自动将构造的每个Class对象添加到ArrayList中

Question

I'm still fairly new in learning Java and I'm in need of a way to put every object that is constructed from a certain class into an ArrayList that I can access later.

Here is my Item class:

import java.util.ArrayList;
import java.util.*;
public class Item
{
    private String name;
    private int quantity;
    private ArrayList<Item> allItems = new ArrayList<Item>(); //the ArrayList I'm trying to fill with every Item object

    /**
     * Constructs an item of a given name and quantity
     * @param name Item name
     * @param quantity How much of an item the player has
     */
    public Item(String name, int quantity)
    {
        this.name = name;
        this.quantity = quantity;
    }

    /**
     * @return Item name
     */
    public String getItemName()
    {
        return name;
    }

    /**
     * @return Quantity of the item
     */
    public int getQuantity()
    {
        return quantity;
    }

    /**
     * @return A list of items that have a quantity > 0
     */
    public ArrayList<Item> getInventory()
    {
        ArrayList<Item> inventory = new ArrayList<Item>();
        for(Item i : allItems)
        {
            if(i.getQuantity() > 0)
                inventory.add(i);
        }
        return inventory;
    }
}

How can I add an Item object to allItems every time one is constructed?

Answer 1

First, the arraylis must be static so it is shared between all the instances. Otherwise, you would have a different variable per instance.

More info on instance/class members here .

private String name;
private int quantity;
private static ArrayList<Item> allItems = new ArrayList<Item>();

Then, you can add the created instance in the constructor, refering to it as 'this'.

About the 'this' keyword .

public Item(String name, int quantity)
{
    this.name = name;
    this.quantity = quantity;
    allItems.add(this);
}

Answer 2

You certainly want one list for all items, and not every item to carry around and maintain its own copy list of all items. When that's your intention, you should define the list as static :

private static ArrayList<Item> allItems = new ArrayList<Item>()

A static variable is shared by all instances of the class.

In the constructor of Item, just add this to the list of all items.

allItems.add(this);

Answer 3

One possible solution would be to declare the items list as static like this:

public static List<Item> allItems = new ArrayList<Item>();

afterwards you could access it using the following snippet:

Item.allItems // example would be System.out.println(Item.allItems)

Additionally within your constructor you will need the following code:

public Item(String name, int quantity) {
    this.name = name;
    this.quantity = quantity;
    this.allItems.add(this);
}

BUT use this approach with caution, since it will add to list every single item created, what could lead to a possible memory leak.

Answer 4

You need a static List and then you can use the class constructor to add this object into the list.

import java.util.ArrayList;
import java.util.*;
public class Item
{
    private String name;
    private int quantity;
    private static ArrayList<Item> allItems = new ArrayList<Item>(); //the ArrayList I'm trying to fill with every Item object

    /**
     * Constructs an item of a given name and quantity
     * @param name Item name
     * @param quantity How much of an item the player has
     */
    public Item(String name, int quantity)
    {
        this.name = name;
        this.quantity = quantity;
        allItems.add(this);
    }

    /**
     * @return Item name
     */
    public String getItemName()
    {
        return name;
    }

    /**
     * @return Quantity of the item
     */
    public int getQuantity()
    {
        return quantity;
    }

    /**
     * @return A list of items that have a quantity > 0
     */
    public static ArrayList<Item> getInventory()
    {
        ArrayList<Item> inventory = new ArrayList<Item>();
        for(Item i : allItems)
        {
            if(i.getQuantity() > 0)
                inventory.add(i);
        }
        return inventory;
    }
}