实现可比较的java

Question

我正在尝试使用可比较的界面按日期对用户输入的任务和日期列表进行排序。我在外部 .txt 文件中有任务，如果我访问正确的变量，我会有点困惑。程序编译，但是当我尝试对任务进行排序时，它似乎删除了文件的内容。这是我所拥有的：

public class DueDate implements Comparable<DueDate>{

    public String addedTask = "";
    public String enteredDueDate;


    public DueDate(String addedTask, String dueDate){

        this.addedTask = addedTask;
        this.enteredDueDate = enteredDueDate;
    }

    public String toString(){
        return addedTask+"\t"+enteredDueDate+"\t";
    }

    @Override
    public int compareTo(DueDate o) {
        return this.enteredDueDate.compareTo(o.enteredDueDate);
    }
}

public class Main {

    public static String fileName = "/Users/eringray/Desktop/tasklistjava/src/javatask.txt";

    public static void main(String[] args) throws IOException {

        int menuItem = -1;
        while(menuItem != 0){
            menuItem = menu();
            switch (menuItem){
                case 1:
                    showTaskList();
                    break;
                case 2:
                    addTask();
                    break;
                case 3:
                    sortList();
                case 4:
                    deleteTasks();
                    break;
                case 0:
                    break;
                default:
                    System.out.println("Invalid Input");

            }
        }
    }



    static int menu(){
        int choice;
        Scanner sc = new Scanner(System.in);
        System.out.println("\n Task List Menu \n");
        System.out.println("0: Exit Menu");
        System.out.println("1: Show Tasks in List");
        System.out.println("2: Add Task to List");
        System.out.println("3: Sort Tasks by Due Date");
        System.out.println("4: Delete Tasks");
        System.out.println();
        System.out.println("Enter a choice: ");
            choice = sc.nextInt();
            return choice;
    }

    static void showTaskList(){
        System.out.println("\nTask List\n");
        try {
            Scanner inFile = new Scanner(new FileReader(fileName));
            String line;
            int number = 1;
            while(inFile.hasNextLine()){
                line = inFile.nextLine();
                System.out.println(number + ". " + line);
                ++number;
            }
            System.out.println();
            inFile.close();
        } catch (FileNotFoundException ioe) {
            System.out.println(ioe);
        }
    }

    static void addTask(){
        System.out.println("\nAdd Task\n");
        try {
            Scanner input = new Scanner(System.in);
            PrintWriter outFile = new PrintWriter(new FileWriter(fileName, true));
            System.out.println("Enter a Task: ");
            String addedTask = input.nextLine();
            System.out.println("Set Due Date for this Task(yyyy-mm-dd): ");
            String dueDate = input.nextLine();
            outFile.println(addedTask + "\t" + dueDate);
            outFile.close();
        } catch (IOException ioe) {
            System.out.println(ioe);
        }
    }
    static void sortList() throws IOException {
        System.out.println("\nSorted List\n");
        try {
            BufferedReader br = new BufferedReader(new FileReader(fileName));
            BufferedWriter bw = new BufferedWriter(new FileWriter(fileName, true));
            ArrayList<DueDate> tasks = new ArrayList<DueDate>();
            String line = "";
            while((line = br.readLine()) != null) {
                String[] values = line.split("\t");

                if(values.length == 2) {
                    String addedTask = values[0];
                    String enteredDueDate = values[1];

                    DueDate d = new DueDate(addedTask, enteredDueDate);

                    tasks.add(d);
                }
            }

            Collections.sort(tasks);

            for(int i = 0; i < tasks.size(); i++){
                DueDate date = tasks.get(i);
                String lineText = date.toString();
                bw.write(lineText);
                bw.newLine();
            }

            br.close();
            bw.close();

        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }


    }
    private static void deleteTasks(){
        PrintWriter writer = null;
        try {
            writer = new PrintWriter(fileName);
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }
        assert writer != null;
        writer.print("");
        writer.close();
        }

    }

对不起，额外的代码，但认为它可能会帮助您了解我正在尝试做什么。任何建议都会很棒。我是Java新手，所以请耐心等待！谢谢！

Answer 1

问题是您的switch 中缺少break 语句，该语句在调用sortList() 之后调用deleteTasks() 方法。将您的代码更改为此，它应该可以正常工作：

int menuItem = -1;
while(menuItem != 0){
    menuItem = menu();
    switch (menuItem){
        case 1:
            showTaskList();
            break;
        case 2:
            addTask();
            break;
        case 3:
            sortList();
            break; //The missing break
        case 4:
            deleteTasks();
            break;
        case 0:
            break;
        default:
            System.out.println("Invalid Input");
    }
}

并且DueDate 类的构造函数缺少enteredDueDate 变量的赋值，因为该参数被命名为dueDate 而不是enteredDueDate。你应该把它改成这样：

public DueDate(String addedTask, String enteredDueDate){
    this.addedTask = addedTask;
    this.enteredDueDate = enteredDueDate;
}

因为您当前正在将成员变量分配给它自己的值。 您还应该考虑更改sortList 方法，尤其是BufferedWriter，否则每次调用它时都会复制列表。像这样的东西可以做到：

BufferedReader br = new BufferedReader(new FileReader(fileName));
ArrayList<DueDate> tasks = new ArrayList<DueDate>();
String line = "";
while((line = br.readLine()) != null) {
    String[] values = line.split("\t");
    if(values.length == 2) {
        String addedTask = values[0];
        String enteredDueDate = values[1];
        DueDate d = new DueDate(addedTask, enteredDueDate);
        tasks.add(d);
    }
}
Collections.sort(tasks);
br.close();
//Changed it to not append but overwrite the old file so it only contains the sorted list
BufferedWriter bw = new BufferedWriter(new FileWriter(fileName, false));
for (DueDate date : tasks) {
    String lineText = date.toString();
    bw.write(lineText);
    bw.newLine();
}
bw.flush();
bw.close();

---

编辑：要打印排序列表，您可以做一些事情。 最简单的方法可能是将showTaskList 方法的调用附加到sortList 方法的末尾，如下所示

    //...
    bw.flush();
    bw.close();
    showTaskList();
} catch (FileNotFoundE
    e.printStackTrace(
}

或者你可以遍历ArrayList 并像这样打印出来：

    //...
    bw.flush();
    bw.close();
    for (int i = 0; i < tasks.size(); i++) {
        DueDate dueDate = tasks.get(i);
        System.out.println(i+". "+dueDate.toString());
    }
} catch (FileNotFoundException e) {
    e.printStackTrace();
}

---

编辑 2：在排序时将所有 Tasks 与空 DueDate 放到底部的最简单方法是更改​​ DueDate 类中的 compareTo 方法：

@Override
public int compareTo(DueDate o) {
    return -this.enteredDueDate.compareTo(o.enteredDueDate);
}

只需将其取反，所有条目都按降序排序，DueDate 为空的条目将位于列表末尾（它根据默认情况下如何比较字符串）。
但是，如果您想使用没有截止日期的Tasks，您将在sortList 方法中遇到问题，因为split("\t") 方法只会返回一个长度为1 的数组，而您的if 条件将失败。解决它的一种方法如下所示：

//...
while ((line = br.readLine()) != null) {
    String[] values = line.split("\t");
    //To ensure it's still valid data
    if (values.length >= 1 && values.length <= 2) {
        String addedTask = values[0];
        String enteredDueDate;
        //Check whether dueDate has a value or is empty
        if (values.length == 1)
            enteredDueDate = "";
        else
            enteredDueDate = values[1];
        DueDate d = new DueDate(addedTask, enteredDueDate);
        tasks.add(d);
    }
}
//...

或更紧凑的版本（完全相同）

//...
while ((line = br.readLine()) != null) {
    String[] values = line.split("\t");
    if (values.length >= 1 && values.length <= 2)
        tasks.add(new DueDate(values[0], values.length == 1 ? "" : values[1]));
}
//...

希望这会有所帮助（：