Void method returns items to the list, but when I try to use this list, it is empty!

Question

 listaPedidoComprador.setLayoutManager(new LinearLayoutManager(this));
    listaPedidoComprador.setHasFixedSize(true);
    adapterPedidoComprador = new AdapterPedidoComprador(itensCarrinho2);
    System.out.println(String.format("quantidade que está no itensCarrinho pro adapter:  %d ", itensCarrinho.size()));
    listaPedidoComprador.setAdapter(adapterPedidoComprador);
    adapterPedidoComprador.setOnItemClickListener(new AdapterPedidoComprador.OnItemClickListener() {
        @Override
        public void onAddClick(int position) {
            ItemPedido itemPedido = itensCarrinho2.get(position);
            Toast.makeText(CompradorPedidos.this, itemPedido.getNomeProduto(), Toast.LENGTH_SHORT).show();
        }
    });
    recuperarItemPedidoJamilton();

    System.out.println(String.format("pedidos recuperados: %d", produtos.size()));
    System.out.println(String.format("quantidade que está no carrinho:  %d nesse exato momento fora do método", itensCarrinho.size()));
    System.out.println(String.format("quantidade que está no carrinho2:  %d nesse exato momento fora do método", itensCarrinho2.size()));
    System.out.println("eae bb");

}

@Override
protected void onStart() {
    super.onStart();
    //recuperarItemPedidoJamilton();
}

private void recuperarItemPedidoJamilton() {
    DatabaseReference produtoref = firebaseRef
            .child("pedidos_usuario")
            .child(idUsuarioLogado);

    produtoref.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            if (dataSnapshot.getValue() != null) {
                pedidoRecuperado = dataSnapshot.getValue(Pedido.class);
                itensCarrinho2 = pedidoRecuperado.getItens();
                adapterPedidoComprador.notifyDataSetChanged();
                System.out.println(String.format("quantidade que está no carrinhoJamilton:  %d nesse exato momento", itensCarrinho2.size()));
            } else {
                //Toast.makeText(CompradorPedidos.this, "Seu carrinho está vazio", Toast.LENGTH_SHORT).show();
            }
        }

        @Override
        public void onCancelled(@NonNull DatabaseError databaseError) {
            // Error
        }

    });
    System.out.println(String.format("quantidade que está no carrinhoJamiltonfora:  %d nesse exato momento", itensCarrinho.size()));

}

As you can see, I’m trying to put this list on the Adapter, but it’s not right because this list is apparently empty. I do not understand what is happening, since I put a println inside the method and it is returning that there are items inside the list!!

I have tried to leave itensCarrinho2 (list I am trying to fill) static, but I can’t work well so there are cases where it is necessary to delete the list, because it is a shopping cart, and when confirming or removing an item from the cart, cannot be removed from the list that is static... I suppose...

Answer 1

I get it now.

Well, understand that itensCarrinhos2 is a reference to a list object, not the object itself. You passed this reference to Adapter and it does not expect it to be changed, just to change the content of the object itself (i.e., the contents of the list).

So when you do itensCarrinho2 = pedidoRecuperado.getItens(); you point this variable to another object, in the case to what is being returned by getItens(), only when it calls notifyDataSetChanged() the Adapter does not know this and checks the object pointed by the reference passed to it previously, which has not changed.

Remember that in Java the arguments are always passed "by value", that is, a copy of the argument is being passed, in this case the reference to the list, and Adapter starts to store a copy of this reference internally. While the two are pointing to the same great object, everything works, but you changed it in your code when you did itensCarrinho2 point to another object.

The solution therefore is to either change the content of the list by the very reference the Adapter already knows:

itensCarrinho2.clear();
itensCarrinho2.addAll(pedidoRecuperado.getItens());
adapterPedidoComprador.notifyDataSetChanged();

...or else look for a method in the Adapter class/base interface that allows updating the reference that was passed to the Adapter, a setDataSet() of life or something, I don’t remember if you have a.