Problems with a Linked List object in Java

I'm learning from the Caelum workbooks data structures, and I started to see Liagada Lists, I followed all the code that it offers, step by step and I understood the logic of the list chained, but what I did not understand was this line of code:

System.out.println(lista); // lista é um objeto to tipo ListaLigada

That has output the array of strings:

  

[Paulo, Rafael]

I do not understand, because in my code the output that it gives is only the name of the class + the hascode of the object. Someone explain to me what happened ??? All the code to avoid bureaucracy is there:

public class Celula {

  private Celula proxima;

  private Object elemento;

  public Celula(Celula proxima, Object elemento) {
    this.proxima = proxima;
    this.elemento = elemento;
  }

  public Celula(Object elemento) {
    this.elemento = elemento;
  }

  public void setProxima(Celula proxima) {
    this.proxima = proxima;
  }

  public Celula getProxima() {
    return proxima;
  }

  public Object getElemento() {
    return elemento;
  }
}

The LinkedList class:

public class ListaLigada {

  private Celula primeira;

  private Celula ultima;

  public void adiciona(Object elemento) {}
  public void adiciona(int posicao, Object elemento) {}
  public Object pega(int posicao) {return null;}
  public void remove(int posicao){}
  public int tamanho() {return 0;}
  public boolean contem(Object o) {return false;}
}

The test code:

    public class TesteAdicionaNoFim {
      public static void main(String[] args) {
        ListaLigada lista = new ListaLigada();
        lista.adiciona("Rafael");
        lista.adiciona("Paulo");

        System.out.println(lista);
       }
     }

NOTE: I DO THE SAME CODE, EQUAL. EVERY LINE EXACTLY THE SAME WITH CAELUM'S APOSTILA