“This is the sixth day of my participation in the Gwen Challenge in November. Check out the details: The Last Gwen Challenge in 2021.”
One, the operation of the file
1. The File class (java.io.File) can represent files or directories (in Java, files and directories belong to this class, and the distinction is not very obvious).
The method under File performs disk operations on files on disk, but cannot read the contents of the files.
Note: Creating a file object and creating a file are two different concepts in JAVA. The former creates a file in the virtual machine, but does not actually create it into the OS’s file system. When the virtual machine is shut down, the created object disappears. Creating a file is actually creating a file in the system.
For example, File f=new File(” 11.txt “); // Create a file object named 11.txt
f.CreateNewFile(); // Actually create the file
2. File method
Boolean createNewFile() // Create a file Boolean mkdir() // Create a directory Boolean mkdirs() // Create multiple directories Boolean delete() // Delete files Boolean deleteOnExit(); // Delete files when the process exits. This operation is usually used to delete temporary files.Copy the codeString[] List() : Returns all File and directory names (relative paths) in the current File object.
File[] ListFiles() : Returns the current File object. All Files objects can be queried with getName().
IsDirectory () and isFile() to determine whether it is a directory or file.
String getParent() gets the file name of the parent class
File getParentFile()…
String getPath()… The path
Exists () Checks whether a file exists
Second, dealing with cross-platform
For the command: File f2=new File (” d: ABC \789\1.txt “)
This command is not cross-platform because file system separators vary from OS to OS.
Use the separtor property of the File class to return the current platform file separator.
File newD = new File("aa"+File.separator+"bb"+File.separator+"cc");
File newF = new File(newD,"mudi.txt");
try{
newD.mkdirs();
newF.createNewFile();
}catch(Exception e){}
Copy the codeObject serialization interface
The Serializable interface has no methods and is an identity interface.
Serialization steps:
1) Implement Serializable interface
2) Instantiate object file output object
3) Output the object to a file
4) Some temporary variables do not need to exceed the lifetime of the virtual machine, need to add: TRANSIENT keyword, this attribute is not serialized.
An object that serializes its internal properties also needs a serialization interface.
I/O flow base
Input/Output: Data is exchanged across the JVM boundary with the outside world.
Note: Input/output is specific to the JVM.
5. Classification of streams
1) From the data type: byte stream and character stream
Byte stream class:
Abstract parent classes: InputStream,OutputStream
Implementation class:
BufferedInputStream Buffered stream – Overstream
BufferedOutputStream
ByteArrayInputStream Byte array stream – node stream
ByteArrayOutputStream
DataInputStream handles the JAVA standard data stream – the overstream
DataOutputStream
FileInputStream handles file IO streams – node streams
FileOutputStream
FilterInputStream implements the stream – byte stream parent class
FilterOutputStream
PipedInputStream pipe flow
PipedOutputStream
PrintStream contains print() and println()
RandomAccessFile supports random files
Abstract parent classes: Reader, Writer
Implementation class:
BufferedReader
BufferedWriter
PrintWriter
CharArrayReader
CharArrayWriter
FileReader
FileWriter
FilterReader
FilterWriter
InputStreamReader
OutputStreamWriter
PipedReader
PipedWriter
StringReader
StringWriter
- From the data direction: input stream and output stream
InputXXXXX , OutputXXXXX
- Functions from flow: nodal flow and filter flow (used to painter mode)
Node streams are used to transfer data.
Filter streams are used to encapsulate node streams or other filter streams, thereby adding functionality to node streams or other filter streams.
I/O input and output
The standard way to write a stream is:
OutputStream os=null; OutputStream os=null; try{ String a="hello"; byte[] b=a.getBytes(); os=new FileOutputStream("D:\aa.txt"); os.write(b); }catch(IOException ioe){ ioe.printStackTrace(); }finally{ if(os! =null){ try { os.close(); } catch (IOException e) { // TODO Auto-generated catch block e.printStackTrace(); }}}Copy the code- InputStream class
All the parent class in the input stream of bytes, such as: FileInputStream, ObjectInputStream, PipedInputStrean
- Three basic read() methods
A. int read(): a byte or -1 read from the stream; (Actually read how long)
B. int read(byte[]) : reads data into an array of bytes and returns the number of bytes read; (Expect to read how long)
C. Int read(byte[], int, int) : The two int arguments specify the subrange of the array to be filled.
- Other methods
A. void close(): Close the stream. If a filter stream is used, closing the stream at the top of the stack will close the rest of the stream.
B. Int available(): Returns the number of bytes that can be read from the stream.
C. Skip (long): Discard the specified characters in the stream.
d. boolean markSupported()
e. void mark(int)
f. void rese()
- OutputStream method
Answer: 1) The three basic read() methods
a. void write():
B. void write (byte []) :
C. Void write(byte[], int, int) :
Write the output stream.
2) Other methods
A. void close(): Close the stream. If a filter stream is used, closing the stream at the top of the stack will close the rest of the stream.
B. void flush(): allows you to force write operations.
Note: The close() method is controlled by the programmer in the flow. Because the input and output streams are beyond the boundaries of the JVM, it may sometimes be impossible to reclaim resources.
Rule: Any resources that cross the boundaries of the virtual machine should be closed by the programmer, not garbage collected.
- FileInputStream and FileOutputStream
A: 1) Node streams, using disk files.
2) To construct a FileInputStream, the associated file must exist and be readable.
3) To construct a FileOutputStream if the output file already exists, it will be overwritten.
FileInputStream infile = new FileInputStream("myfile.dat"); FIleOutputStream outfile = new FileOutputStream("results.dat"); FileOutputStream outfile = new FileOutputStream(" results.dat ",true);Copy the codeThe output is add when the argument is true, and overwrite when the argument is false.
FileOutputStream class code:
Public FileOutputStream(String name){ This(name! = null? new File(String):null,false); }Copy the codeKeyboard flow
PrintWriter : System.in
4. A DataInputStream and DataOutputStream
Is the filter flow. To read and write Java base classes through streams, note that the DataInputStream and DataOutputStream methods are paired.
Filtering flow. Output input various data types.
WriteBoolean (Boolean b) —— send 1bit data
WriteByte (int) —— Send 1 byte data
WriteBytes (String s) ——– Send data in byte sequence
WriteChar (int v) — in 2 bytes
WriteChars (String s)————- in 2 byte sequence
WriteDouble (double D) ——- in 8 bytes
writeInt(int v)
writeLong(long l)
writeShort(short s)
WriteUTF (String)———– can output Chinese!
6. ObjectInputStream and ObjectOutputStream
Filtering flow. Handles persistence of objects
Object o = new Object(); FileOutputStream fos=new FileOutputStream("Object.txt"); ObjectOutputStream oos=new ObjectOutputStream(fos); oos.writeObject(o); oos.close(); FileInputStream fis = new FileInputStream (" Object. TXT "); ObjectInputStream ois =new ObjectInputStream(fis); Object o = (Object)Ois.readObject(); ois.close();Copy the code- BufferInputStream and BufferOutputStream
Filtering flows improves I/O operation efficiency
Used to add a buffer to the node stream.
A buffer is created inside the VM. Data is written to the buffer first and written out once the buffer is full, which is very efficient.
The speed of using buffered input/output streams is greatly improved, and the larger the buffer, the more efficient it is. (This is the typical sacrifice of space for time)
Remember: when using a buffered stream, flush the buffer to the external data source once the data has been entered. Use close() to achieve the same effect, because each close is flush. Be sure to close the external filter stream.
- PipedInputStream and PipedOutputStream
Used to communicate between threads.
PipedOutputStream pos=new PipedOutputStream();
PipedInputStream pis=new PipedInputStream();
try
{
pos.connect(pis);
new Producer(pos).start();
new Consumer(pis).start();
}
catch(Exception e)
{
e.printStackTrace();
}
Copy the code9.RandomAccessFile
You can get a file pointer.
Long getFilePointer() gets the position from the start of the file to the file pointer.
Seek (long point) moves the file pointer here.
10, Reader and Writer
1) Java technology uses Unicode to represent strings and characters, and provides a 16-bit version of the stream to handle characters in a similar way.
2) InputStreamReader and OutputStreamWriter serve as the interface between the byte stream and the character stream.
3) If a Reader and Writer are constructed to connect to the stream, the conversion rules switch between using the byte encodings defined by the default platform and Unicode.
4) The difference between byte stream and character stream:
Encoding is converting characters into numbers and storing them in a computer. The process of converting numbers into corresponding characters is called decoding.
Classification of coding methods:
ASCII (numeric, English) :1 character in a byte (all encoding sets are ASCII compatible)
Iso8859-1 (Europe) : 1 character in a byte
Gb-2312 /GBK: A character consists of two bytes
Unicode: 1 character in two bytes (slow network transmission)
Utf-8: Variable-length bytes, one byte for English, two or three bytes for Chinese characters.
BufferedReader method: readLine():String
PrintWriter method: println(… .string,Object, etc.) and write()
- Random access file
1) Two interfaces are implemented: DataInput and DataOutput;
2) As long as the file can be opened, it can read and write;
3) Through the file pointer can read and write file specified location;
4) Access all read() and write() operations in DataInputStream and DataOutputStream;
5) Move method in file:
A. long getFilePointer(): Returns the current position of the file pointer.
B. Void seek(long pos): Sets the file pointer to a given absolute position.
C. long length(): Returns the length of the file.
12. Coding problems:
Encoding method:
Each character corresponds to an integer.
Different countries have different codes. When the encoding and decoding methods are not unified, garbled codes will be generated.
Because the United States was the first to develop software, so every code is upward compatible with ASCII so there is no gibberish in English.
Iso-8859-1 Western characters
GB2312…… rong
GBK
Big5
Unicode
UTF-8
Supplement:
Byte stream end returns -1
The end of a character stream returns NULL
EOFException is returned at the end of the object stream
——— > Exceptions are often used in process control and are also a return form of a method.