10.file/io stream -bite

File object

Directories are also called folders , A folder is also a file , Specific directories and files can be used File Object to represent .\ Indicates the folder where the layer is located

Byte stream object and character stream object

Byte stream object :
read :InputStream( abstract class )->FileInputStream( Implementation subclass )
Write :OutputStream( abstract class )->FileOutputStream( Implementation subclass )

FileInputStream:

read Three versions of overloads are provided .
1) Parameterless version : Read one byte at a time . The return value is the byte read : The return value is the corresponding byte ASCII Code value , After reading, go back -1

2) A parameter version : Read several bytes at a time , Put the read result into the array specified in the parameter , The return value is the number of bytes read .
Returns the length of the filled byte array , Read and return -1

3) Three parameter versions : Read several bytes at a time , Put the read result into the array specified in the parameter , The return value is the number of bytes read . Not from the beginning of the array , But from the middle (off The position of this subscript )len Indicates how many elements can be placed at most ( byte )

        //  In the construction method, you need to specify the path to open the file .
        //  The path here can be an absolute path ,  It could be a relative path ,  It can also be  File  object 
        InputStream inputStream = null;
        try {
    
            // 1.  Create objects ,  At the same time, it is also opening the file .
            inputStream = new FileInputStream("d:/test.txt");
            // 2.  Try reading one byte at a time ,  Finish reading the whole document .
            while (true) {
    
                int b = inputStream.read();
                if (b == -1) {
    
                    //  Read to the end of the file 
                    break;
                }
                System.out.println(b);
            }
        } catch (IOException e) {
    
            e.printStackTrace();
        } finally {
    
            // 3.  Remember to close the file after reading ,  Release resources ~
            try {
    
                inputStream.close();
            } catch (IOException e) {
    
                e.printStackTrace();
            }
        }

Another way of writing : from try Automatically call close

        try (InputStream inputStream = new FileInputStream("d:/test.txt")) {
    
            while (true) {
    
                int b = inputStream.read();
                if (b == -1) {
    
                    break;
                }
                System.out.println(b);
            }
        } catch (IOException e) {
    
            e.printStackTrace();
        }

Read several bytes at a time and put them in the array :

try (InputStream inputStream = new FileInputStream("d:/test.txt")) {
    
            //  Read several bytes at a time .
            while (true) {
    
                byte[] buffer = new byte[1024];
                int len = inputStream.read(buffer);
                if (len == -1) {
    
                    //  If you return  -1  It means that the reading is finished 
                    break;
                }
                for (int i = 0; i < len; i++) {
    
                    System.out.println(buffer[i]);
                }
            }
        } catch (IOException e) {
    
            e.printStackTrace();
        }

FileOutputStream: Each time a file is written, the contents of the original file will be cleared , You can add a parameter to the constructor :true, Make it not empty , Then write... After the original file

//  Use byte stream ,  The case of writing documents .
public class Demo9 {
    
    public static void main(String[] args) {
    
        try (OutputStream outputStream = new FileOutputStream("d:/test.txt")) {
    
// outputStream.write(97);
// outputStream.write(98);
// outputStream.write(99);

            byte[] buffer = new byte[]{
    97, 98, 99};
            outputStream.write(buffer);
        } catch (IOException e) {
    
            e.printStackTrace();
        }
    }
}

Character stream object :
read :Reader->FileReader

//  Read and write according to characters 
public class Demo10 {
    
    public static void main(String[] args) {
    
        try (Reader reader = new FileReader("d:/test.txt")) {
    
            //  Read by character .
            while (true) {
    
                char[] buffer = new char[1024];
                int len = reader.read(buffer);
                if (len == -1) {
    
                    break;
                }
// for (int i = 0; i < len; i++) {
    
// System.out.println(buffer[i]);
// }
                //  If it comes in here   Array   yes  byte  Array ,  You can also manually specify  utf8  Character sets avoid garbled code .
                String s = new String(buffer, 0, len);
                System.out.println(s);
            }
        } catch (IOException e) {
    
            e.printStackTrace();
        }
    }
}

Write :Writer->FileWriter

//  Write in characters 
public class Demo11 {
    
    public static void main(String[] args) {
    
        try (Writer writer = new FileWriter("d:/test.txt")) {
    
            writer.write("xyz");
        } catch (IOException e) {
    
            e.printStackTrace();
        }
    }
}