File operation io-part2

Catalog

How to operate the file

Key method

Construction method

How to modify the file tree structure

Create new file method

Create directory

Delete file method

Mobile nodes （ rename ） 

Determine file properties

Determine whether there is a method

Judge that the file exists && It's a folder.

Judge that the file exists && It's an ordinary document

Get file standard path

Return the general absolute path of the file  

Return the standard absolute path of the file

For catalog

  Traverse this path

Get the file name method

Get the filename （ Current filename , No path other information ）

Get the parent class file name

Get the time when the file was last modified

All children who get the document

Relative paths

Where is the working directory of the process runtime ？

About the reading and writing of content —— About reading

Read the classes and models used in the file

1. Input stream

2. Use concrete classes

3. About abstract models

4. Remember to turn off

The specific methods

Turn off the signal

Use byte Input method of array —— read(byte[] b)

Direct reception ——read() Method

utilize InputStream Direct reading of binary data

String Construction method of ( Not recommended )

  utilize Scanner Method

About the reading and writing of content —— About what's written

buffer —>  Balanced write speed

Buffer flushing

OutputStreamWriter

PrintWriter

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How to operate the file

Key method

Construction method

You can use absolute paths and relative paths , But I usually use the absolute path .

File file = new File("C:\\Users\\DELL\\Desktop\\exercise");

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How to modify the file tree structure

Create new file method

file.createNewFile();

Create directory

All nonexistent directories under this path will be created

file.mkdirs();

Delete file method

file.delete();

Mobile nodes （ rename ） 

        File file = new File("D:\\ Course \\2022-06-29-2022 Rocket class -IO\\src\\nihao.txt");
        File dest = new File("D:\\ Course \\2022-06-29-2022 Rocket class -IO\\src\\bye.txt");
        // This directory will realize the cutting effect , The files in the original directory will be moved to the new directory 
        //File dest = new File("D:\\ Course \\2022-06-29-2022 Rocket class -IO\\dest\\nihao.txt");
        file.renameTo(dest);

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Determine file properties

Determine whether there is a method

file.exists()

Judge that the file exists && It's a folder.

file.isDirectory();

Judge that the file exists && It's an ordinary document

file.isFile();

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Get file standard path

Return the general absolute path of the file  

String absolutePath = file.getAbsolutePath();

Return the standard absolute path of the file

String canonicalPath = file.getCanonicalPath();

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For catalog

file.listFiles();

  Traverse this path

It is generally used for the overall deletion of directory folders

Only depth traversals are listed here , Breadth traversal requires the help of queues .

    private static void traversalDepthFirst(File dir) throws Exception {
        // 1.  Find all the children in this directory 
        File[] files = dir.listFiles();
        if (files == null) {
            return;
        }

        // 2.  For every child , Determine whether it is a directory or a file 
        for (File file : files) {
            if (file.isDirectory()) {
                //  If it is a directory , Then continue to recursively traverse the processing 
                System.out.println("[D] " + file.getCanonicalPath());
                traversalDepthFirst(file);
            } else {
                System.out.println("[F] " + file.getCanonicalPath());
            }
        }
    }

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Get the file name method

Get the filename （ Current filename , No path other information ）

file.getName();

Get the parent class file name

// Get the parent class file name 
file.getParent();
// Want to get the standard writing , You need to get the parent file first , Then get the standard absolute path .
String ret = file.getParentFile().getCanonicalPath();

Get the time when the file was last modified

file.lastModified();

All children who get the document

File[] files = file.listFiles();

Relative paths

About relative paths , It needs to be clear

Where is the working directory of the process runtime ？

working directory ：current working diretory(cwd)

stay JVM The following is usually the directory when the process starts .

That is to say, your idea The startup directory is where the folder is placed

About the reading and writing of content —— About reading

There are two ways to read ：

1). Direct reading ( Read as binary data , That's in the code byte In units of );

2). Text reading

Read the classes and models used in the file

1. Input stream

When reading a document , We use it java.io.InputStream Input as an input stream .

2. Use concrete classes

InputStream It's an abstract class , We use subclasses FileInputStream File input .

3. About abstract models

The input process can be represented by a model , We can think of the input stream we build as a tap , The file we need to input is regarded as a water tower , Last , We also need a bucket to catch the water from the water tower flowing through the faucet .

Water tower ： Specific documents

water tap ：FileInputStream class

Buckets ：byte[] Array

 

4. Remember to turn off

Remember to turn off the tap after you use it

The specific methods

We use FileInputStream Class read() Method to carry the input stream

Turn off the signal

We use... In our code -1 To express EOS（ Turn off the signal ）

Use byte Input method of array —— read(byte[] b)

This method will return how much water it received .

This method can receive a lot of water at a time , We just need to pick up the water in the bucket , Take another container that just can hold this water and put it in , Then traverse the container .

public static void main(String[] args) throws Exception {
        InputStream is = new FileInputStream("./hello.txt");
        // FileInputStream  I can assign a value to  InputStream, Because   There is an inheritance relationship  && FileInputStream  yes  InputStream  Child class of 

        //  Prepare a bucket 
        byte[] buf = new byte[1024];        // 1024  The representative can answer  1024  drop （ byte ） water , We have the capacity of the barrel ready 

        //  Take the prepared bucket to the tap to collect water 
        int n = is.read(buf);
        //  there  n  Represents how many drops we really received this time （ byte ） water 
        // n  Must be less than or equal to  1024 && n >= 0
//        System.out.println(n);  // 28
        //  Real data on  buf  from  [0, 28)
        byte[] bytes = Arrays.copyOf(buf, n);
        for (byte b : bytes) {
            System.out.printf("%02x\n", b);
        }

        is.close();
    }

Direct reception ——read() Method

This method directly catches every drop of water flowing through the tap , No buckets and containers , There is no water to return int The value will change to -1, At this time, it is necessary to quit the water connection process and turn off the tap .

public static void main(String[] args) throws Exception {
        InputStream is = new FileInputStream("./hello.txt");
        while (true) {
            int data = is.read();
            if (data == -1) {
                //  All the data has been read 
                break;
            }

            //  otherwise ,data  It's the data we get 
            byte b = (byte) data;
            System.out.printf("%02x\n", b);
        }
        is.close();
    }

utilize InputStream Direct reading of binary data

String Construction method of ( Not recommended )

May adopt String The construction method of... Can be byte[] The array is directly converted to the text in the text file .

Put it into the code , There's no need to do it again byte Array traversal , But directly byte Just pass in the array

try (InputStream is = new FileInputStream("./hello.txt")) {
            byte[] buf = new byte[1024];
            int n = is.read(buf);
//            System.out.println(n);
//            for (int i = 0; i < n; i++) {
//                System.out.printf("%02x ", buf[i]);
//            }
            String s = new String(buf, 0, n, "UTF-8");
            System.out.println(s);
        }

  utilize Scanner Method

We can use Scanner Method , Direct will Scanner Compared to a bucket , The input stream goes directly into Scanner that will do

Scanner In fact, we also need close, So add one try It's just that we don't close it at ordinary times .

try (InputStream is = new FileInputStream("./hello.txt")) {
            try (Scanner scanner = new Scanner(is, "UTF-8")) {
                while (scanner.hasNextLine()) {
                    String line = scanner.nextLine();   //  Line breaks are removed by default 
                    System.out.println("|" + line + "|");
                }
            }
        }

About the reading and writing of content —— About what's written

buffer —>  Balanced write speed

Writing content is actually through data through memory to hard disk , Because the writing speed of memory is much faster than that of hard disk ,

We need one “ buffer buffer” To balance the difference in speed .

Buffer flushing

Flushing the buffer is to flush the data in the buffer into the hard disk . The following situations will cause disk brushing ：

1. The buffer is full or reaches a certain threshold .

2. Time between reaching

3. The process takes the initiative to brush the disk

Before closing

There will still be some data left in memory , At this time, all the data will be flushed into the hard disk through a flushing .

In the use of OutputStreamWriter Implement class time , Pay attention to the last use .flush() To flush the data in the buffer .

OutputStreamWriter

We use Writer Subclasses of OutputStreamWriter To complete the write operation .

This step is to Do character set encoding

public static void main1(String[] args) throws Exception {
        try (OutputStream os = new FileOutputStream("./world.txt")) {
            try (Writer writer = new OutputStreamWriter(os, "UTF-8")) {
                writer.write(" Hello China \r\n");
                writer.write(" It's nothing ");
                writer.flush();
            }
        }
    }

PrintWriter

coordination PrintWriter, And its construction method is introduced writer Object can It's similar to print Effect of the method .

public static void main(String[] args) throws Exception {
        try (OutputStream os = new FileOutputStream("./world.txt")) {
            try (Writer writer = new OutputStreamWriter(os, "UTF-8")) {
                try (PrintWriter printWriter = new PrintWriter(writer)) {
                    printWriter.println(1 + 1);
                    printWriter.print(3);
                    printWriter.printf("%s + %d", " I ", 3);

                    printWriter.flush();
                }
            }
        }
    }