Java 数组搜索和排序未按预期工作

Question

我目前正在学习 Java 简介类(class)，其中我们目前正在讨论数组主题。我们有一个类实验室，假设创建一个由两个类(Passenger.java 和 Demo.java)组成的简单数组程序。该数组可以是任意大小的(最小 4)，我们可以将详细信息硬编码到几个元素中。我们应该创建两个按 ID 和姓名(名字 + 姓氏)搜索的方法，以及两个按 ID 和姓氏升序排序的方法(冒泡和插入)。

在 ID 和名称的搜索方法中，当我输入数组中不存在的 ID 时，出现 java.lang.NullPointerException 错误。如果没有找到，它应该返回我编写的错误消息。

在我的 ID 排序方法中，我还在第二个循环后收到 java.lang.NullPointerException 错误。对于我的姓氏排序方法，我不知道如何实现。

因此我可能知道如何使用完整的代码示例来解决这两个问题。

注意:

• 我无法使用 ArrayList 和任何 Java Arrays 类
• 请将数组的元素 3 到 8 视为使用值进行初始化
• 我查看过这个 initialize all variables but still get NullPointerExceptions error 、这个 What is a NullPointerException, and how do I fix it? 、这个 Array Sorting and Searching Java 以及最后这个 Initialized elements getting NullPointerException error

Passenger.java

import java.util.Scanner;

public class Passenger {

    private String title;
    private String firstName;
    private String lastName;
    private long id;

    public Passenger() {

    }

    public Passenger(String title, String firstName, String lastName, long id) {
        this.title = title;
        this.firstName = firstName;
        this.lastName = lastName;
        this.id = id;
    }

    public void enterDetails() {
        Scanner keyboard = new Scanner(System.in);
        System.out.print("Enter your title: ");
        title = keyboard.next();
        System.out.print("Enter your first name: ");
        firstName = keyboard.next();
        System.out.print("Enter your last name: ");
        lastName = keyboard.next();
        System.out.print("Enter your ID: ");
        id = keyboard.nextLong();
    }

    public void outputDetails() {
        System.out.print();
        System.out.print("Passenger name: " + title + " " + firstName + " " + lastName);
        System.out.print("Identify Number: " + id);
        System.out.print();
    }

    public String getFirstName() {
        return firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public long getId() {
        return id;
    }

}

演示.java

import java.util.Scanner;

public class Demo {

    public static Passenger[] passengers = new Passenger[10];
    public static Scanner kb = new Scanner(System.in);

    public static void main(String[] args) {

        int option = 0;

        passengers[0] = new Passenger("Mr", "Benjamin", "Parker", 12345678);
        passengers[1] = new Passenger("Mrs", "Mary", "Long", 96421368);
        passengers[2] = new Passenger(....);
        passengers[3] = new Passenger(....);
        passengers[4] = new Passenger(....);
        passengers[5] = new Passenger(....);
        passengers[6] = new Passenger(....);
        passengers[7] = new Passenger(....);

        while(option != 7) {
            System.out.println("Please select an option:"
                + "\n1) Enter passenger particulars to array"
                + "\n2) Display passenger particulars from array"
                + "\n3) Search passenger particulars using ID"
                + "\n4) Search passenger particulars using both surname and name"
                + "\n5) Display passenger particulars in ascending order of ID"
                + "\n6) Display passenger particulars in ascending order of surname"
                + "\n7) Quit
            )
            option = kb.nextInt();

            switch(option) {
                case 1:
                    inputDetails();
                    break;
                case 2:
                    outputDetails();
                    break;
                case 3:
                    searchDetailsID();
                    break;
                case 4:
                    searchDetailsName();
                    break;
                case 5:
                    sortDetailsID();
                    break;
                case 6:
                    sortDetailsName();
                    break;
                case 7:
                    System.out.println("You quitted the program!");
                    break;
                default:
                    System.out.println("Invalid input!");
                    break;
            }
        }
    }

    public static void inputDetails() {
        int element = 0;
        for (int i = 0; i < passengers.length; i++) {
            if(passengers[i] != null)
                element++;
        }
        passengers[element] = new Passenger();
        passengers[element].enterDetails();
        element++;
    }

    public static void outputDetails() {
        for(int i = 0; i < passengers.length; i++) {
            passengers[i].outputDetails();
        }
    }

    public static void searchDetailsID() {
        boolean isFound = false;
        System.out.print("Please enter an passenger id: ");
        long id = kb.nextLong();

        for(int i = 0; i < passengers.length; i++) {
            if(id == passengers[i].getid()) {
                passengers[i].outputDetails();
                isFound = true;
                break;
            }
        }

        if(!isFound) {
            System.out.println("No passenger with that id!");
        }
    }

    public static void searchDetailsName() {
        boolean isFound = false;
        System.out.print("Please enter an passenger first name: ");
        String firstName = kb.next();
        System.out.print("Please enter an passenger last name: ");
        String lastName = kb.next();

        for(int i = 0; i < passengers.length; i++) {
            if(firstName.equalsIgnoreCase(passengers[i].getFirstName()) && lastName.equalsIgnoreCase(passengers[i].getLastName())) {
                passengers[i].outputDetails();
                isFound = true;
                break;
            }
        }

        if(!isFound) {
            System.out.println("No passenger with that name!");
        }
    }

    public static void sortDetailsID() {
        long temp = 0;
        for(int i = 0; i < passengers.length; i++) {
            for(int j = 0; j < passengers.length - 1; j++) {
                if(passengers[i].getId() > passengers[j].getId()) {
                    temp = passengers[i].getId();
                    passengers[i] = passengers[j];
                }
            }
        }
    }

    public static void sortDetailsName() {

    }

}

Answer 1

您的 NPE 堆栈跟踪是:

Exception in thread "main" java.lang.NullPointerException
    at com.Demo.searchDetailsID(Demo.java:90)
    at com.Demo.main(Demo.java:46)

如果你深入评估你的代码，你会发现你初始化了 passengers具有 10 的数组，但您仅在 main 中实例化 8 索引方法Demo类(class)。换句话说，当循环想要从数组中读取 9 或 10 索引时，会抛出 NPE。 因此，如果您替换 Demo 的第一行类包含以下行，您的代码将正常工作:

public static Passenger[] passengers = new Passenger[8];

解决方案 2:

当然，您可以使用 if ，如下所示:

public static void searchDetailsID() {
        boolean isFound = false;
        System.out.print("Please enter an passenger id: ");
        long id = kb.nextLong();

        for(int i = 0; i < passengers.length; i++) {
            if(passengers[i] != null /* I append this check*/ && id == passengers[i].getId()) {
                passengers[i].outputDetails();
                isFound = true;
                break;
            }
        }

        if(!isFound) {
            System.out.println("No passenger with that id!");
        }
    }