如何在内存中存储arrayList的不同对象并稍后检索它们？

Question

I am new to Java, and I am stuck at a place here. I have a student class that has some variables like name, contact no, etc. I have created a method addStuDetails to store the details of student in main class by taking values of these variables from user using scanner.

public class Student extends Person {

private int rollNumber, marks;

public Student(String name, String add, int contactNo, int roll, int mark) {
    super(name, add, contactNo);
    this.rollNumber = roll;
    this.marks = mark;
}

public String toString() {
    return "Name: " + Name + " ,Address: " + Address + " ,Contact no:" + ContactNumber + " ,Roll no.: " + rollNumber
            + " ,Marks :" + marks;

}

public void addStuDetails(Student s) {
    List<Student> stu = new ArrayList<Student>();
    Student sObj = new Student(Name, Address, ContactNumber, rollNumber, marks);
    stu.add(sObj);
    System.out.println(stu);
}

}

Following in the main class where the user enters the values:

public class Main {

 public static void main(String[] args) {

    Scanner input = new Scanner(System.in);
    System.out.println("Enter the details of student: ");

    System.out.print("Enter name: ");
    String name = input.nextLine();
    System.out.println("Enter address: ");
    String address = input.nextLine();
    System.out.println("Enter contact no: ");
    int no = input.nextInt();
    System.out.println("Enter rollno: ");
    int roll = input.nextInt();
    System.out.println("Enter marks");
    int marks = input.nextInt();
    Student s = new Student(name,address,no,roll, marks);
    s.addStuDetails(s);     
}

The details are added in the above created arrayList and gets printed. Now, I want to store all the students that user created in the mail class in the memory and then retrieve the same using roll no. Can anyone please help on how to do the same? Please note that I don't want to store these students in ant database or file, only in the memory.

Answer 1

It doesn't make sense for Student to contain a List of students, it's beyond it's scope of responsibility.

Instead, List should be independent of it, for example...

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

public class Main {

    public static void main(String[] args) {
        new Main();
    }

    private List<Student> students = new ArrayList<Student>();

    public Main() {

        Scanner input = new Scanner(System.in);
        System.out.println("Enter the details of student: ");

        System.out.print("Enter name: ");
        String name = input.nextLine();
        System.out.println("Enter address: ");
        String address = input.nextLine();
        System.out.println("Enter contact no: ");
        int no = input.nextInt();
        System.out.println("Enter rollno: ");
        int roll = input.nextInt();
        System.out.println("Enter marks");
        int marks = input.nextInt();
        Student s = new Student(name, address, no, roll, marks);
        students.add(s);
    }
}

nb: I have a personal dislike for static , as it tends to lead new developers to bad habits 😝

Equally, if you wanted to encapsulate the "student" functionality, you could create a student "manager" class, which maintained the list internally and exposed functionality that could be performed upon it, but that might be beyond the scope of this question