打印 ArrayList 的子列表（基于用户输入的待办事项列表）会导致 ArrayList 为空

Question

一个很简单的待办事项列表，要求输入，然后以ArrayList的形式打印出列表，分成几个部分（子列表）（我的视力很差，所以我必须使用大字体，并且当列表太长时，问题是列表的末尾超出了页面。尽管我可以使用 home/end 按钮快速查看页面，但这不是最佳情况。我会而是将 ArrayList 分成子列表，并打印出子列表，每行一个，如下所示：

这是今天的待办事项：【起床，遛狗，吃早餐】 [铺床，扫背，学习Java]

进口java.util.Scanner；导入 java.util.ArrayList;

/**
 * @author Troy
 *
 */
public class HelloWorld {


    public static void main(String[] args) {
        // I chose an ArrayList because the size does not have to be predetermined.
        ArrayList<String> to_do = new<String>ArrayList();
        System.out.println("What would you like to add to your to-do list?");
        Scanner user_input = new Scanner(System.in);
        //While the user_input still has entries, perform the following:
        while (user_input.hasNextLine()) {
            //Add next entry in the to-do list(user_input) to the ArrayList
            String input = user_input.nextLine();
            //If input = remove, remove the last item in the to_do list.(ArrayList)
            if ("remove".equals(input)) {
                if (to_do.size() > 0) {
                to_do.remove(to_do.size() -1);
            }}
            /**If the user types in "exit", when prompted for the next item in their
             * to_do list, close user_input, and print out... 
             */
            if ("exit".equals(input)) {
                user_input.close();
                System.out.println("Your to-do list is complete!");

                ArrayList<String> sect1 = new ArrayList<String>(to_do.subList(0, to_do.size()));                
                if (to_do.size() <= 5) {
                    System.out.println(sect1 + "\n");
                    break;
                }
                ArrayList<String> sect2 = new ArrayList<String>(to_do.subList(6, to_do.size()));
                if (to_do.size() > 5 && to_do.size() <=10) {
                    System.out.println(sect1 + "\n" + sect2);
                    break;
            }
            //If input does NOT equal "remove", add user_input to the to_do list.
                if (!"remove".equals(input)) {
                    to_do.add(input);
                }

            System.out.println("\n");
            /**Print the ArrayList called "to_do" split into sections AFTER writing, 
             * "Here is today's to-do list:"
             *  */
            System.out.println("Here is today's to-do list: " + "\n");
            if (to_do.size() <= 5) {
                System.out.println(sect1 + "\n");
            }
            if (to_do.size() > 5 && to_do.size() <=10) {
                System.out.println(sect1 + "\n" + sect2);
            }

        }
    }
}}

Answer 1

问题在于括号的位置。 if (!"remove".equals(input)) { 行在 if ("exit".equals(input)) { 块内。我移动了 if 语句：导入 java.util.ArrayList；导入 java.util.Scanner;

public class HelloWorld {


    public static void main(String[] args) {
        // I chose an ArrayList because the size does not have to be predetermined.
        ArrayList<String> to_do = new<String>ArrayList();
        System.out.println("What would you like to add to your to-do list?");
        Scanner user_input = new Scanner(System.in);
        //While the user_input still has entries, perform the following:
        while (user_input.hasNextLine()) {
            //Add next entry in the to-do list(user_input) to the ArrayList
            String input = user_input.nextLine();
            //If input = remove, remove the last item in the to_do list.(ArrayList)
            if ("remove".equals(input)) {
                if (to_do.size() > 0) {
                to_do.remove(to_do.size() -1);
            }}

            /**If the user types in "exit", when prompted for the next item in their
             * to_do list, close user_input, and print out... 
             */
            if ("exit".equals(input)) {
                user_input.close();
                System.out.println("Your to-do list is complete!");

                ArrayList<String> sect1 = new ArrayList<String>(to_do.subList(0, to_do.size()));                
                if (to_do.size() <= 5) {
                    System.out.println(sect1 + "\n");
                    break;
                }
                ArrayList<String> sect2 = new ArrayList<String>(to_do.subList(6, to_do.size()));
                if (to_do.size() > 5 && to_do.size() <=10) {
                    System.out.println(sect1 + "\n" + sect2);
                    break;
            }
            //If input does NOT equal "remove", add user_input to the to_do list.
            if (!"remove".equals(input) && !"exit".equals(input)) {
                to_do.add(input);
            }

            System.out.println("\n");
            /**Print the ArrayList called "to_do" split into sections AFTER writing, 
             * "Here is today's to-do list:"
             *  */
            System.out.println("Here is today's to-do list: " + "\n");
            if (to_do.size() <= 5) {
                System.out.println(sect1 + "\n");
            }
            if (to_do.size() > 5 && to_do.size() <=10) {
                System.out.println(sect1 + "\n" + sect2);
            }

        }
    }
}}

Answer 2

正如另一位发帖人所说，您的代码的问题是 if 块的嵌套不正确。这会导致您的 to_do.add 位于 if ("exit".equals(input)) 块内，因此您的列表仍然为空。我建议使用 IDE 并让它重新缩进（格式化）你的代码，然后这个问题会变得更加明显。

但除此之外，您的代码中还有另一个问题：您的 sect1 需要 subList(0, to_do.size())，这是您的整个列表。这将导致它在一行上打印整个列表，这就是您所看到的。我建议您改为使用循环并将列表分成相等的块。由于subList已经returns一个列表，你也不必再用另一个ArrayList包装它，你可以直接打印它。

所以我将您的代码更正为：

import java.util.Scanner;
import java.util.List;
import java.util.ArrayList;

/**
 * @author Troy
 */
public class HelloWorld {
    public static void main(String[] args) {
        // I chose an ArrayList because the size does not have to be predetermined.
        List<String> toDo = new ArrayList<String>();
        System.out.println("What would you like to add to your to-do list?");
        Scanner userInput = new Scanner(System.in);

        // While the userInput still has entries, perform the following:
        while (userInput.hasNextLine()) {
            // Get the next line entered by the user
            String input = userInput.nextLine();

            //If input is "remove", remove the last item in the toDo list. (ArrayList)
            if ("remove".equals(input)) {
                if (toDo.size() > 0) {
                    toDo.remove(toDo.size() -1);
                }
            }
            /*
             * If the user types in "exit", when prompted for the next item in their
             * toDo list, close userInput, and print out... 
             */
            else if ("exit".equals(input)) {
                userInput.close();
                System.out.println("Your to-do list is complete!");
                System.out.println("Here is today's to-do list: ");

                final int perLine = 3;
                int i = 0;
                while(i < toDo.size()) {
                    // Print from the start of our current chunk (i)
                    //  to the end (i+3), or to the size of the list if our last chunk is smaller than "perLine".
                    System.out.println(
                        toDo.subList(i, Math.min(toDo.size(), i+perLine))
                    );
                    i+=perLine;
                }

                break;
            }
            /*
             * If input is neither "remove" nor "exit", add input to the list
             */
            else {
                toDo.add(input);
            }
        }
    }
}

我还将一些变量更改为驼峰式而不是 snake_case，这是 Java 中的约定。

打印 ArrayList 的子列表（基于用户输入的待办事项列表）会导致 ArrayList 为空

Printing sublist of an ArrayList (To-Do List based on user input) results in empty ArrayList

java

printing

arraylist

sublist

empty-list