如何在Java中使用扫描仪扫描特定行？

Question

When using Scanner utility in Java, how to scan a specific line or scan a line twice? Like when I using scanner.nextLine(), how do I go back to this line again? Or how to read any line twice? Is that possible?

Answer 1

Unfortunatly no

Scanner was created to read streams and they don't "store" the information they read. Meaning should your source be reading some transmission over the internet it can't go back and read it again for example. Scanner handles files in a similar way to those internet transmissions by creating an input stream from the file and handling all the fancy encodings and all that for you.

If you want to read a line from a file again or hop back to another line your best option is to close the scanner, create a new scanner object and go all the way down again to the preferred line.

Quick example:

Let's say we're looking for a name that's above the name we ask it to search for in a file containing names:

One approach would be to loop through the file in a while loop, reading a line(name) and seeing if it matches our name, if it does we store the line we read it from, close the scanner, create a new scanner object and start the looping process again, this time stopping when we reached the line before the name we were searching for and bam!

Another (and better approach in my opinion) would be to store all the information we read from scanner (in our example names) in a list of some sort (in our example an arraylist would be perfect). We can then use that list to read a line twice, read the 5th line (5th element in the list) or go back to a previous line

Hope this helps and if you need some code feel free to ask :)

Answer 2

Scanner wasn't built to do that - Plus, the Scanner class does not keep any history of any lines at all

.nextLine() would just give you the whole line (the first one) and .next() would just give you the first word. It does not store any lines/words/characters.

Answer 3

我认为您不能使用扫描仪本身来执行此操作，但是可以使用基础BufferedInputStream mark()和reset()方法返回到先前读取的位置。