通过 Java I/O API在 Java 中执行文件处理。

在 Java 中，自动为我们创建了 3 个流。所有这些流都与控制台相连。

1) System.out：标准输出流

2) System.in：标准输入流

3) System.err：标准错误流

OutputStream：Java application uses an output stream to write data to a destination; it may be a file, an array, peripheral device or socket.

InputStream：Java application uses an input stream to read data from a source; it may be a file, an array, peripheral device or socket.

Java IO

OutputStream class is an abstract class. It is the superclass of all classes representing an output stream of bytes. An output stream accepts output bytes and sends them to some sink.

Useful methods of OutputStream

MethodDescription
1) public void write(int)throws IOException is used to write a byte to the current output stream.
2) public void write(byte[])throws IOException is used to write an array of byte to the current output stream.
3) public void flush()throws IOException flushes the current output stream.
4) public void close()throws IOException is used to close the current output stream.

Java output stream hierarchy

InputStream class is an abstract class. It is the superclass of all classes representing an input stream of bytes.

Useful methods of InputStream

MethodDescription
1) public abstract int read()throws IOException reads the next byte of data from the input stream. It returns -1 at the end of the file.
2) public int available()throws IOException returns an estimate of the number of bytes that can be read from the current input stream.
3) public void close()throws IOException is used to close the current input stream.

InputStream Hierarchy

Java input stream hierarchy

Java FileReader 类

Java FileReader 类用于从文件中读取数据。它以字节格式返回数据，如FileInputStream类。它是面向字符的类，用于在java中进行文件处理。

public class FileReader extends InputStreamReader

FileReader 类的构造函数

ConstructorDescription
FileReader(String file) It gets filename in string. It opens the given file in read mode. If file doesn't exist, it throws FileNotFoundException.
FileReader(File file) It gets filename in file instance. It opens the given file in read mode. If file doesn't exist, it throws FileNotFoundException.

FileReader 类的方法

MethodDescription
int read() It is used to return a character in ASCII form. It returns -1 at the end of file.
void close() It is used to close the FileReader class.

Java FileReader Example

In this example, we are reading the data from the text file testout.txt using Java FileReader class.

 import java.io.FileReader; 
 public class FileReaderExample { 
 public static void main(String args[])throws Exception{  
     FileReader fr=**new*FileReader("D:\\testout.txt");  
     int i;  
     while((i=fr.read())!=-1)  
     System.out.print((char)i);  
     fr.close();  
     }  
 }

假设您在“testout.txt”文件中有以下数据：Welcome to java

输出：Welcome to java

Java BufferedReader 类

Java BufferedReader 类用于从基于字符的输入流中读取文本。它可以用于通过 readLine() 方法逐行读取数据，性能快速。它继承了Reader 类。

public class BufferedReader extends Reader

Java BufferedReader 类构造函数

构造函数描述
BufferedReader(Reader rd) 它用于创建使用输入缓冲区的默认大小的缓冲字符输入流。
BufferedReader(Reader rd, int size) 它用于创建一个缓冲字符输入流，该流使用输入缓冲区的指定大小。

methods：

MethodDescription
int read() It is used for reading a single character.
int read(char[] cbuf, int off, int len) It is used for reading characters into a portion of an array.
boolean markSupported() It is used to test the input stream support for the mark and reset method.
String readLine() It is used for reading a line of text.
boolean ready() It is used to test whether the input stream is ready to be read.
long skip(long n) It is used for skipping the characters.
void reset() It repositions the stream at a position the mark method was last called on this input stream.
void mark(int readAheadLimit) It is used for marking the present position in a stream.
void close() It closes the input stream and releases any of the system resources associated with the stream.

eg：

 import java.io.*;  
 public class BufferedReaderExample {  
     public static void main(String args[])throws Exception{    
           FileReader fr=new FileReader("D:\\testout.txt"); 
           BufferedReader br=new BufferedReader(fr); 
           int i;    
           while((i=br.read())!=-1){  
           System.out.print((char)i);  
           }  
           br.close();    
           fr.close();    
     }    
 }    

data in "testout.txt" file: Welcome to java.

Output: Welcome to java.

通过 InputStreamReader 和 BufferedReader 从控制台读取数据

 import java.io.*;  
 public class BufferedReaderExample{    
 public static void main(String args[])throws Exception{       InputStreamReader r=new InputStreamReader(System.in); 
     BufferedReader br=new BufferedReader(r);            
     System.out.println("Enter your name");    
     String name=br.readLine();    
     System.out.println("Welcome "+name);    
 }    
 }  
 Output:
 Enter your name
 Nakul Jain
 Welcome Nakul Jain

从控制台读取数据直到用户写入停止的另一个示例:读取和打印数据，直到用户停止打印。

 import java.io.*;  
 public class BufferedReaderExample{    
 public static void main(String args[])throws Exception{
      InputStreamReader r=new InputStreamReader(System.in); 
      BufferedReader br=new BufferedReader(r);           
      String name="";    
      while(!name.equals("stop")){    
       System.out.println("Enter data: ");    
       name=br.readLine();    
       System.out.println("data is: "+name);    
      }              
     br.close();    
     r.close();    
     }    
 }  

Output:

 Enter data: Nakul
 data is: Nakul
 Enter data: 12
 data is: 12
 Enter data: stop
 data is: stop