片是没有字符编码的，读取的二进制流就不能转化成字符信息，变为乱码
所有的文件存在硬盘上都是二进制流，但文本文件有字符编码，图片没有字符编码，字节流可以读取任何文件

public static void main(String[] args) throws IOException {
    Reader reader = new FileReader("src/main/resources/demo.txt");
    Writer writer = new FileWriter("src/main/resources/out.txt");
    int length = 0;
    while ((length = reader.read()) != -1) {
        writer.write(length);
        writer.flush();
    }
    reader.close();
    writer.close();
}

3. 字符缓冲流

可读写一行，支持输入换行符

public class BufferedReader extends Reader {
    public String readLine() throws IOException {
        return readLine(false);
    }
}

public class BufferedWriter extends Writer {
    // Writes a line separator
    public void newLine() throws IOException {
        write(lineSeparator);
    }
}

3.1 BufferedReader

    BufferedReader reader = new BufferedReader(new FileReader("src/main/resources/demo.txt"));
    char[] buf = new char[3];
    int length = 0;
    while ((length = reader.read(buf)) != -1) {
        System.out.println(new String(buf, 0, length));
    }
    reader.close();

一行一行的读取

    BufferedReader reader = new BufferedReader(new FileReader("src/main/resources/demo.txt"));
    String line = null;
    while ((line = reader.readLine()) != null) {
        System.out.println(line);
    }
    reader.close();

3.2 BufferedWriter

    BufferedWriter writer = new BufferedWriter(new FileWriter("src/main/resources/out.txt"));
    for (int i = 0; i < 10; i++) {
        writer.write("写入测试");
        writer.newLine();
        writer.flush();
    }
    writer.close();

4. 打印流（PrintWriter）

• 封装 print() / println() , 支持写入后换行
• 支持数据原样打印

    PrintWriter printWriter = new PrintWriter("src/main/resources/print.txt");
    printWriter.println(97);
    printWriter.println(true);
    printWriter.print(3.14);
    printWriter.print('a');
    printWriter.close();

5. 转换流

• 桥转换流：InputStreamReader / OutputStreamWriter
• 可将字节流转换为字符流
• 可设置字符的编码方式
• 内存中的字符与硬盘中的字节之间的转换

    InputStreamReader isr = new InputStreamReader(new FileInputStream("src/main/resources/demo.txt"), "UTF-8");
    int data = 0;
    while ((data = isr.read()) != -1) {
        System.out.println((char) data);
    }
    isr.close();

    OutputStreamWriter osw = new OutputStreamWriter(new FileOutputStream("src/main/resources/out.txt"), StandardCharsets.UTF_8);
    for (int i = 0; i < 10; i++) {
        osw.write("这是一个测试");
    }
    osw.close();

5.1 InputStreamReader / OutputStreamWriter