[英]What is the fastest way to find out how many non-empty lines are in a file, using Java?
使用Java查找文件中有多少個非空行的最快方法是什么?
最簡單的方法是使用BufferedReader,並檢查哪些行為空。 但是,這是一種相對較慢的方法,因為它需要為文件中的每一行創建一個String對象。 一種更快的方法是使用read()將文件讀入數組,然后遍歷數組以計算換行符。
這是兩個選項的代碼; 第二個大約花了我機器50%的時間。
public static void timeBufferedReader () throws IOException
{
long bef = System.currentTimeMillis ();
// The reader buffer size is the same as the array size I use in the other function
BufferedReader reader = new BufferedReader(new FileReader("test.txt"), 1024 * 10);
int counter = 0;
while (reader.ready())
{
if (reader.readLine().length() > 0)
counter++;
}
long after = System.currentTimeMillis() - bef;
System.out.println("Time: " + after + " Result: " + counter);
}
public static void timeFileReader () throws IOException
{
long bef = System.currentTimeMillis();
FileReader reader = new FileReader("test.txt");
char[] buf = new char[1024 * 10];
boolean emptyLine = true;
int counter = 0;
while (reader.ready())
{
int len = reader.read(buf,0,buf.length);
for (int i = 0; i < len; i++)
{
if (buf[i] == '\r' || buf[i] == '\n')
{
if (!emptyLine)
{
counter += 1;
emptyLine = true;
}
}
else emptyLine = false;
}
}
long after = System.currentTimeMillis() - bef;
System.out.println("Time: " + after + " Result: " + counter);
}
關於NIO的建議,我支持Limbic System。 我在Daphna的測試代碼中添加了NIO方法,並在他的兩種方法中進行了標記:
public static void timeNioReader () throws IOException {
long bef = System.currentTimeMillis();
File file = new File("/Users/stu/test.txt");
FileChannel fc = (new FileInputStream(file)).getChannel();
MappedByteBuffer buf = fc.map(MapMode.READ_ONLY, 0, file.length());
boolean emptyLine = true;
int counter = 0;
while (buf.hasRemaining())
{
byte element = buf.get();
if (element == '\r' || element == '\n') {
if (!emptyLine) {
counter += 1;
emptyLine = true;
}
} else
emptyLine = false;
}
long after = System.currentTimeMillis() - bef;
System.out.println("timeNioReader Time: " + after + " Result: " + counter);
}
以下是89MB文件的預熱結果:
timeBufferedReader Time: 947 Result: 747656
timeFileReader Time: 670 Result: 747656
timeNioReader Time: 251 Result: 747656
NIO比FileReader快2.5倍,比BufferedReader快4倍!
盡管預熱時間要長得多,但使用6.4MB的文件時效果甚至更好。
//jvm start, warming up
timeBufferedReader Time: 121 Result: 53404
timeFileReader Time: 65 Result: 53404
timeNioReader Time: 40 Result: 53404
//still warming up
timeBufferedReader Time: 107 Result: 53404
timeFileReader Time: 60 Result: 53404
timeNioReader Time: 20 Result: 53404
//ripping along
timeBufferedReader Time: 79 Result: 53404
timeFileReader Time: 56 Result: 53404
timeNioReader Time: 16 Result: 53404
隨心所欲。
最簡單的方法是使用掃描儀(是的,我喜歡冗長的代碼...您可以使其物理上更短)。 Scanner()還需要File,Reader等...,因此您可以隨便傳遞它。
import java.util.Scanner;
public class Main
{
public static void main(final String[] argv)
{
final Scanner scanner;
final int lines;
scanner = new Scanner("Hello\n\n\nEvil\n\nWorld");
lines = countLines(scanner);
System.out.println("lines = " + lines);
}
private static int countLines(final Scanner scanner)
{
int lines;
lines = 0;
while(scanner.hasNextLine())
{
final String line;
line = scanner.nextLine();
if(line.length() > 0)
{
lines++;
}
}
return lines;
}
}
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