Java object contains list of other objects. how to create object with empty list instaed of null-CodePudding

I have below code sample, if i create Cart object , im getting Items list as NULL.

@Data
class Cart{
    int cartId;
    List<Item> items;

}

@Data
class Item{
    int itemId;
    String itemName;
}


public class Test {

    public static void main(String[] args) {
        Cart cart = new Cart();
        System.out.println(cart); // Cart(cartId=0, items=null)
    }
}

How can i create Cart object with Items as empty list {}, instead of NULL.

If my class has 10 no.of List Items, How can we create Cart Object with empty list.

Can we create with ObjectMapper or any other API ?

CodePudding user response：

Override public no argument constructor to initialize the variable

Public Cart(){     
  this.items=new ArrayList<>();
}

or have a default value

@Data
class Cart{
    int cartId;
    List<Item> items=new ArrayList<>();;
}

CodePudding user response：

You have 2 options:

new Cart("id", new ArrayList<>()); // items is mutable
new Cart("id", List.of()); // items is immutable

or set default values:

@Data
class Cart{
    int cartId = 0;  // optional, because int is 0 by default  
    List<Item> items = new ArrayList<>();
}

You use Lombok. In case you have such questions, I reccomend you not to use it for a while.