Question

每次迭代我添加一个新的学生对象时，它会覆盖以前的对象并在整个列表中包含相同的对象。我尝试使用链接和数组列表，但继续遇到同样的问题。如果有人能够指引我朝着正确的方向前进，我将不胜感激。谢谢。

public static void main(String[] args) {

        // Scanner and LinkedList
        Scanner keyboard = new Scanner(System.in);
        LinkedList<Student> studentList = new LinkedList<Student>();
        ArrayList<Student> stdList = new ArrayList<Student>();

        int choice;
        boolean flag = true;
        do {
            Student.showMenu();
            choice = keyboard.nextInt();
            switch (choice) {
            case 1: {
                System.out.println("What is the students last name?");
                String last = keyboard.next();

                System.out.println("What is the students first name?");
                String first = keyboard.next();

                System.out.println("What is the students course code?");
                int courseCode = keyboard.nextInt();

                System.out.println("What is the students course grade?");
                String grade = keyboard.next();

                Student st = new Student(last, first, courseCode, grade);
                studentList.add(st);
                stdList.add(st);
                break;
            }
            case 2: {
                System.out
                        .println("Please enter the students last name you wish to delete.");
                String last = keyboard.next();
                int index = Student.indexOf(studentList, last);
                if (index != -1)
                    studentList.remove(index);
                else
                    System.out.println("Student does not exist.");
                break;
            }
            case 3: {
                System.out
                        .println("Please enter the students last name you wish to search.");
                String last = keyboard.next();
                int index = Student.indexOf(studentList, last);
                if (index != -1)
                    studentList.get(index).toString();
                else
                    System.out.println("Student does not exist.");
                break;
            }
            case 4: {
                System.out.println("");
                break;
            }
            case 5:
                System.out.println("");
                break;
            case 6:
                Student.displayList(studentList);
                break;
            case 7:
                flag = false;
                break;
            }
        } while (flag);

    }




public class Student {
    private static String last;
    private static String first;
    private static int courseCode;
    private static String grade;

public Student(String last, String first, int courseCode, String grade) {
    this.last = last;
    this.first = first;
    this.courseCode = courseCode;
    this.grade = grade;
}

public String getLast() {
    return last;
}

public void setLast(String last) {
    this.last = last;
}

public String getFirst() {
    return first;
}

public void setFirst(String first) {
    this.first = first;
}

public int getCourseCode() {
    return courseCode;
}

public void setCourseCode(int courseCode) {
    this.courseCode = courseCode;
}

public String getGrade() {
    return grade;
}

public void setGrade(String grade) {
    this.grade = grade;
}

public static void showMenu() {
    System.out.println("Welcome to the database menu!\n");
    System.out.println("Press 1 to insert a new record");
    System.out.println("Press 2 to delete a record");
    System.out.println("Press 3 to search the database (by last name)");
    System.out.println("Press 4 to print a range in the database");
    System.out.println("Press 5 to find the class average for a course");
    System.out.println("Press 6 to print the list");
    System.out.println("Press 7 to quit");
}

public String toString() {
    String str = " Last name: " + last + " First name: " + first
            + " Course code: " + courseCode + " Course grade: " + grade;
    return str;

}

static int indexOf(LinkedList<Student> list, String last) {
    int i = 0;
    for (Student std : list) {
        if (std.getLast().equals(last))
            return i;
        i++;
    }
    return -1;
}

static void displayList(LinkedList<Student> list) {
    for (int i = 0; i < list.size(); i++) {
        System.out.println(list.get(i).toString() + " ");

    }
}

static void displayList(ArrayList<Student> stdList) {
    for (int i = 0; i < stdList.size(); i++) {
        System.out.println(stdList.get(i).toString() + " ");

    }

   }

}

Answer 1

这是一个相当特殊的问题。我最好的建议是改变

Student st = new Student(last, first, courseCode, grade);
studentList.add(st);
stdList.add(st);

到

studentList.add(new Student(last, first, courseCode, grade));
stdList.add(new Student(last, first, courseCode, grade));

Answer 2

问题在于您的商家信息代码而不是添加方法，检查您的迭代代码，如果您使用常规for进行迭代

for（int i = 0; i

然后你必须提交经典的初学者错误，这个错误是每次迭代的第一项，而不是使用分配的变量，如

List.get(0)而不是List.get(i)

我建议您使用增强 a.k.a foreach来实现此类实例

Answer 3

问题是您已将class Student的私人成员定义为static。有任何理由将其标记为static。请删除静态，您的代码应该可以正常工作。

每次迭代循环时都会覆盖对象

3 个答案: