在利用LineNumberReader读取文件指定行的时候,遇到了一个很有意思的问题,该问题极易忽略,而且极易犯错,先看代码:
// 文件内容的总行数
static int getTotalLines(File file) throws IOException {
FileReader in = new FileReader(file);
LineNumberReader reader = new LineNumberReader(in);
String s = reader.readLine();
int lines = 0;
while (s != null) {
lines++;
s = reader.readLine();
}
reader.close();
in.close();
return lines;
}
// 读取文件指定行
static String readLineNumber(File sourceFile, int lineNumber) throws IOException {
FileReader in = new FileReader(sourceFile);
LineNumberReader reader = new LineNumberReader(in);
String s = null;
int line = 1;
if (lineNumber < 0 || lineNumber > getTotalLines(sourceFile)) {
System.out.println("传入行数" + lineNumber + "有误,不在范围之内");
} else {
reader.setLineNumber(lineNumber);
int i = reader.getLineNumber();
while (reader.readLine() != null) {
line++;
if (i == line) {
s = reader.readLine();
break;
}
}
}
reader.close();
in.close();
return s;
}
public static void main(String[] args) {
File sourceFile = new File("E:\\ceshi\\1.txt");
try {
int lineNum = getTotalLines(sourceFile);
// 获取文件的内容的总行数
System.out.println("总行数=" + lineNum);
// 读取指定的行
int lineNumber = 1;
String content = readLineNumber(sourceFile, lineNumber);
System.out.println("第" + lineNumber + "行的内容是=" + content);
} catch (IOException e) {
e.printStackTrace();
}
}
调用main方法,从第2行读到第8行,都没有问题,都能正常输出,可是输入参数为1的时候,竟然输出为null?
我们点进去查看LineNumberReader的源码,一看源码,真有点哭笑不得(为了方便,删减很多,重点看readLine()方法):
package java.io;
public class LineNumberReader extends BufferedReader {
/** The current line number */
private int lineNumber = 0;
/** The line number of the mark, if any */
private int markedLineNumber; // Defaults to 0
/** If the next character is a line feed, skip it */
private boolean skipLF;
/** The skipLF flag when the mark was set */
private boolean markedSkipLF;
public LineNumberReader(Reader in) {
super(in);
}
public LineNumberReader(Reader in, int sz) {
super(in, sz);
}
public void setLineNumber(int lineNumber) {
this.lineNumber = lineNumber;
}
public int getLineNumber() {
return lineNumber;
}
public String readLine() throws IOException {
synchronized (lock) {
String l = super.readLine(skipLF);
skipLF = false;
if (l != null)
lineNumber++;
return l;
}
}
}
看了源码,再来理解为什么参数是1的时候,获取的内容为null了,代码中我们用reader.setLineNumber(lineNumber)设置了行号,而line初始值是1,line++后,line肯定大于lineNumber了,自然读取为null了,也就是说,这个代码,从逻辑上来说,只能从第二行开始读取。那么,就有人问了,line的初始值设为0如何?我们就改改代码,再测试一下:
发现第一行的内容竟然是李逍遥,而不是仙剑奇侠传?而且第8行竟然没有内容??这就不得不重点提源码了,我们虽然在代码中reader.setLineNumber(lineNumber)标记了行号,但是实际读取调用的是父类(BufferedReader)的readLine()方法,即:String l = super.readLine(skipLF);所以,需要注意的是,setLineNumber只能改变行号,却不能改变读的位置。
指定读取文件第几行,我们可以这样:
// 读取文件指定行内容
static String readLineNumberString(File sourceFile, int lineNumber) throws IOException {
String s = null;
if (lineNumber < 0 || lineNumber > getTotalLines(sourceFile)) {
System.out.println("传入行数" + lineNumber + "有误,不在范围之内");
} else {
InputStreamReader inputReader = null;
BufferedReader bufferReader = null;
OutputStream outputStream = null;
try {
InputStream inputStream = new FileInputStream(sourceFile);
inputReader = new InputStreamReader(inputStream, "UTF-8");
bufferReader = new BufferedReader(inputReader);
// 读取一行
String lineContent = null;
StringBuffer strBuffer = new StringBuffer();
int currentReadLine = 0;
while ((lineContent = bufferReader.readLine()) != null) {
currentReadLine++;
if (currentReadLine == lineNumber) {
strBuffer.append(lineContent);
break;
}
}
s = strBuffer.toString();
} catch (IOException e) {
System.out.println("" + e.getMessage());
} finally {
IOCloseUtil.closeAll(outputStream, bufferReader, inputReader);
}
}
return s;
}