在Java中如何以相反的順序逐行讀取文件 - 完整教程

已發表: 2013-03-30

這是一個使用 Java8 API 以相反順序逐行讀取文件的最簡單 Java 程序。 試試看，如果您發現任何問題，請告訴我。

  • 用於反轉文件內容的java程序
  • 編寫一個程序，用它的反向替換文件的每一行

我將編寫 Java 面試系列，您將在其中找到更多相關的問題和答案。 請繼續關注以獲取更多信息。

在本教程中，我們將介紹以下步驟：

  1. 我們將讀取位於 Document 位置的文件crunchify.txt 。 我使用的是 Macbook pro，所以如果您使用不同的操作系統和筆記本電腦/台式機，請在程序CrunchifyReverseLineReader.java中相應地更改文件路徑
  2. 我們將首先以簡單順序讀取文件。
  3. 然後我們將以相反的順序讀取文件。
  4. Please note ：默認情況下，以下程序會從反向輸出中刪除所有空行。 如果您想保留空白行，請查看程序本身的註釋。
  5. 創建文件CrunchifyReverseLineReaderTest.java並將下面的代碼複製到測試結果。

Java 代碼：

CrunchifyReverseLineReaderCore.java
Java
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package crunchify . com . tutorials ;
 
import java . io . ByteArrayOutputStream ;
import java . io . File ;
import java . io . IOException ;
import java . io . RandomAccessFile ;
import java . io . UnsupportedEncodingException ;
import java . nio . ByteBuffer ;
import java . nio . channels . FileChannel ;
 
/**
* Read a file from end to start
*
* @author Crunchify.com
*/
 
public class CrunchifyReverseLineReader {
private static final int BUFFER_SIZE = 8192 ;
private final FileChannel channel ;
private final String encoding ;
private long filePos ;
private ByteBuffer buf ;
private int bufPos ;
private ByteArrayOutputStream baos = new ByteArrayOutputStream ( ) ;
private RandomAccessFile raf ;
private byte lastLineBreak = '\n' ;
 
 
public CrunchifyReverseLineReader ( File file ) throws IOException {
this ( file , null ) ;
}
 
public CrunchifyReverseLineReader ( File file , String encoding ) throws IOException {
raf = new RandomAccessFile ( file , "r" ) ;
channel = raf . getChannel ( ) ;
filePos = raf . length ( ) ;
this . encoding = encoding ;
}
 
public void close ( ) throws IOException {
raf . close ( ) ;
}
 
public String readLine ( ) throws IOException {
byte c ;
while ( true ) {
if ( bufPos < 0 ) {
if ( filePos == 0 ) {
if ( baos == null ) {
return null ;
}
String line = bufToString ( ) ;
baos = null ;
return line ;
}
 
long start = Math . max ( filePos - BUFFER_SIZE , 0 ) ;
long end = filePos ;
long len = end - start ;
 
buf = channel . map ( FileChannel . MapMode . READ_ONLY , start , len ) ;
bufPos = ( int ) len ;
filePos = start ;
 
// Ignore Empty New Lines
c = buf . get ( -- bufPos ) ;
if ( c == '\r' | | c == '\n' )
while ( bufPos > 0 && (c == '\r' || c == '\n')) {
bufPos--;
c = buf . get ( bufPos ) ;
}
if ( ! ( c == '\r' | | c == '\n' ) )
bufPos ++ ; // IS THE NEW LENE
}
 
/*
* This will ignore all blank new lines.
*/
while ( bufPos -- > 0 ) {
c = buf . get ( bufPos ) ;
if ( c == '\r' | | c == '\n' ) {
// skip \r\n
while ( bufPos > 0 && (c == '\r' || c == '\n')) {
c = buf.get(--bufPos);
}
// restore cursor
if ( ! ( c == '\r' | | c == '\n' ) )
bufPos ++ ; // IS THE NEW Line
return bufToString ( ) ;
}
baos . write ( c ) ;
}
/*
*  If you don't want to ignore new line and would like
*  to print new line too then use below code
*  and comment out above while loop
while (bufPos-- > 0) {
byte c1 = buf.get(bufPos);
if (c1 == '\r' || c1 == '\n') {
if (c1 != lastLineBreak) {
lastLineBreak = c1;
continue;
}
lastLineBreak = c1;
return bufToString();
}
baos.write(c1);
}
*/
}
}
 
private String bufToString ( ) throws UnsupportedEncodingException {
if ( baos . size ( ) == 0 ) {
return "" ;
}
 
byte [ ] bytes = baos . toByteArray ( ) ;
for ( int i = 0 ; i < bytes . length / 2 ; i ++ ) {
byte t = bytes [ i ] ;
bytes [ i ] = bytes [ bytes . length - i - 1 ] ;
bytes [ bytes . length - i - 1 ] = t ;
}
 
baos . reset ( ) ;
if ( encoding ! = null )
return new String ( bytes , encoding ) ;
else
return new String ( bytes ) ;
}
}

根據 francesco 的評論，更新了代碼以從文件中刪除空白行。

其他必讀文章：https://crunchify.com/why-and-for-what-should-i-use-enum-java-enum-examples/

現在讓我們測試一下。 創建測試文件。

CrunchifyReverseLineReaderTest.java
Java
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package crunchify . com . tutorials ;
 
import java . io . BufferedReader ;
import java . io . File ;
import java . io . FileReader ;
import java . io . IOException ;
 
/**
* @author Crunchify.com
*/
 
public class CrunchifyReverseLineReaderTest {
 
@SuppressWarnings ( "resource" )
public static void main ( String [ ] args ) throws IOException {
 
/*
* If you want to have blank file and would like to add some lines then use below code
*
* File file = new File("/Users/appshah/Documents/crunchify.txt"); RandomAccessFile raf = new RandomAccessFile(file,
* "rw"); for (int i = 0; i <= 10; i++) { // Adding line from 1 to 10 //raf.writeBytes("Adding Line " + i + "\n"); }
* raf.close();
*
*/
 
/*
* Test - 1: In this tutorial we will read exsiting file crunchify.txt
* Reading file in Simple Order
*/
FileReader logReader = new FileReader ( "/Users/appshah/Documents/crunchify.txt" ) ;
BufferedReader buffer = new BufferedReader ( logReader ) ;
System . out . println ( "~~~~~~~~~~~~~~~~~~~Simple way to read file in Java without Reversing ~~~~~~~~~~~~~~~~~~\n" ) ;
for ( String line1 = buffer . readLine ( ) ; line1 ! = null ; line1 = buffer . readLine ( ) ) {
System . out . println ( line1 ) ;
}
 
/*
* Test - 2: In this tutorial we will read exsiting file crunchify.txt
* Reading file in Reverse Order
*/
 
File file = new File ( "/Users/appshah/Documents/crunchify.txt" ) ;
CrunchifyReverseLineReader reader = new CrunchifyReverseLineReader ( file , "UTF-8" ) ;
String line ;
System . out . print ( "\n~~~~~~~~~~~~~~~~~~~ Reading a file in Reverse Order ~~~~~~~~~~~~~~~~~~~ \n\n" ) ;
while ( ( line = reader . readLine ( ) ) ! = null ) {
System . out . println ( line ) ;
}
}
}

Note:文件 crunchify.txt total 11 lines ，包括中間的 1 個空白行，如下面的結果所示。

輸出：

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~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ Simple way to read file in Java without Reversing ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
 
—————— Crunchify Tutorials Start ——————
How To Troubleshoot Higher CPU , Memory & Process Usage ?
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Detailed Guide On How To Install SSL On WordPress Site
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Simplest Hello World Spring MVC Tutorial And JDBC MySQL Tutorial
NEW Start Embedded HTTP Jersey Server During Java Application Startup
NEWTop 10 Java Interview Questions Answers
Java Properties File : How To Read Config . Properties Values In Java ?
—————— Crunchify Tutorials End ——————
 
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ Reading a file in Reverse Order ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
 
—————— Crunchify Tutorials End ——————
Java Properties File : How To Read Config . Properties Values In Java ?
NEWTop 10 Java Interview Questions Answers
NEW Start Embedded HTTP Jersey Server During Java Application Startup
Simplest Hello World Spring MVC Tutorial And JDBC MySQL Tutorial
List Of All Genesis WordPress Framework Tips
Detailed Guide On How To Install SSL On WordPress Site
Top 7 Tips To Speed Up WordPress & Boost Performance
How To Troubleshoot Higher CPU , Memory & Process Usage ?
—————— Crunchify Tutorials Start ——————