如何通过主类调用另一个类的方法？

Question

抱歉标题不佳，但我不知道如何表达我的问题。在将一些学生放入 ArrayList 之后，我试图从我的 BubbleSorter 类中的 Student 类调用 getSSN 方法。如何调用特定的 SSN 变量。我理解为什么会发生错误，但不知道如何更改它以使其工作。报错信息如下：SearchandSort类型的getSSN()方法未定义。

我的学生班级：

public class Student {

    private String firstName, lastName;
    private int SSN;

    public Student(String first, String last, int ssn) {

        firstName = first;
        lastName = last;
        SSN = ssn;
    }

    public String toString() {
        return lastName + ", " + firstName + " " + SSN + "\t";
    }

    public boolean equals(Object other) {

        return (lastName.equals(((Student)other).getLastName()) && firstName.equals(((Student)other).getFirstName()) && (SSN == (((Student)other).getSSN())));
    }

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName (String lastName) {
        this.lastName = lastName;
    }

    public int getSSN() { 
        return SSN;
    }

    public void setSSN(int SSN) {
        this.SSN = SSN;
    }
}

我的气泡排序器：

import java.lang.reflect.Array;

// “On my honor, I have neither received nor given any unauthorized assistance on this examination.”
public class BubbleSorter {

    public static void bubbleSort(SortandSearch Students[]) {

        int lastPos;
        int index;
        int temp;

        for (lastPos = Students.length-1; lastPos >= 0; lastPos--) {
            for (index = 0; index <= lastPos - 1; index++) {

                if (((Students[index]).getSSN()) - ((Students[index+1].getSSN())) > 0) {
                    temp = Students[index].getSSN();
                    Students[index].getSSN() = Students[index+1].getSSN();
                    Students[index+1].getSSN() = temp;
                }
            }
        }
    }
}

我的主要课程：

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

public class SortandSearch {

    public static void main(String[] args) {

        Student a = new Student("Ryan", "Jones", 123456);
        Student b = new Student("Jolie", "Decker", 123457);

        ArrayList<Student> Students = new ArrayList<Student>();

        Students.add(a);
        Students.add(b);

        System.out.println(Students);
        bubbleSort(Students);
        System.out.println(Students);

        System.out.println("Please enter the SSN of the student you are searching for:");
        Scanner scan = new Scanner(System.in);
        int searchKey = scan.nextInt();

        for (int index = 0; index < Students.size(); index++) {
            if (Students.get(index).getSSN() == searchKey) {
                System.out.print("The student with SSN " + searchKey + " is located at " + index);
            }
        }
    }
}

编辑：

错误现在显示：表达式的类型必须是数组类型，但它解析为 ArrayList。

新的 BubbleSorter：

import java.util.ArrayList;

// “On my honor, I have neither received nor given any unauthorized assistance on this examination.”
public class BubbleSorter {

    public static void bubbleSort(ArrayList<Student> studentList) {

        int lastPos;
        int index;
        int temp;

        for (lastPos = studentList.size()-1; lastPos >= 0; lastPos--) {
            for (index = 0; index <= lastPos - 1; index++) {

                if ((studentList.get(index).getSSN()) - ((studentList.get(index).getSSN())) > 0) {
                    temp = studentList[index];
                    studentList[index] = studentList[index+1];
                    studentList[index+1] = temp;
                }
            }
        }
    }
}

Answer 1

这段代码似乎有很多问题。我认为您正在尝试将您的数组列表不必要地转换为其他内容

首先，我认为您的函数签名应该更改为

 public static void bubbleSort(ArrayList<Student> studentList) 

然后您可以通过以下方式访问元素：

studentList.get(index).getSSN()

Answer 2

在SortandSearch 中，您将Student 的实例放入ArrayList，并且您的bubbleSort 方法还必须采用这种类型的参数或至少采用List，然后将其转换为数组并返回排序列表：

public static List<Student> bubbleSort(List<Student> studentsList) {
    Student[] Students = studentsList.toArray(new Student[studentsList.size()]);
// ...
    return Arrays.asList(Students);
}

并且在调用 main 方法中你需要存储排序列表：

    List<Student> sorted = BubbleSorter.bubbleSort(Students);
    System.out.println(sorted);