我有一个文本文件，其中包含多个条目，例如：
hello
there
my
name
is
JoeBloggs
我如何按降序读取最后五个条目，即来自JoeBloggs - 那里
我目前只有代码才能阅读最后一行：
public class TestLastLineRead {
public static void main(String[] args) throws Exception {
FileInputStream in = new FileInputStream(file.txt);
BufferedReader br = new BufferedReader(new InputStreamReader(in));
String strLine = null, tmp;
while ((tmp = br.readLine()) != null) {
strLine = tmp;
}
String lastLine = strLine;
System.out.println(lastLine);
in.close();
}
}
答案 0 :(得分：13)
您可以将这些行添加到List，例如一个LinkedList。当列表超过五行时，删除第一个/最后一个。
List<String> lines = new LinkedList<String>();
for(String tmp; (tmp = br.readLine()) != null;)
if (lines.add(tmp) && lines.size() > 5)
lines.remove(0);
答案 1 :(得分：9)
一种非常简单的方法是使用Apache Commons Collections库中的CircularFifoBuffer类。它基本上是一个固定大小的列表，它会在旧元素已满时丢弃旧元素并添加新元素。因此，您要创建一个大小为5的CircularFifoBuffer，然后将所有行添加到其中。最后，它只包含文件的最后五行。
答案 2 :(得分：5)
我们可以使用MemoryMappedFile打印最后5行：
private static void printByMemoryMappedFile(File file) throws FileNotFoundException, IOException{
FileInputStream fileInputStream=new FileInputStream(file);
FileChannel channel=fileInputStream.getChannel();
ByteBuffer buffer=channel.map(FileChannel.MapMode.READ_ONLY, 0, channel.size());
buffer.position((int)channel.size());
int count=0;
StringBuilder builder=new StringBuilder();
for(long i=channel.size()-1;i>=0;i--){
char c=(char)buffer.get((int)i);
builder.append(c);
if(c=='\n'){
if(count==5)break;
count++;
builder.reverse();
System.out.println(builder.toString());
builder=null;
builder=new StringBuilder();
}
}
channel.close();
}
RandomAccessFile打印最后5行：
private static void printByRandomAcessFile(File file) throws FileNotFoundException, IOException{
RandomAccessFile randomAccessFile = new RandomAccessFile(file, "r");
int lines = 0;
StringBuilder builder = new StringBuilder();
long length = file.length();
length--;
randomAccessFile.seek(length);
for(long seek = length; seek >= 0; --seek){
randomAccessFile.seek(seek);
char c = (char)randomAccessFile.read();
builder.append(c);
if(c == '\n'){
builder = builder.reverse();
System.out.println(builder.toString());
lines++;
builder = null;
builder = new StringBuilder();
if (lines == 5){
break;
}
}
}
}
答案 3 :(得分：0)
尝试此代码，扫描所有行中的长度为5的列表，最后列表反转。我编辑/修改了你的代码，测试它以确保它完全正常工作。
import java.io.BufferedReader;
import java.io.FileInputStream;
import java.io.InputStreamReader;
import java.util.ArrayList;
public class Test
{
public static void main(String[] args) throws Exception
{
ArrayList<String> bandWidth = new ArrayList<String>();
FileInputStream in = new FileInputStream("file.txt");
BufferedReader br = new BufferedReader(new InputStreamReader(in));
String tmp;
while ((tmp = br.readLine()) != null)
{
bandWidth.add(tmp);
if (bandWidth.size() == 6)
bandWidth.remove(0);
}
ArrayList<String> reversedFive = new ArrayList<String>();
for (int i = bandWidth.size() - 1; i >= 0; i--)
reversedFive.add(bandWidth.get(i));
in.close();
}
}
答案 4 :(得分：0)
如果它确实需要做的就是打印最后5行：
ArrayList<String> lines = new ArrayList<String>();
String tmp="";
while ((tmp = br.readLine()) != null) {
lines.add(tmp);
}
for (int i = lines.size()-5; i < lines.size(); i++) {
System.out.println(lines.get(i-1));
}
答案 5 :(得分：0)
试试这个。 这给出了最后5行。
public static void main(String[] args) throws IOException {
List<String > list =new ArrayList<String>();
FileInputStream in = new FileInputStream("C:/adminconsole.txt");
BufferedReader br = new BufferedReader(new InputStreamReader(in));
String strLine ="", tmp;
while ((tmp = br.readLine()) != null){
//strLine =tmp+"\n"+strLine;
list.add(tmp);
}
if(list.size()>5){
for (int i=list.size()-1; i>=(list.size()-5); i--) {
System.out.println(list.get(i));
}
}else{
for (int i=0; i<5; i++) {
System.out.println(list.get(i));
}
}
}
}
答案 6 :(得分：0)
首先，您必须逐行读取文件并将每行添加到列表中。完全读取文件后，您可以按相反的顺序打印列表中的每个元素，如下所示：
import java.io.BufferedReader;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.List;
public class FileReader {
public static List<String> readFile() throws IOException {
List<String> fileContents = new ArrayList<String>();
FileInputStream fileInputStream = new FileInputStream("C:/Users/compaq/Desktop/file.txt");
InputStreamReader inputStreamReader = new InputStreamReader(fileInputStream);
BufferedReader bufferedReader = new BufferedReader(inputStreamReader);
String strLine = null;
while((strLine=bufferedReader.readLine())!=null) {
fileContents.add(strLine);
}
fileInputStream.close();
return fileContents;
}
public static void printFileInReverse(List<String> fileContents, int numberOfLines) {
int counter = 0;
for(int i=(fileContents.size()-1);i>=0;i--) {
if(counter==numberOfLines) { break; }
System.out.println(fileContents.get(i));
counter++;
}
}
public static void main(String[] args) throws IOException {
List<String> fileContents = new ArrayList<String>();
fileContents = FileReader.readFile();
int numberOfLines = 5;// Number of lines that you would like to print from the bottom of your text file.
FileReader.printFileInReverse(fileContents, numberOfLines);
}
}
答案 7 :(得分：0)
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.Scanner;
public class REVERSE {
public static void main(String[] args) {
ArrayList<String> al = new ArrayList<String>();
try {
Scanner sc = new Scanner(new FileReader("input.txt"));
while (sc.hasNextLine()) {
al.add(sc.nextLine());
}
System.out.println(al.get(0));
System.out.println(al.get(1));
System.out.println(al.get(2));
System.out.println(al.get(3));
System.out.println(al.get(4));
Collections.reverse(al);
/*
* for (String s : al) { System.out.println(s); }
*/
System.out.println(al.get(0));
System.out.println(al.get(1));
System.out.println(al.get(2));
System.out.println(al.get(3));
System.out.println(al.get(4));
/*
* for (int i = 0; i < al.size(); i++) {
* System.out.println(al.get(i)); }
*/
} catch (FileNotFoundException e) {
}
}
}