[英]Deleting the last line of a file with Java
I have a .txt
file, which I want to process in Java. 我有一个.txt
文件,我想用Java处理它。 I want to delete its last line. 我想删除它的最后一行。
I need ideas on how to achieve this without having to copy the entire content into another file and ignoring the last line. 我需要有关如何实现这一点的想法,而无需将整个内容复制到另一个文件中并忽略最后一行。 Any suggestions? 有什么建议?
您可以通过扫描文件找到最后一行的开头,然后使用FileChannel.truncate
或RandomAccessFile.setLength
截断它。
By taking RandomAccessFile you can: 通过使用RandomAccessFile,您可以:
Code would be something like this: 代码将是这样的:
LinkedList<String> lines=null;
int howMuch = 1; // one line to read and delete
try{
RandomAccessFile raf = new RandomAccessFile(inputFileName, "rw");
System.out.println("File Length="+raf.length());
long step = 20; // here I advice to write average length of line
long jump = raf.length()<step*howMuch?
0:
raf.length()-step*howMuch;
raf.seek(jump);
lines = new LinkedList<String>();
LinkedList<Long> pos = new LinkedList<Long>();
Entry<LinkedList<String>,LinkedList<Long>> rLinesRead = getRemainingLines(raf,
new AbstractMap.SimpleEntry<LinkedList<String>,LinkedList<Long>> (lines,pos));
while(rLinesRead.getKey().size()<howMuch){
if(jump<step)
if(jump<=0)
break;
else{
jump=0;
raf.seek(jump);
rLinesRead=getRemainingLines(raf,rLinesRead);
break;
}
else
jump=jump-step;
raf.seek(jump);
rLinesRead=getRemainingLines(raf,rLinesRead);
}
int originalSize=rLinesRead.getKey().size();
lines=rLinesRead.getKey();
pos=rLinesRead.getValue();
for (int i=0;i<originalSize-howMuch;++i){
lines.removeFirst();
pos.removeFirst();
}
if(!pos.isEmpty())
raf.setLength(pos.getFirst()); // before last(from EOF) returned fully read lines in file
}catch(Exception ex){
ex.printStackTrace();
} finally{
try { raf.close(); } catch (Exception e) { e.printStackTrace(); }
}
//returns line to EOF with their begin file pointer positions
private Entry<LinkedList<String>,LinkedList<Long>> getRemainingLines(RandomAccessFile raf,
Entry<LinkedList<String>,LinkedList<Long>> linesAlreadyLoadedFromEnd) throws IOException{
LinkedList<String> pLines = linesAlreadyLoadedFromEnd.getKey();
LinkedList<Long> pPos = linesAlreadyLoadedFromEnd.getValue();
long init=raf.getFilePointer();
String str = raf.readLine();
if(pPos.size()>0?pPos.getFirst()==0:false || str==null)
return linesAlreadyLoadedFromEnd;
LinkedList<String> lines = new LinkedList<String>();
LinkedList<Long> pos = new LinkedList<Long>();
if(init==0L ){
lines.add(str);
pos.add(0L);
}
Long tmpPos = raf.getFilePointer();
while ((str = raf.readLine())!=null && !pPos.contains(tmpPos)){
lines.add(str);
pos.add(tmpPos);
tmpPos = raf.getFilePointer();
}
pLines.addAll(0,lines);
pPos.addAll(0,pos);
return new AbstractMap.SimpleEntry<LinkedList<String>,LinkedList<Long>> (pLines,pPos);
}
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