Java file I/O(java 文件输入输出)

Posted 2020-10-18 CaiCongyu

File Overview

I/O = Input/Output, here it is input to and output from programs

　　Inputs can be from: Keyboard Files Etc.

　　Output can be: Screen Files Etc.

Advantages of file I/O

-permanent copy

-output from one program can be input to another

-input can be automated (rather than entered manually)

Streams

　　Stream: an object that either delivers data to its destination (screen, file, etc.) or that takes data from a source (keyboard, file, etc.) it acts as a buffer between the data source and destination

　　Input stream: a stream that provides input to a program E.g., System.in is an input stream

　　Output stream: a stream that accepts output from a program E.g., System.out is an output stream

　　Examples: System.out.println(“Hi”)    connects a program to the screen

Streams作为电子设备与程序传输过程中的传输工具

 

Binary vs. Text Files

　　All data and programs are ultimately just zeros and ones each digit can have one of two values (0 or 1),

　　hence binary 1 byte = 8 bits

　　Text files: the bits represent printable characters

　　one byte per character for ASCII, the most common code

　　for example, Java source files are text files so is any file created with a "text editor"

　　Binary files: the bits represent other types of encoded information, such as executable instructions or numeric data

Text Files vs. Binary Files

　　Numbe r: 127 (decimal)十进制

　　Text file Three bytes: “1”, “2”, “7” ASCII (decimal): 49, 50, 55

　　ASCII (binary): 00110001, 00110010, 00110111

　　Binary file: One byte (byte): 01111111     Two bytes (short): 00000000 01111111    Four bytes (int): 00000000 00000000 00000000 01111111

Buffering

　　Not buffered: each byte is read/written from/to disk as soon as possible “little” delay for each byte A disk operation per byte--- higher overhead

　　Buffered: reading/writing in “chunks” Some delay for some bytes,

　　 e.g.: Assume 16-byte buffers

　　Reading: access the first 4 bytes, need to wait for all 16 bytes are read from disk to memory

　　Writing: save the first 4 bytes, need to wait for all 16 bytes before writing from memory to disk

　　A disk operation per a buffer of bytes---lower overhead

 

Java I/O

　　Standard IO: System.out.println("a line"); to send a line to the standard output, i.e., the screen, unless you redirect standard output

　　public class RedirectSysOut {//重定向输出

　　　　public static void main(String[] args) throws FileNotFoundException { //文件找不到时要抛出异常

　　　　PrintStream stream1 = System.out;     //新建一个打印对象

　　　　File aFile=new File("sysout.log");

　　　　PrintStream stream2 = new PrintStream(new FileOutputStream(aFile));

　　　　System.out.println("1");

　　　　System.setOut(stream2);   //重定项屏幕输出到stream2对象中

　　　　System.out.println("2");

　　　　System.setOut(stream1);

　　　　System.out.println("3");

　　　　}

　　}

 

Text File I/O

　　Important classes for text file output (to the file)

　　　1)FileWriter

　　　2)FileOutputStream

　　　3)BufferedWriter

　　　4)PrintWriter

　　Important classes for text file input (from the file):

　　　1)FileReader

　　　2)BufferedReader

　　FileWriter and FileReader take file names as arguments.

　　PrintWriter and BufferedReader provide useful methods for easier writing and reading.

                                              

 

对 TXT 文件的输入输出，使用 FileReader和FileWriter。

 

 

                                

因为PrintWriter的范围最大，所以使用起来也最方便？？

 

 

Text File Output

To open a text file for output: connect a text file to a stream for writing FileOutputStream

　　fOutStream = new FileOutputStream("out.txt");

　　PrintWriter pWriter = new PrintWriter(fOutStream );

Or:

　　PrintWriter outputStream = new PrintWriter(new FileOutputStream("out.txt"));

　　What happened is: create a PrintWriter object which uses FileOutputStream to open a text file FileOutputStream “connects” PrintWriter to a text file.

　　　　

exa:

　　public static void main(String[] args) {

　　　　PrintWriter outputStream = null;

　　　　try {

　　　　　　outputStream = new PrintWriter(new FileOutputStream("out.txt"));  //open the file

　　　　　　for (int count = 1; count <= 3; count++)

　　　　　　　　outputStream.println(count); //write the file

　　　　　　System.out.println("... written to out.txt.");

　　　　　　outputStream.close();

　　　　 } catch(FileNotFoundException e){

　　　　　　 System.out.println("Error opening the file out.txt."+ e.getMessage());

　　　　　　 }

　　　　 }

　　 }

 

Overwriting a File

　　In the previous example:

　　　　-Opening an output file creates a new file if it does not already exist

　　　　- If the file is existing,it will overwrite the file, i.e., data in the original file is lost

 

因为之前都是overwrite

Appending to a Text File

　　To add/append to a file instead of replacing it, use a different constructor for FileOutputStream:

　　　　outputStream = new PrintWriter(new FileOutputStream("out.txt", true));

 

　　　　outputStream = new PrintWriter(new FileOutputStream("out.txt”));

　　　　for (int count = 1; count <= 10; count++)

　　　　　　outputStream.println(count);

　　　　outputStream.close();

　　　　outputStream = new PrintWriter(new FileOutputStream("out.txt”, true));

　　　　 for (int count = 1; count <= 10; count++)

　　　　　　outputStream.println(count);

　　　　outputStream.close();

 

Closing a File

　　After writing a file, you should close the FileOutputStream and the PrintWriter

　　fileOutputStream.close();

　　outputStream.close();

-Why close the file?

　　To make sure it is closed if a program ends abnormally (it could get damaged if it is left open) Flush the stream Calling close() on a wrapper stream should close the child stream.  E.g., if a PrintWriter is closed, the FileOutputStream it linked with is also closed.

-outputStream.close();

　　Both the PrintWriter and FileOutputStream have been closed!!!  程序是通过PrintWriter——》FileOutputStream 接触到TXT（比如），如果关闭了 FileOutputStream 就直接断开了程序与文件的联系，就等于两者都关闭了。

 

 

 

 

 

 

 

Text File Input

　　To open a text file for input: connect a text file to a stream for reading a BufferedReader object, which uses FileReader to open a text file FileReader “connects” BufferedReader to the text file

　　　　FileReader s = new FileReader(“input.txt");

　　　　BufferedReader inStream = new BufferedReader(s);

Or:

　　　　BufferedReader inStream = new BufferedReader(new FileReader(“input.txt"));

             

            

 

 

try {

　　 FileReader fr=new FileReader("input.txt");

　　 BufferedReader inputStream = new BufferedReader(fr);

　　 String line = null;

　　 while((line=inputStream.readLine())!=null) // Check whether reached the end

　　　　System.out.println(line); // Read a line from the file

　　　　 inputStream.close();

　　} catch(FileNotFoundException e) {

　　System.out.println("File not found.");

　　} catch(IOException e) {

　　System.out.println("Error reading from file " + fileName);

　}

 

 

StringTokenizer

There are methods to read a line and a character, but not just a single word

　　StringTokenizer:

　　　　Token: a block of (useful) text

　　　　Delimiter: is a character used to separate items of data

　　　　E.g., CSV files: comma-delimited 

　　Create a StringTokenizer instance you can specify delimiters (the character or characters that separate words)

　　　　the default delimiters are: space, tab, and newline

　　　　　　　　StringTokenizer(String str, String delim, boolean returnDelims) //str--The string to be separated      delim--The delimiters     returnDelims--Whether return delimiters as tokens

Read an integer by using BufferedReader

　　No methods to read numbers directly Read numbers as Strings and then convert them:

　　try{

　　　　String line =inputStream.readLine();

　　　　int value = Integer.parseInt(line);

　　　　} catch(java.lang.NumberFormatException e){

　　　　　　…

　　　　　} catch(IOException e){ …}

Creating Scanner objects

　　We can create a Scanner object by invoking several different constructors.

　　Scanner(File source) 

　　Constructs a new Scanner that produces values scanned from the specified file.

　　Scanner(InputStream source)          

　　Constructs a new Scanner that produces values scanned from the specified input stream.

　　Scanner(String source)          

　　Constructs a new Scanner that produces values scanned from the specified string. …

　　Example (read a line from keyboard):

　　 Scanner sc = new Scanner (System.in);

Next Methods/hasNext methods

　　Example:

　　Scanner sc = new Scanner(System.in);

　　int i=sc.nextInt();

 

　　Scanner sc = new Scanner (System.in);

　　System.out.print ("Enter first int: ");

　　while (sc.hasNextInt()) {

　　　　int i = sc.nextInt();

　　　　System.out.println("You entered " + i);

　　　　System.out.print ("Enter another int: ");

　　}

 

Delimiters in Scanner 

　　Default delimiters are: space, tab, new line