Ignore existing item in the LInkedList

I am trying to ignore the existing item that I added in the duplicate.

Normally, if the item does not exist it will eventually added the item to the
LinkedList
When I try to added item again, I just wanted to ignore the adding process and the increment the value by 1.
But the problem is it keep adding the items to the LinkedList.

Can someone explain to me?

class Item{
   Store store;
   String name;
   String code;
   String number;
   public Item(String name, String code){
     this.name = name;
     this.code = code;
     number = 0;
   }
   public boolean itemExists(String name, String code){
    return this.name.equals(name) && this.code.equals(code);
   }
   
   public void increment(){ number++; }
   
   @Override
   public String toString(){ return store.getName()+ " " + name + " " + code + " " +number; }
}

Items will be added to the factory.

class Factory{
  private LinkedList<Item> items = new LinkedList<Item>():
  private String name;
  private String number;
  
  public Factory(String name, String number){
    this.name = name;
    this.number = number;
  }
  public void getName(){
   return name;
  }
  
  public void addItem(String name, String code){
     items.add(new Item(this, name, code));
  }
  
  @Override
  public String toString(){ return name + " " + number; }

  public List<Item> getItems{
    return items;
  }
}

The factory then delivery to the store.

class Store{
  private LinkedList<Factory> factories = new LinkedList<>();

  public Store(){
     factories.add(new Factory("MayFlower", "01");
     factories.add(new Factory("SunFlower", "02");
     factories.get(0).addItem("GTA", "001A");
     factories.get(0).addItem("GTA", "002A");
     factories.get(0).addItem("GTA", "003A");
     
     factories.get(1).addItem("Sonic", "022A");
     factories.get(1).addItem("Sonic", "023B");
     factories.get(1).addItem("Sonic", "024C");
  
  }
  
  public List<Item> getItemFromFact(){
    List<Item> temp = new ArrayList<>();
    for(Factory factory: factories) 
        for(Item item: factory.getItems())
            temp.add(item);
    return temp;
  }

}

The customer buy items at the store.

class Customer{
  private LinkedList<Item> items = new LinkedList<>();
  
  public static void main(String args[]){
     new Customer.view();
  }

  private void view(){
    for(Item item: items)
       System.out.println(item);
  }
  
  private void adding(){
    String name = "GTA";
    String code = "001A":
    List<Item> item = item(name, code);
    if(!item.isEmpty()){
      items.add(item);
      item.increment(); // will increment the value;
    }   
    else{
     System.out.println("Item does not exists");
    }
  }
  
  private List<Item> item(String name, String code){
    List<item> temp = new ArrayList<>();
    List<item> fromStore = new Store().getItemFromFact();
    for(Item item: fromStore) 
        if(item.itemExists(name, code))
           temp.add(item)
    return temp;
  }
}

The main problem is in the item class under item(). If I try with the same item again, it will just add another it become like this.

MayFlower GTA 001A 1
MayFlower GTA 001A 1

The result should be

MayFlower GTA 001A 2

after I added another item.

I problem I have is that I don't know how to match the item from exisiting.
If someone know the solution.
That's would be very helpful thanks.

There are so many problems in your design and code. I've discussed some of them as given below:

1. I do not understand why you need a reference to Store in Item. An Item should not know which Store or Factory it is going to belong to.
2. I also didn't understand the purpose of the attribute, number in Store. An Item shouldn't know how many numbers of it is present in a Store or Factory. If you keep it for any reason, it should be of a numeric type (e.g. int, double etc.) so that you can perform arithmetic operations on it.
3. Instead of a LinkedList of Item objects in Factory, you should have a variable of type, Map<String, Integer> and you can call it stock. The key in this Map is the unique identifier of the item, which is the combination code and name as per your requirement and the value will be the available number/quantity of the Item. Given below is a minimal verifiable example of how you can maintain stock:

import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;
class Item {
String name;
String code;
public Item(String name, String code) {
this.name = name;
this.code = code;
}
public String getName() {
return name;
}
public String getCode() {
return code;
}
@Override
public String toString() {
return "Name: " + name + "Code :" + code;
}
}
class Factory {
private Map<String, Integer> stock = new HashMap<String, Integer>();
private String name;
public Factory(String name) {
this.name = name;
}
public void addItem(Item item) {
if (item != null) {
String key = item.getName() + ":" + item.getCode();
stock.put(key, stock.getOrDefault(key, 0) + 1);
}
}
public void showStock() {
for (Entry<String, Integer> entry : stock.entrySet()) {
System.out.println("Item = " + entry.getKey() + ", Available quantity = " + entry.getValue());
}
}
}
public class Main {
public static void main(String[] args) {
Factory factory = new Factory("MayFlower");
factory.addItem(new Item("GTA", "001A"));
factory.addItem(new Item("GTA", "001A"));
factory.addItem(new Item("GTA", "002A"));
factory.addItem(new Item("GTA", "003A"));
factory.addItem(new Item("GTA", "003A"));
factory.addItem(new Item("GTA", "003A"));
factory.showStock();
}
}

Output:

Item = GTA:002A, Available quantity = 1
Item = GTA:003A, Available quantity = 3
Item = GTA:001A, Available quantity = 2