java 您可以将扫描仪跳转到文件中的某个位置或向后扫描吗？
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Can you jump a scanner to a location in file or scan backwards?
提问by Mike
I have a very large text file and I need to gather data from somewhere near the end. Maybe Scanner isn't the best way to do this but it would be very wasteful to start at the top and grab 6000 lines before getting to the part of the file I am interested in. Is there a way to either tell Scanner to jump to say 7/8ths down the document or start from the bottom and scan upwards grabbing line by line?
我有一个非常大的文本文件，我需要从接近尾声的某个地方收集数据。也许 Scanner 不是执行此操作的最佳方法，但是在到达我感兴趣的文件部分之前从顶部开始并抓取 6000 行会非常浪费。有没有办法告诉 Scanner 跳转到说文档的 7/8ths 还是从底部开始并逐行向上扫描？
Thanks
谢谢
采纳答案by polygenelubricants
The underlying input source for a java.util.Scanneris a java.lang.Readable. Beyond the Scanner(File)constructor, a Scannerneither knows nor cares of the fact that it's scanning a file.
a 的底层输入源java.util.Scanner是 a java.lang.Readable。除了Scanner(File)构造函数之外，aScanner既不知道也不关心它正在扫描文件的事实。
Also, since it's regex based on java.util.regex.*, there's no way it can scan backward.
此外，由于它是基于 的正则表达式java.util.regex.*，因此无法向后扫描。
To accomplish what you want to do, it's best to do it at the input source level, e.g. by using InputStream.skipof the source before passing it to the constructor of Scanner.
要完成您想做的事情，最好在输入源级别执行此操作，例如InputStream.skip在将源传递给Scanner.
On Scanner.skip
在 Scanner.skip
Scanneritself does have a skip, and a pattern like "(?s).{10}"would skip 10 characters (in (?s)single-line/Pattern.DOTALLmode), but this is perhaps a rather roundabout way of doing it.
Scanner本身确实有一个skip, 并且一个模式"(?s).{10}"会跳过 10 个字符（在(?s)单行/Pattern.DOTALL模式下），但这可能是一种相当迂回的方式。
Here's an example of using skipto skip a given number of lines.
这是skip用于跳过给定行数的示例。
String text =
"Line1 blah blah\n" +
"Line2 more blah blah\n" +
"Line3 let's try something new \r\n" +
"Line4 meh\n" +
"Line5 bleh\n" +
"Line6 bloop\n";
Scanner sc = new Scanner(text).skip("(?:.*\r?\n|\r){4}");
while (sc.hasNextLine()) {
System.out.println(sc.nextLine());
}
This prints (as seen on ideone.com):
这打印（如在 ideone.com 上看到的）：
Line5 bleh
Line6 bloop
回答by RonK
回答by Ben S
You should probably use RandomAccessFileinstead.
您可能应该改用RandomAccessFile。
