Java中的scanner skip是什么，为什么要用它？

Question

scanner.skip("(\r\n|[\n\r\u2028\u2029\u0085])?"); 

Can someone explain, what does the above code mean.

I'm pretty new to Java.

Any help would be greatly appreciated.

Answer 1

Scanner.skip

public Scanner skip(Pattern pattern)

Skips input that matches the specified pattern, ignoring delimiters. This method will skip input if an anchored match of the specified pattern succeeds.
If a match to the specified pattern is not found at the current position, then no input is skipped and a NoSuchElementException is thrown.

Since this method seeks to match the specified pattern starting at the scanner's current position, patterns that can match a lot of input (".*", for example) may cause the scanner to buffer a large amount of input.

So this allows you to "move" the scanner position using a regex.

Example :

Skip the start of the line :

Scanner scan = new Scanner("Hello world");
scan.skip("Hello ");

System.out.println(scan.nextLine());
scan.close();

world

Since this is using a regex, you skip until a work in the middle of a line :

Scanner scan = new Scanner("Hello world, I am happy to see you");
scan.skip(".*I am ");

System.out.println(scan.nextLine());
scan.close();

happy to see you

Answer 2

The skip() method of this class does exactly that. It will skip input matching the pattern. In this case, the pattern is saying to skip over carriage returns (\\r) and new lines (\\n), plus some unicode caracters.

So, when the line is read, it will ignore that pattern and return just the rest of the string. A simpler example would be like this. Let's say you have a string:

String s = "Hello world, this is my scanner!";

Then if you have a scanner such as:

Scanner scan = new Scanner(s);
scan.skip(", this is my scanner");

Then when you do:

System.out.println("" + scanner.nextLine());

The output to the console will be simply

Hello world!

What happened is the pattern ", this is my scanner" was matched, and ignored from the input. Then all that was left is the "Hello world!" string.

Answer 3

import java.util.Scanner;
public class Main {
    public static void main(String[] args) {

                                String s = "Hello world, this is my scanner!";

//Then if you have a scanner such as:

Scanner scan = new Scanner(s);
scan.skip("world");
//Then when you do:

System.out.println("" + scan.nextLine());
scan.close();
    }
}

Output: Exception thrown

When I passed Hello to scan.skip(), the code worked and printed everything past Hello.

You can only pass a prefix to.skip(). Don't pass a substring starting from anywhere in it. The string passed has to start the input/or the string S in this case.

Scanner scan = new Scanner(System.in);
scan.skip("a");
//Then when you do:
System.out.println("err");
System.out.println("" + scan.nextLine());
scan.close();

If I enter anything starting with a, it skips a and displays rest of the line.

However when I enter anything that doesn't start with a, Then following System.out.println() is never executed and I get stuck at the input. It won't take input.