How do I add a different object to a list whenever using the add?

Question

I’m having trouble adding a different object to a list of objects whenever I use her add, I know what the problem is but I don’t know how to fix it, follow the code List class

public class Lista {

    NodeLista inicio;
    NodeLista fim;

    public Lista (){
        inicio = null;
        fim = null;
    }

    public boolean isEmpty(){
        return inicio == null;
    }

    public int size(){
        if(isEmpty()){
            return 0;
        }
        NodeLista aux = inicio;
        int cont = 1;
        while(aux.proximo != inicio){
            cont++;
            aux = aux.proximo;
            if(aux == inicio){
                break;
            }
        }
        return cont;
    }

    public void add(NodeTree arvore){
        NodeLista novo = new NodeLista(arvore);
        if(isEmpty()){
            inicio = novo;
            fim = novo;
            inicio.proximo = null;
            fim.proximo = null;
        }else {
             fim.proximo = novo;
             fim = novo;
             fim.proximo = null;


        }
    }

    public boolean validarElemento(String elemento){
         NodeLista aux = inicio;
         for (int i = 0; i < size(); i++) {
             if(aux.arvore.elemento.equals(elemento)){
                 return true;
             }
             aux = aux.proximo;
         }
         return false;
     }
 }

Nodetree class

public class NodeTree {
    String elemento;
    Lista filhos;

    public NodeTree(String elemento){
        this.elemento = elemento;
        filhos = new Lista();
    }

}

Nodelist class

public class NodeLista {
    NodeTree arvore;
    NodeLista proximo;
    NodeLista anterior;

    public NodeLista(NodeTree arvore){
        proximo = null;
        anterior = null;
        this.arvore = arvore;
    }

}

Main

public class MainTree {


    public static void main(String[] args) {
        Lista lista = new Lista();
        System.out.println(lista.size());
        NodeTree arvore = new NodeTree("1");
        lista.add(arvore);
        NodeTree arvore2 = new NodeTree("2");
        lista.add(arvore2);
        arvore2.elemento = "3";
        System.out.println(lista.inicio.arvore.elemento);
        System.out.println(lista.inicio.proximo.arvore.elemento);
        System.out.println(lista.size());
    }

}

Console

0 1 3 2

Desired exit

0 1 2 2

Every time I add a nodetree obj to the list and change its element by the object itself, I am changing in the list tbm, there is some way not to change in the list?

Answer 1

As it is generating some problems, I will put the excerpt of the comment (edited) that possibly helped solve the problem:

I think if it can be supplemented, best for all.

When you did: tree 2.element = "3", you changed the attribute node element that was already inside the list, not just the node external. The list stores references for objects that are in Heap (logo: list.inicio == tree). In Java, if you pass an object like parameter, it is always the object reference and not the value, other than C that there is the possibility of passing by parameter and by value.

Reading recommendation: What are and where are the "stack" and "heap"