如何将ArrayLists添加到ArrayList？

Question

So, I have already created an ArrayList<>, say Staff List.

private List<Staff> staffs = new ArrayList<Staff>();

public StaffFacade() {
    staffs.add(new Staff("1", "James"));
    staffs.add(new Staff("2", "Mike"));
    staffs.add(new Staff("3", "Lina"));
    staffs.add(new Staff("4", "John"));
}

public List<Staff> getStaffs() {
    return staffs;
}

And I want to create another List that contains Staff List (by adding), so that I don't have to add the same Staff in StaffFacade?

I already created this BorrowFacade :

private List<Borrow> borrows = new ArrayList<Borrow>();

public BorrowFacade() {
    borrows.add(new Borrow()) //How do I add this?
}

public List<Borrow> getBorrows() {
    return borrows;
}

Referring to my question above, I don't know how to add new Staff List that has already been created.

This is the constructor for the Borrow List:

public Borrow(Date dueDate, Staff staff, Book books) {
    this.dueDate = dueDate;
    this.staff = staff;
    this.books = books;
}

Of course, I put Date there because I wanted to add Date inside the List too.

MAJOR EDIT

Okay so let me try to put it this way. I have 4 classes which is StaffFacade, BorrowFacade, Borrow and Staff .

This is what I wrote inside StaffFacade :

public class StaffFacade {
    private List<Staff> staffs = new ArrayList<Staff>();

    public StaffFacade() {
staffs.add(new Staff("1", "James"));
staffs.add(new Staff("2", "Mike"));
staffs.add(new Staff("3", "Lina"));
staffs.add(new Staff("4", "John"));
    }

    public List<Staff> getStaffs() {
        return staffs;
    }
}

BorrowFacade :

public class BorrowFacade {
    private List<Borrow> borrows = new ArrayList<Borrow>();

    public BorrowFacade() {
        borrows.add(staffsList);
    }

    public List<Borrow> getBorrows() {
        return borrows;
    }
}

Borrow (parts of it, the rest are just setters and getters)

public class Borrow {
    String id;
    Date dueDate;
    Staff staff;
    Book books;

    public Borrow(String id, Date dueDate, Staff staff, Book books) {
        this.id = id;
        this.dueDate = dueDate;
        this.staff = staff;
        this.books = books;
    }

Staff :

public class Staff{
    String id, name;

    public Staff(String id, String name) {
        this.id = id;
        this.name = name;
    }

The problem is in BorrowFacade. I don't know how to add List that has been created in StaffFacade into BorrowFacade's List which is List<Borrow> borrows;

I'm very sorry for the confusion. If anything please ask me. I really want this program to work.

Answer 1

If you have a collection you can addAll(someOtherCollection); but I am not sure I fully understand your question: you refer to a 'constructor for the Borrow List' but you show a constructor for a Borrow class which is not a list.

You seem to be mixing up an instance of an individual class (eg Book ) with a collection or plurality of that class: Book books (why is it plural? What are you trying to express?)

Edit:

Based on you comment, I think you're trying to understand how to construct the Borrow objects to be placed in the list.

The difference between constructed the Staff List is that you 'know' the staff ahead of time - albeit these are canned values.

The Borrow object seems to express a particular person borrowing a particular book due back on a certain date. If so, you need to have these details somewhere, for example from a database. The reason you're having trouble is you're trying to construct these objects in your Facade instead of just encapsulating ones that already exist.

public class Facade {
    private List<Borrow> borrows = new ArrayList<Borrow>();

    // Pass the items in to the constructor
    public Facade(List<Borrow> borrows) {
        this.borrows.addAll(borrows);
    }

    // You could call this externally in a loop to populate one by one
    public void addBorrow(Borrow borrow) {
        borrows.add(borrow);
    }

}

To restate: your Staff and your Borrow objects have to come from somewhere, so if they're already in a collection, use addAll , if not, just iterate the list and call add . Don't construct the objects in your Facades.

Edit 2:

In response to your amended question, you can't do this. You're trying to add a list of a particulr object (Staff) in to a list of another type of object (Borrow). This is just inherently wrong. I don't quite know how else to say it. If you asked me to give you a List of my favourite Stack Overflow questions, would you expect to find my favourite Stack Overflow user in that list? This is the fundamental nature of type safety. Now, if you asked me to give you a list of my Favourite Things then it is perfectly reasonable to expect to find various types of things in there - Stack Overflow Questions, Wines, Foods, etc. because they would conceptually share a common Favourite parent class or interface.

To be frank, I think you neeed to (re-)read up on the basic nature of Java generics and type safety, but in pursuit of the almighty reputation, here goes:

Note: I'm using StaffMember and BorrowedItem as names here to try to illustrate the value of good naming conventions.

You seem to want a Facade class for reasons none of us understand. Okay, we can accept that. Your Facade class seems to contain a list of objects. You have created multiple classes to accomplish this, with no discernable difference between the two except which objects are listed inside. Generics to the rescue:

public class Facade<T> {

    private List<T> list = new ArrayList<T>();

    public Facade(List<T> existingList) {
        list.addAll(existingList);
    }

}

This facade holds a list of objects, meaning you can do this:

List<StaffMember> staffMembers= new ArrayList<StaffMember>();
// .. assume list is populated here
Facade<StaffMember> staffMembersFacade = new Facade<StaffMember>(staffMembers);

Likewise, with the same facade class:

List<BorrowedItem> borrowedItems = new ArrayList<BorrowedItem>();
// ... populate borrowed items
Facade<BorrowedItem> borrowedItemsFacade = new Facade<BorrowedItem<(borrowedItems);

But you aren't adding StaffMember objects to the borrowedItemsFacade . At least not directly - in your example a BorrowedItem has a Date and it also points to which StaffMember borrowed it.

So at this point you have two lists - a list of StaffMember s, and a list of BorrowedItem s but you really have to ask yourself what purpose does this serve? Doesn't it make more sense for a single StaffMember to have a List<BorrowedItem> to keep track of all the items they borrowed?

class BorrowedItem {
    Date dueDate;
    StaffMember borrower;
}

class StaffMember {
    String name;
    List<BorrowedItem> borrowedItems;
}

Now this provides the opportunity to add a function to the StaffMember like this:

List<BorrowedItem> getOverdueItems() {
    List<BorrowedItem> overdueItems = new ArrayList<BorrowedItem>();

    Date today = getTodaysDate(); // Calendar.getInstance etc.

    for (BorrowedItem borrowedItem : borrowedItems) {
        Date dueDate = borrowedItem.getDueDate();
        if (today.after(dueDate)) {
            overdueItems.add(borrowedItem);
        }
    }

    return overdueItems;
}

Do you see how you need to create meaningful relationships between these classes in order for there to be anything useful to happen?

Then you can add functions to let someone lend an item to another person, or take something from someone, etc.

So yeah, Collections.addAll is what you're looking for, I think.