IO - 常用类

IO - 常用类

IO流定义

狭义流方向定义

这里把硬盘理解为内存之外的源即可

input 输入流(程序读入数据): 硬盘--->内存

output 输出流(程序写到文件): 内存--->硬盘

流分类

• 按数据单位：字节流(1Byte = 8bit)、字符流(字节长度取决于编码方式)

• 按数据流向：输入流、输出流

• 按流的角色：字节流、处理流(包装流)

(抽象基类)	字节流	字符流
输入流	InputStream	Reader
输出流	OutputStream	Writer

Java 流(Stream)、文件(File)和IO

常用类

FileInputStream

该流用于从文件读取数据

文本文件推荐使用字符流, 不推荐使用字节流

1. 测试文件 D:\\file.txt

hello, world!
hello, java.

2. FileInputStream 构造器// throws FileNotFoundException

public FileInputStream(String name) {}
public FileInputStream(File file) {}

3. 逐个字节 public int read() throws IOException

String filePath = "D:\\file.txt";
int read;
try {
    FileInputStream fileInputStream = new FileInputStream(filePath);
    while ((read = fileInputStream.read()) != -1) {// 结尾(int) = -1
        System.out.print((char)read);
    }
    fileInputStream.close();
} catch (IOException e) {
    e.printStackTrace();
}

4. 字节数组 public int read(byte b[]) throws IOException

String filePath = "D:\\file.txt";
byte[] buffer = new byte[8];
int l = 0;
try {
    FileInputStream fileInputStream = new FileInputStream(filePath);
    while ( (l = fileInputStream.read(buffer)) != -1) {
        System.out.print(new String(buffer, 0, l));
    }
    fileInputStream.close();
} catch (IOException e) {
    e.printStackTrace();
}

FileOutputStream

用来创建一个文件并向文件中写数据

1. 构造器// throws FileNotFoundException

   public FileOutputStream(String name) {}
   public FileOutputStream(String name, boolean append) {}
   public FileOutputStream(File file) {}
   public FileOutputStream(File file, boolean append) {}
   

2. 常用写方法// throws IOException

   private native void write(int b, boolean append) ;
   public abstract void write(int b) ;
   public void write(byte b[]) ;
   public void write(byte b[], int off, int len) ;// off 从1开始
   

3. 测试用例

   String filePath = "D:\\file_out.txt";
   try {
       FileOutputStream fileOutputStream = new FileOutputStream(filePath);
       fileOutputStream.write('#');
       fileOutputStream.write("\nhello, outputStream\n".getBytes(StandardCharsets.UTF_8));
       fileOutputStream.write('#');
       fileOutputStream.write("\npartFind\n".getBytes(StandardCharsets.UTF_8), 5, 4);
       fileOutputStream.close();
       /*
       上述的操作是覆盖操作, 即先清空文件(如果存在) 后执行各创建操作
       #
       hello, outputStream
       #Find
        */
       fileOutputStream = new FileOutputStream(filePath, true);// 追加方式
       fileOutputStream.write("\nappend\n".getBytes());
       fileOutputStream.close();
       /*
       这里执行了追加方式的写入操作
       #
       hello, outputStream
       #Find
       append
   
        */
   } catch (IOException e) {
       e.printStackTrace();
   }
   

4. 综合案例

   读源文件 D:\pic.png 486字节

   写到目标文件 D:\pic_new.png 保持字节一致

   String source = "D:\\pic.png";
   String target = "D:\\pic_new.png";
   try {
       FileInputStream fileInputStream = new FileInputStream(source);
       FileOutputStream fileOutputStream = new FileOutputStream(target, true)
       byte[] buffer = new byte[8]; // 长度随便写的
       int l = 0;
       while ((l = fileInputStream.read(buffer)) != -1) {
           fileOutputStream.write(buffer, 0, l);
           // 如果不处理空字节, 则目标文件的字节数为 8 的倍数 -> 488 字节
       }
       fileInputStream.close();
       fileOutputStream.close();
   } catch (IOException e) {
       e.printStackTrace();
   }
   

FileReader

1. 构造器 // throws FileNotFoundException

   public FileReader(String fileName) {}
   public FileReader(File file) {}
   

2. 常用读取方法 // throws IOException

   public int read()
   public int read(char cbuf[]) {}
   

3. 测试文件 D:\\file.txt

   hello, world!
   hello, java.
   

4. 案例

   String filePath = "D:\\file.txt";
   char[] buffer = new char[8];
   int l = 0;
   try {
       FileReader fileReader = new FileReader(filePath);
       StringBuilder sb = new StringBuilder();
       while ((l = fileReader.read(buffer)) != -1) {
           sb.append(buffer, 0, l);
       }
       fileReader.close();
       System.out.println(sb.toString());
   } catch (IOException e) {
       e.printStackTrace();
   }
   

FileWriter

注意，使用 FileWriter 时，必须要关闭（close）或者刷新（flush），否则写入不到指定文件

1. 构造器 // throws IOException

   public FileWriter(String fileName) {}
   public FileWriter(String fileName, boolean append) {}
   public FileWriter(File file) {}
   public FileWriter(File file, boolean append) {}
   

2. 常用方法 // throws IOException

   public void write(int c) {}
   public void write(char cbuf[], int off, int len) {}
   public void write(String str, int off, int len) {}
   public void write(char cbuf[]) {}
   public void write(String str) {}
   

3. 案例

   String targetPath = "D:\\out.txt";
   String content = "山远天高烟水寒，相思枫叶丹。";
   try {
       FileWriter fileWriter = new FileWriter(targe
       fileWriter.write(content);
       fileWriter.flush();// 不关闭保存
       /*
       山远天高烟水寒，相思枫叶丹。
       */
       fileWriter = new FileWriter(targetPath, true
       fileWriter.write("\n松花酿酒，春水煎茶。");
       fileWriter.close(); // 等价于保存并关闭
       /*
       山远天高烟水寒，相思枫叶丹。
       松花酿酒，春水煎茶。
        */
   } catch (IOException e) {
       e.printStackTrace();
   }