Create data validation and page sequence

Use CardLayout that allows you to work with multiple JPanel in of Container . With this layout you can easily control the flow of the panels using methods to toggle them:

first(C)    // Alterna para o primeiro painel adicionado no container (C).
last(C)     // Alterna para o último painel inserido no container (C).
next(C)     // Muda para o painel seguinte em (C).
previous(C) // Muda para o painel anterior em (C).
show(C, N)  // Alterna para o painel com nome (N) dentro do container (C).

This would avoid creating an application with multiple JDialog and / or JFrame .

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In your code you are inserting all components directly into JFrame . One improvement to make is to separate the two views (Registry and Login) you want into JPanel different.

// Painel de criação do usuário
JPanel registerView = new JPanel();

// Painel de login
JPanel loginView = new JPanel();

To avoid adding everything directly to JFrame , a "root" panel can be created where everything will happen inside it (and it will have CardLayout as layout).

/* 
 * É importante armazenar em uma variável para podermos controlar o fluxo
 * dentro do JPanel através dos métodos listados no início dessa resposta.
 */
CardLayout card = new CardLayout();

// JPanel onde serão inseridos os JPanels
JPanel rootPanel = new JPanel();
rootPanel.setLayout(card); // definindo o layout 

Finally, just add the registration and login panels within rootPanel . Here, the way to add is slightly different from the simple jframe.add(jpanel) , in addition to the component we want to add, we also need to pass a unique name so we can identify it.

//Inserindo os painéis de Registro e Login,
rootPanel.add(registerView, "register");
rootPanel.add(loginView, "login");

// Painel que será exibido primeiro
card.show(rootPanel, "register"); // mostrará o painel com nome "register"

Test code

//Window.java

import java.awt.CardLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;

public final class Window extends JFrame {

    public Window(String title){
        super(title);
        init();
    }

    // Inicializa os componentes
    public void init(){
        setSize(300,300);
        setLocationRelativeTo(null);
        setDefaultCloseOperation(EXIT_ON_CLOSE);

        // Painel de criação do usuário
        JPanel registerView = new JPanel();
        registerView.setLayout(null);

        // Botão que avançará para o próximo painel.
        JButton btnNext = new JButton("Me Registrei! Próximo Painel");
        btnNext.setBounds(50, 100, 200, 35);
        registerView.add(btnNext);

        // Painel de login
        JPanel loginView = new JPanel();
        loginView.setLayout(null);

        JLabel lblLoginPanel = new JLabel("Esse é o painel de login");
        lblLoginPanel.setBounds(70, 100, 150, 35);
        loginView.add(lblLoginPanel);

        CardLayout card = new CardLayout();

        // JPanel onde serão inseridos os JPanels
        JPanel rootPanel = new JPanel();
        rootPanel.setLayout(card); // definindo o layout
        getContentPane().add(rootPanel);

        // Inserindo os painéis de Registro e Login,
        rootPanel.add(registerView, "register");
        rootPanel.add(loginView, "login");

        card.show(rootPanel, "register"); // mostrará o painel de registro

        // Quando clicado...
        btnNext.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                card.next(rootPanel); // mostra o próximo JPanel dentro do container
            }
        });
    }
}

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//Main.java
public class Main {
   public static void main(String[] args) throws IOException {
      new Window("StackOverflow").setVisible(true);
   }
}

Result

In your code, you can have the panel advanced when the user clicks, for example, a log completion button after filling in the required fields. The issue of validating the user is apparently already implemented in your code.