如何将元素添加到数组列表中？

Question

public class Student {
    private String name;
    private String id;
    private String email;
    private  ArrayList<Student> s;

    public Student( String n, String i, String e)
    {
        n = name; i= id; e = email;
    }
}


public class Library {

    private  ArrayList<Student> s;

    public void addStudent(Student a)
    {
        s.add(a);
    }


    public static void main(String[] args)
    {
        Student g = new Student("John Elway", "je223", "j@gmail.com");
        Student f = new Student("Emily Harris", "emmy65", "e@yahoo.com");
        Student t = new Student("Sam Knight", "joookok", "fgdfgd@yahoo.com");

        s.addStudent(g);
        s.addStudent(f);
        s.addStudent(t);
    }

}

It seems like that my student objects would be added to the Student s arraylist, but it is not working that way. Is it not working due to the arraylist being in the library class instead of the Student class?

Answer 1

Shouldn't be your constructor like this?

public Student( String n, String i, String e)
{
    name = n; id = i; email = e;
}

Answer 2

There are a couple of problems with your code:

1. main is a static method, meaning that it executes outside the context of any instance of Library . However, s is an instance field of Library . You should either make s a static field or create an instance of Library and reference the field through that instance:

    public static void main(String[] args) { Library lib = new Library(); . . . lib.addStudent(g); // etc. } 

2. addStudent is not a member function of ArrayList ; it is a member function of Library . Thus, you should not be coding s.addStudent(f); , etc.

3. You don't initialize s , so the first time your code tries to add an element, you will get a NullPointerException . You should initialize it either inline:

    private ArrayList<Student> s = new ArrayList<Student>(); 

   or write a constructor for Library and initialize the field there.

4. Your latest change—to add private ArrayList<Student> s; to the Student class—is on the wrong track. You will end up with a separate list of students for each student you create; surely not what you want! The list of students belongs with the Library where it was.

5. Your constructor for Student looks like it has the assignments backwards.

Answer 3

You're trying to add directly to an instance ArrayList from a static method, something that you can't do, and more importantly, someething that you shouldn't do. You need to first create a Library instance within your main method before you can call methods on it.

Library myLibrary = new Library();
myLibrary.add(new Student("John Elway", "je223", "j@gmail.com"));
// ... etc...

Answer 4

public class Library {

private  ArrayList<Student> s = new ArrayList<Student>(); //you forgot to create ArrayList

public void addStudent(Student a)
{
    s.add(a);
}


public static void main(String[] args)
{
    Student g = new Student("John Elway", "je223", "j@gmail.com");
    Student f = new Student("Emily Harris", "emmy65", "e@yahoo.com");
    Student t = new Student("Sam Knight", "joookok", "fgdfgd@yahoo.com");
    Library  library = new Library ();
    library.addStudent(g);
    library.addStudent(f);
    library.addStudent(t);
}

and change your constructor like this

public Student( String n, String i, String e)
{
        this.name = n;
        this.id = i; 
        this.email = e;
}