Firebase object to Java class

Question

I am trying to read from relatime database the following object:

.. to a this Java Category.class:

public class Category {
    private String nombre;
    private String descripcion;
    private String estado;
    private List<String> productos;
    private List<Page> paginas;

    public Category(){}

    public Category(String name) {
        this.nombre = "";
        this.descripcion = "";
        this.estado = "";
        productos = new ArrayList<>();
        paginas = new ArrayList<>();
    }

    public String getCategoryName() {
        return nombre;
    }

    public void setCategoryName(String nombre) {
        this.nombre = nombre;
    }

    public String getCategoryDescription() {
        return descripcion;
    }

    public void setCategoryDescription(String descripcion) {
        this.descripcion = descripcion;
    }

    public String getCategoryState() {
        return estado;
    }

    public void setCategoryState(String estado) {
        this.estado = estado;
    }

    public List<String> getProducts() {
        return this.productos;
    }

    public void setProducts(List<String> productos) {
        this.productos = productos;
    }

    public List<Page> getPages() {
        return this.paginas;
    }

    public void setPages(List<Page> pages) {
        this.paginas = paginas;
    }

    @Override
    public String toString() {
        return nombre;
    }
}

I have searched and read from different sources and from what I understand I should use Maps, but the truth is, I don't know how to do it ...

This is the code of the reading, and if I debug it, I can see that the dataSnaphot has everything I need, but the Category c is null:

FirebaseDatabase.getInstance().getReference().child("categorias").addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
            Category c = snapshot.getValue(Category.class);
            list.add(c);
        }
        AdapterCategories adapter = new AdapterCategories(list);
        recycleView.setAdapter(adapter);
    }

    @Override
    public void onCancelled(DatabaseError error) {
        // Failed to read value
        Toast.makeText(getContext(), error.getMessage(), Toast.LENGTH_LONG).show();
    }
});

This is the JSON exported from FIREBASE:

"categorias" : {
    "Telas de Hogar" : {
      "descripcion" : "",
      "estado" : "invisible",
      "nombre" : "Telas de Hogar",
      "paginas" : {
        "Loneta" : {
          "descripcion" : "",
          "estado" : "invisible",
          "nombre" : "Loneta"
        }
      }
    }
  }

Please could anybody help me to solve this problem.

Answer 1

You can store both a List and a Map in the database, but they result in a different JSON structure.

---

If you store an object with List<String> products in it, it will end up like this in the database:

"products": {
  "0": "Product one",
  "1": "Product two",
  "2": "Product three"
}

You'll notice that the keys that Firebase generates are sequential numbers, stored as string value (because all keys are stored as strings). This is how Firebase represents a List or array in the database.

So if you read a JSON like this from the database, it maps to a List<String>.

---

If you have a structure with keys that are no sequential numbers, for example keys that are generated by calling push(), it may look like this:

"paragraphs": {
  "-M...1": "He was an old man who fished alone in a skiff in the Gulf Stream...",
  "-M...2": "The old man was thin and gaunt with deep wrinkles in...",
  "-M...3": "Everything about him was old except his..."
}

This JSON structure maps to a Map<String, String> in Java, since the keys can not be converted to sequential numbers.