How do I create an Arraylist of objects with each position containing a new object?

Question

public class Produto {

       String marca;
       int quant;

       Produto prod = new Produto();
       ArrayList <Produto> lista_de_produtos = new ArrayList<>(); 
       lista_de_produtos.add(new Produto());

       //É dessa forma?

}

Answer 1

You should not do so. The code compiles and executes, but this form is considered wrong.

The class calls Produto, So what do you expect to have in it? A product, right? Why do you find a list of products in that object? Even worse, there will be a different list in each product. This doesn’t make any sense. If at least the list were static, and then belonging to the class and not to the object, it would even have a partial sense, but it would still be conceptually wrong because it should be in another class of stock control and not the product.

Conceptually, there should be another class that handles the list of products. Nor will I get into the merits of the application to be able to run in a concurrent environment and this would be a problem, I will consider it a non-existent possibility.

Another problem of this class is that the constructor does not accept parameters, i.e., it works as a builder in fact. This is another conceptual error. I also didn’t talk about encapsulation and abstraction, because this is even more confusing and there are controversies, and I’m not even saying that I should.

Answer 2

Following your question and the comments between you and the @Maniero, I will leave my answer below, but I make it clear that you must pay attention to the following @Maniero:

• to) Read the answer carefully.
• b) Carefully read the comments in your answer to your own question.

Well, come on... how come you didn’t detail your question, like for example: "What you really want?", "What are the business rules?" among others, I will leave below the explanations of concepts and how they should be used in the correct way.

What is an Arraylist?

The Class ArrayList is a scalable matrix , which can be found in the package java.util. The difference between a internal matrix and a ArrayList in Java is the following:

• The size of an array cannot be modified (if you want to add or remove elements from/to an array, you will need to create a new one).

• While in a ArrayList elements can be added and removed from one whenever you want.

• The syntax between a ArrayList and a internal matrix is also a little different

How to create an Arraylist?

You must import the library (package) from it and after this instantiate the object, inserting its type and its name, for example:

// importa a biblioteca (pacote) do ArrayList
import java.util.ArrayList;

// Cria um Objeto de ArrayList chamado cars e do tipo de armazenamento String
ArrayList<String> cars = new ArrayList<String>(); 

// Cria um Objeto de ArrayList chamado cars e do tipo de armazenamento Car
Car carObj = new Car();
ArrayList<Car> cars = new ArrayList<Car>();

How to store data in an Arraylist?

ArrayList<String> cars = new ArrayList<String>();
    cars.add("Volvo");
    cars.add("BMW");
    cars.add("Ford");
    cars.add("Mazda");

How to access the index of an Arraylist?

cars.get(0);
cars.get(1);
cars.get(2);

Now giving example using your question, using a simple situation and only for knowledge gains, you should do the following:

• Creating a Class for Products
• Creating a Class for Product Lists

Listaproduct.class

import java.util.ArrayList;

public class ListaProduto {
    private ArrayList<Produto> arrProd;
    
    public ListaProduto() {
        this.arrProd = new ArrayList<Produto>();
    }
    
    public void addProd(Produto prod) {
        this.arrProd.add(prod);
    }
    
    public void getAllProd() {
        for(int i = 0; i < this.arrProd.size(); i++) {
            System.out.println("\nREF DO OBJ: " + this.arrProd.get(i) + " -- "  + "MARCA: " + this.arrProd.get(i).getMarca() + " -- QUANTIDADE: " + this.arrProd.get(i).getQuantidade());
        }
    }

    
}

Java product.

public class Produto {
    private String marca;
    private int quantidade;
    
    public Produto(String marca, int quantidade) {
        this.marca = marca;
        this.quantidade = quantidade;
    }
    
    public String getMarca() {
        return this.marca;
    }
    
    public int getQuantidade() {
        return this.quantidade;
    }

}

Main java.

public class Main {
    public static void main(String[] args) {
        ListaProduto lp = new ListaProduto();
        
        for(int i = 0; i < 4; i++) {
            lp.addProd(new Produto("PRODUTO " + i, i));
        }
        
        lp.getAllProd();
    }

}

OUTPUT:

REF DA CLASS: Produto@15db9742 -- MARCA: PRODUTO 0 -- QUANTIDADE: 0
REF DA CLASS: Produto@6d06d69c -- MARCA: PRODUTO 1 -- QUANTIDADE: 1
REF DA CLASS: Produto@7852e922 -- MARCA: PRODUTO 2 -- QUANTIDADE: 2
REF DA CLASS: Produto@4e25154f -- MARCA: PRODUTO 3 -- QUANTIDADE: 3

NOTE:

The above code was given for the purpose of gaining knowledge, there are several other ways and mainly best and best practices to do the same.

OFFICIAL SOURCE - W3SCHOOLS

Answer 3

That’s basically how it is. It’s important to remember the context of your application. More generally this is the code I used to validate.

import java.util.ArrayList;

public class Produto {

    public Produto(){

    }

    public static void main(String[] args) {

     String marca;
     int quant;

     Produto prod = new Produto();
     ArrayList<Produto> lista_de_produtos = new ArrayList<>(); 
     lista_de_produtos.add(new Produto());
     lista_de_produtos.add(new Produto());

    }

}