Question

Can someone provide a concrete (actual Java code) example of MVP in action?

This would include the following 3 types of classes and how they call each other’s methods to achieve the pattern and process/respond to a client-side response:

• Model – some kind of value object (VO)
• View – represents or generates the UI
• Presenters – business logic

Answer 1

MVP is my favorite design pattern to create a UI.

The big difference between MVP and MVC is how to handle the view.

• In MVC, the Controller manipulates the view, taking care of how to render in the view parts of the user actions and model. That means that the Controller and View have ‘tight coupling’.
• In MVP, the Presenter takes care of the user tasks, the model is shared between the Presenter and View. So the view renders UI according to the model, sometimes the view can have actions to be called from the Presenter. The Presenter and View can have a defined interface contracts to make them ‘loose coupling’. For example you can create a View for Java Swing UI and another for JavaFX UI. Or if the connection to the data source changes, then you just need update the presenter.

There are many styles to program the MVP.

In a formal way, consists in create interfaces for each element of the design pattern.

/*-- file: Application.java --*/
import javax.swing.JOptionPane;

/**
 *
 * @author danLeon
 */
interface LoginModel {

    String getUser();

    void setUser(String user);
}

class MyLoginModel implements LoginModel {

    String user;

    @Override
    public String getUser() {
        return user;
    }

    @Override
    public void setUser(String user) {
        this.user = user;
    }
}

interface LoginView {

    LoginPresenter getPresenter();

    void setPresenter(LoginPresenter loginPresenter);

    void updateModelFromView();

    void updateViewFromModel();

    void open();

    void close();

    void userRejected();
}

class MyLoginView extends javax.swing.JFrame implements LoginView {

    private LoginPresenter loginPresenter;

    /**
     * Creates new form MyLoginView
     */
    public MyLoginView() {
        initComponents();
    }


    @SuppressWarnings("unchecked")
    private void initComponents() {
        java.awt.GridBagConstraints gridBagConstraints;

        jLabel1 = new javax.swing.JLabel();
        jTextField1 = new javax.swing.JTextField();
        jLabel2 = new javax.swing.JLabel();
        jButton1 = new javax.swing.JButton();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
        setBounds(new java.awt.Rectangle(0, 0, 0, 0));
        java.awt.GridBagLayout layout = new java.awt.GridBagLayout();
        layout.columnWidths = new int[] {0, 7, 0};
        layout.rowHeights = new int[] {0, 7, 0, 7, 0};
        getContentPane().setLayout(layout);

        jLabel1.setFont(new java.awt.Font("Tahoma", 1, 11)); // NOI18N
        jLabel1.setText("Welcome");
        gridBagConstraints = new java.awt.GridBagConstraints();
        gridBagConstraints.gridx = 0;
        gridBagConstraints.gridy = 0;
        gridBagConstraints.gridwidth = 3;
        getContentPane().add(jLabel1, gridBagConstraints);

        jTextField1.setColumns(13);
        gridBagConstraints = new java.awt.GridBagConstraints();
        gridBagConstraints.gridx = 2;
        gridBagConstraints.gridy = 2;
        gridBagConstraints.fill = java.awt.GridBagConstraints.HORIZONTAL;
        getContentPane().add(jTextField1, gridBagConstraints);

        jLabel2.setText("User");
        gridBagConstraints = new java.awt.GridBagConstraints();
        gridBagConstraints.gridx = 0;
        gridBagConstraints.gridy = 2;
        getContentPane().add(jLabel2, gridBagConstraints);

        jButton1.setText("Login");
        jButton1.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                jButton1ActionPerformed(evt);
            }
        });
        gridBagConstraints = new java.awt.GridBagConstraints();
        gridBagConstraints.gridx = 2;
        gridBagConstraints.gridy = 4;
        getContentPane().add(jButton1, gridBagConstraints);

        pack();
    }

    private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {
        getPresenter().login();
    }

    private javax.swing.JButton jButton1;
    private javax.swing.JLabel jLabel1;
    private javax.swing.JLabel jLabel2;
    private javax.swing.JTextField jTextField1;


    @Override
    public void updateModelFromView() {
        getPresenter().getModel().setUser(jTextField1.getText());
    }

    @Override
    public void updateViewFromModel() {
        jTextField1.setText(getPresenter().getModel().getUser());
    }

    @Override
    public void open() {
        setVisible(true);
        jTextField1.selectAll();
        jTextField1.requestFocus();
    }

    @Override
    public void close() {
        dispose();
    }

    @Override
    public void userRejected() {
        jLabel1.setText("Try again!");
        jTextField1.selectAll();
        jTextField1.requestFocus();
    }

    @Override
    public LoginPresenter getPresenter() {
        return loginPresenter;
    }

    @Override
    public void setPresenter(LoginPresenter loginPresenter) {
        this.loginPresenter = loginPresenter;
    }
}

interface LoginPresenter {

    LoginModel getModel();

    void setModel(LoginModel loginModel);

    LoginView getView();

    void setView(LoginView loginView);

    void setOnLogin(Runnable onLogin);

    void run();

    void login();
}

class MyLoginPresenter implements LoginPresenter {

    LoginModel loginModel;
    LoginView loginView;
    private Runnable onLogin;

    @Override
    public LoginModel getModel() {
        return loginModel;
    }

    @Override
    public void setModel(LoginModel loginModel) {
        this.loginModel = loginModel;
    }

    @Override
    public LoginView getView() {
        return loginView;
    }

    @Override
    public void setView(LoginView loginView) {
        this.loginView = loginView;
    }

    @Override
    public void setOnLogin(Runnable onLogin) {
        this.onLogin = onLogin;
    }

    @Override
    public void run() {
        loginModel.setUser("previousUser");
        loginView.setPresenter(this);
        loginView.updateViewFromModel();
        loginView.open();
    }

    @Override
    public void login() {
        loginView.updateModelFromView();
        if (loginModel.getUser().equalsIgnoreCase("root")) {
            loginView.close();
            loginView.setPresenter(null);// for memory sanity. 
            onLogin.run();
        } else {
            loginView.userRejected();
        }
    }
}

public class Application {

    /**
     * @param args the command line arguments
     */
    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {

            public void run() {
                LoginModel loginModel = new MyLoginModel();
                LoginPresenter loginPresenter = new MyLoginPresenter();
                loginPresenter.setModel(loginModel);
                LoginView loginView = new MyLoginView();
                loginPresenter.setView(loginView);
                loginPresenter.setOnLogin(new Runnable() {

                    @Override
                    public void run() {
                        JOptionPane.showMessageDialog(null, "Welcome user!");
                    }
                });
                loginPresenter.run();
            }
        });
    }
}