EDIT:

Sorry forr the misspellings and typos, I didn't want to put my code here so I tried to make a new look a like code to express my question.

Here is the actual code I'm using, I just removed some parts of it as they are not related to my question, I think, otherwise just ask me and I'll put it here as well.

Heres the actual code:

public class Dados {

    private String sta;
    private String ap;
    private int startTime;
    private int endTime;
    private int repetitionSTA;
    private int pingPong;
    private int tt_previous;
    private int tt_next;
    private int id;

    public Dados(int id, int startTime, int endTime, String ap, String sta, int repetitionSTA, int ttprevious, int ttnext, int ppong)
    {
        this.sta = sta;
        this.ap = ap;
        this.startTime = startTime;
        this.endTime=endTime;
        this.pingPong = ppong;
        this.tt_next = ttnext;
        this.tt_previous = ttprevious;
        this.id = id;
        this.repetitionSTA = repetitionSTA;
    }

    // SET

    public void setPingPong()
    {
        this.pingPong = 1;
    }


    //GET

    public int getPingPong()
    {
        return this.pingPong;
    }

}

//another class from now on

public class Queries extends LigarBD{

    String dbtime = null; 

    int id = 1;

    TreeMap<Integer, ArrayList> tmValores = new TreeMap<>();
    ArrayList<Dados> listaObjectos = new ArrayList<>();
    ArrayList<Dados> listaObjectos2 = new ArrayList<>();


    public ArrayList getUniqueStations(String server)
    {
        ArrayList<String> listaSTA = new ArrayList<>();

        String query = "SELECT distinct calling_station_id FROM java_logs;";

        try
        {
            super.ligar(server);
            Statement s = super.getConexao().createStatement();
            ResultSet rs = s.executeQuery(query);
            while (rs.next())
            {
                listaSTA.add(rs.getString(1));
            }
            rs.close();
            s.close();
            super.desligar(super.getConexao());

        }
        catch (Exception e)
        {
            JOptionPane.showMessageDialog(null, "Error at listing all unique stations. Reason -> "+e.getMessage());
            System.out.println("Error at listing all unique stations. Reason ->  "+e.toString());
        }

        return listaSTA;
    }


    public ArrayList getStationData(String mac, String server)
    {

        try 
        {
            super.ligar(server);
            Statement s = getConexao().createStatement();

            ResultSet rs = s.executeQuery("SELECT timestamp-acct_session_time, timestamp, called_station_id, calling_station_id "
                                        + "FROM java_logs where calling_station_id = '"+mac+"';"); // retirar STA da query *******************
            //System.out.println("Executing the Query on+"+server+" - UniqueSTA - Query number: 1?");
            int repetitionSTA=1;
            while (rs.next()) 
            {              
                Dados d = new Dados(id, rs.getInt(1), rs.getInt(2), rs.getString(3), rs.getString(4), repetitionSTA, 0, 0, 0);

                listaObjectos2.add(d);
                repetitionSTA++;
                id++;
            }

            rs.close();
            s.close();
            super.desligar(super.getConexao());
        }
        catch (Exception e) 
        {
            JOptionPane.showMessageDialog(null,"Error at Select Query. Reason -> "+e.getMessage());
        }

        return listaObjectos2;
    }

}

Another class:

public class Pingpong {

    ArrayList<Dados> dadosArray = new ArrayList<>();



    Queries q = new Queries();

    TreeMap<Integer, ArrayList> mapa = new TreeMap<>();
    ArrayList<Dados> arrayDeDados = new ArrayList<>();


    public ArrayList detectPingPongArray(int threshold_access_session_time, int threshold_transition_time, ArrayList<Dados> dadosSTA)
    {
        dadosArray=dadosSTA;
        for(int i = 1; i<arrayDeDados.size()-1; i++)
        {
            dadosArray.get(i).setPingPong();
        }
        return dadosArray;
    }


}

And here is where I'm printing each object one by one:

ArrayList<Dados> dadosSTA = new ArrayList<>();
        ArrayList<Dados> dataForPPong = new ArrayList();

        ArrayList uniqueSTA = q.getUniqueStations("localserver");

        for(int i = 0; i<uniqueSTA.size(); i++)
        {
            dadosSTA = q.getStationData(uniqueSTA.get(i).toString(), "localserver");
            dataForPPong = p.detectPingPongArray(5, 3, dadosSTA);

        }

        for(int i=0; i<dataForPPong.size(); i++)
        {
            System.out.println("ID: "+dataForPPong.get(i).getId()+" STA: "+dataForPPong.get(i).getStation()
                    + " PingPong: "+dataForPPong.get(i).getPingPong());
        }

So I was expecting it to change the value of pingPong in all objects to 1 but it doesn't.

I think the problem is with the returning from the method detectPingPongArray but I don't know where is the mistake.

Anyone can pinpoint the problem here?