Java 用换行符分割字符串的程序和没有输入的停止不工作？

Question

This is the problem I'm trying to solve:

My code is the following:

import java.util.Scanner;

public class LineByLine {

    public static void main(String[] args) {
        while (true) {
            Scanner scanner = new Scanner(System.in);
            String sentence = String.valueOf(scanner.nextLine());
            String[] pieces = sentence.split(" ");

            for (int i = 0; i < pieces.length; i++) {
                System.out.println(pieces[i]);
            }
            if (sentence.equals("")) {
                break;
            }
        }
    }
}

My code is showing as wrong and I'm unsure why. Any explanations?

Answer 1

You should arrange your code like:

public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);
    
    while (true) { 
        String sentence = String.valueOf(scanner.nextLine());
        String[] pieces = sentence.split(" ");

        for (int i = 0; i < pieces.length; i++) {
            System.out.println(pieces[i]);
        } 
        if (sentence.equals("")) {
            break;
        }
   }
   scanner.close();
}

Also you could use hasNext method instead of while(true) part:

public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);
    
    while (scanner.hasNext()) { 
        String sentence = scanner.nextLine();
        String[] pieces = sentence.split(" ");

        for (int i = 0; i < pieces.length; i++) {
            System.out.println(pieces[i]);
        } 
   }
   scanner.close();
}

Answer 2

You'll need to place:

Scanner scanner = new Scanner(System.in);

outside of the while loop.

The reason:

You only need to create a single Scanner object. This has to be done before you enter the loop since the instantiation will consume the standard input - this means that after the first iteration of the loop, there will be no standard input left to instantiate the object once again.

You can also think about it more mechanically than that:

If you had a loop that was meant to iterate through numbers, you wouldn't want to be resetting your loop counter each time, right? Its quite a similar thing in this case.