如何使用对象作为值构成HashMap，而对象又具有它自己的HashMap。 爪哇

Question

I have been working on this problem for quite some time now, searching around the internet to see if there was any answers, but to no luck unfortunately.

As the title says I am trying to make a HashMap which uses an Object (another class) as a value. Then, I want that Object , to hold another HashMap inside it self which also uses another Object as a value.

Here is how I would have done it:

BookAdmin.java

import java.util.HashMap;

public class BookAdmin
{
    private static HashMap<String, Customer> customer = new Customer<String, Customer>();

    public static void main(String [] args)
    {
        customer.put("James", new Customer("James"));
        customer.put("Peter", new Customer("Peter"));

        customer.get("James").newBook("Flying 101");

        customer.get("James").seeBooks();
    }
}

Customer.java

import java.util.HashMap;

public class Customer
{
    private static HashMap<String, Book> book = new Book<String, Book>();
    private static String name = "";

    public Customer (String nameIn)
    {
        name = nameIn;
    }

    public void newBook (String title)
    {
        book.put(title, new Book(title));
    }

    public void seeBooks()
    {
        for (String l: book.keySet())
        {
            System.out.println(book.get(l).toString());
        }
    }
}

Book.java

public class Book
{
    private static String title = "";

    public Book (String titleIn)
    {
        title = titleIn;
    }

    public String toString()
    {
         return title;
    }
}

So it would seem to be pretty alright, but if I check "Peter"'s books too, it shows that he too owns "Flying 101". It's like they share the same book hashmap.

Answer 1

The cause for that issue is that you maintain your map in a static context. This means the map is shared across all instances of a class, see also JLS 8.3.1.1. - static Fields

If a field is declared static, there exists exactly one incarnation of the field, no matter how many instances (possibly zero) of the class may eventually be created.

So when you say "It's like they share the same book hashmap.", that is because they actually do :)

Remove all the static modifiers from the posted code and you should be good to go