如何从arrayList中删除特定对象，以及如何检查其中是否包含该对象？

Question

I have an arrayList type shopping cart. When I add an new item into the shopping cart, I will check first if the shopping cart already has this item. But the cart.contains(item) method didn't work, it return false even there is a same item in the cart. The second problem is I was not able to remove this item object from the shopping cart arrayList. My code shows as below:

@Controller
@RequestMapping("/addTo.htm")
public class AddToController{
    @SuppressWarnings("unchecked")
    @RequestMapping(method=RequestMethod.GET)
    protected ModelAndView handleRequestInternal(HttpServletRequest request, HttpServletResponse response) throws Exception {
        HttpSession session = request.getSession();
        String action = request.getParameter("action");
        System.out.println(action);
        ModelAndView mv = new ModelAndView();
        ArrayList<Item> cart;
        if(session.getAttribute("cart") != null) {
            cart = (ArrayList<Item>) session.getAttribute("cart");
        }else {
            cart = new ArrayList<Item>();
        }
        if(action.equals("addToCart")) {
            long itemId = Long.parseLong(request.getParameter("itemId"));
            ItemDAO itemDao = new ItemDAO();
            Item item = itemDao.get(itemId);
            System.out.println("111"+cart.contains(item));
            if (!cart.contains(item)) {
                cart.add(item);
            }
            double total = 0;
            int count = 0;
            for (Item i : cart) {
                total = total + i.getPrice();
                count += 1;
            }
            session.setAttribute("cart", cart);
            mv.addObject("total", total);
            mv.addObject("count", count);
            mv.setViewName("User/viewCart");
        }
        if(action.equals("remove")){
            System.out.println("cart size is" + cart.size());
            Long itemId = Long.parseLong(request.getParameter("item"));
            ItemDAO itemDao= new ItemDAO();
            Item item = itemDao.get(itemId);
            System.out.println(cart.contains(item));
            session.setAttribute("cart", cart);
            System.out.println(cart.size());
        }
        return mv;
    }
}

Can anyone help me solve this problem? Thanks!!

Answer 1

You will need to add a .equals method to the Item class so ArrayLists know how to compare two different objects together. While we are at it we should add a hashCode method as well. This is mainly useful for Sets but always good to have it as a backup in case we need it.

We can use the .indexOf(Item) method to get the position of an object in the list. If the number returns if -1. Then it's not in the list. If it is 0 or greater then it's in there and we can use the index to remove the item.

public class Item{
  private String type;

  public Item(String type){
    this.type = type;
  }

  public String getType(){
    return type;
  }

  @Override
  public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + ((type == null) ? 0 : type.hashCode());
    return result;
  }

  @Override
  public boolean equals(Object obj) {
    if (this == obj)
      return true;
    if (obj == null)
      return false;
    if (!(obj instanceof Item))
      return false;
    Item other = (Item) obj;
    if (type == null) {
      if (other.type != null)
        return false;
    } else if (!type.equals(other.type))
      return false;
    return true;
  }
}

Now that we have a .equals and hashcode. We can now compare them in the ArrayList.

ArrayList<Item> itemList = new ArrayList<Item>();

// Fill the list
itemList.add(new Item("Banana"));
itemList.add(new Item("Toaster"));
itemList.add(new Item("Screw Driver"));

Item item = new Item("Hand Grenade");
itemList.add(item);

int index = itemList.indexOf(item);
if( index != -1 ){
  System.out.println("The item is in index " + index);

  // Remove the item and store it in a variable
  Item removedItem = itemList.remove(index);
  System.out.println("We removed " + removedItem.getType() + " from the list.");
}

Answer 2

您必须重写.equals项的.equals方法。默认情况下，Java通常将比较对象引用，而不是对象的值。