Java Java.io.LineNumberReader用法及代码示例

跟踪行号的缓冲 character-input 流。该类定义了方法 setLineNumber(int) 和 getLineNumber() 分别用于设置和获取当前行号。

• 默认情况下，行编号从 0 开始。读取数据时，该数字在每个行终止符处递增，并且可以通过调用 setLineNumber(int) 进行更改。
• 但请注意，setLineNumber(int) 实际上并不更改流中的当前位置；而是更改了流中的当前位置。它仅更改getLineNumber()返回的值。
• 一行被视为以换行符 (‘\n’)、回车符 (‘\r’) 或回车符后紧跟换行符之一终止。

构造函数：

• LineNumberReader(读入)：使用默认的 input-buffer 大小创建新的 line-numbering 读卡器。
• LineNumberReader(读取器，int sz)：创建一个新的 line-numbering 读取器，将字符读取到给定大小的缓冲区中。

方法：

• int getLineNumber()：获取当前行号。

  Syntax :public int getLineNumber()
  Returns:
  The current line number

• 无效标记(int readAheadLimit)：标记流中的当前位置。对 reset() 的后续调用将尝试将流重新定位到该点，并且还将相应地重置行号。

  Syntax :public void mark(int readAheadLimit)
            throws IOException
  参数：
  readAheadLimit - Limit on the number of characters that may be read 
  while still preserving the mark. After reading this many characters, 
  attempting to reset the stream may fail.
  Throws:
  IOException

• int read()：读取单个字符。行终止符被压缩为单个换行符 (‘\n’)。每当读取行终止符时，当前行号就会递增。

  Syntax :public int read()
           throws IOException
  返回：
  The character read, or -1 if the end of the stream has been reached
  Throws:
  IOException

• int 读取(char[] cbuf，int 关闭，int len)：将字符读入数组的一部分。每当读取行终止符时，当前行号就会递增。

  Syntax :public int read(char[] cbuf,
         int off,
         int len)
           throws IOException
  参数：
  cbuf - Destination buffer
  off - Offset at which to start storing characters
  len - Maximum number of characters to read
  返回：
  The number of bytes read, or -1 if the end of the stream has already been reached
  Throws:
  IOException

• 字符串readLine()：读取一行文本。每当读取行终止符时，当前行号就会递增。

  Syntax :public String readLine()
                  throws IOException
  Returns:
  A String containing the contents of the line, not including any line 
  termination characters, or null if the end of the stream has been reached
  Throws:
  IOException

• 无效reset()：将流重置为最新标记。

  Syntax :public void reset()
             throws IOException
  Throws:
  IOException

• void setLineNumber(int lineNumber):设置当前行号。

  Syntax :public void setLineNumber(int lineNumber)
  参数：
  lineNumber - An int specifying the line number

• 长跳过(长n)：跳过字符。

  Syntax :public long skip(long n)
            throws IOException
  参数：
  n - The number of characters to skip
  返回：
  The number of characters actually skipped
  Throws:
  IOException
  IllegalArgumentException

程序：

//Java program demonstrating LineNumberReader methods 
import java.io.FileReader; 
import java.io.IOException; 
import java.io.LineNumberReader; 
class LineNumberReaderDemo 
{ 
    public static void main(String[] args) throws IOException  
    { 
        FileReader fr = new FileReader("file.txt"); 
        LineNumberReader lnr = new LineNumberReader(fr); 
        char c[] = new char[20]; 
  
        //illustrating setLineNumber() 
        lnr.setLineNumber(0); 
          
        //illustrating set 
        System.out.println(lnr.getLineNumber()); 
          
        //illustrating markSupported() method 
        if(lnr.markSupported()) 
        { 
            System.out.println("mark() method is supported"); 
            //illustrating mark method 
            lnr.mark(100); 
        } 
          
        /*File Contents 
        * This is first line 
        this is second line 
        This is third line 
        */
          
        //skipping 19 characters 
        lnr.skip(19); 
  
        //illustrating ready() method 
        if(lnr.ready()) 
        { 
            //illustrating readLine() method 
            System.out.println(lnr.readLine()); 
  
            //illustrating read(char c[],int off,int len) 
            lnr.read(c); 
            for (int i = 0; i <20 ; i++) 
            { 
                System.out.print(c[i]); 
            } 
              
            //illustrating reset() method 
            lnr.reset(); 
              
            for (int i = 0; i <18 ; i++) 
            { 
                //illustrating read() method 
                System.out.print((char)lnr.read()); 
            } 
            int ch; 
              
            //illustrating read() method 
            System.out.println(lnr.readLine()); 
            while((ch = lnr.read())==1) 
                System.out.print((char)ch); 
        } 
          
        //close the stream 
        lnr.close(); 
    } 
} 

输出：

0
mark() method is supported
this is second line
This is third line
This is first line