Java 尝试获取方法时返回 Null

Question

first of all I know this is duplicated question. But I've search and tried from stackoverflow listed on Google to quora but still cant resolve my Get method still return null.

This is my class loginModel.java under package com.hello.model

public class loginModel {
  public String username;

  public void setUsername(String username) {
      this.username = username;
  }

  public String getUsername() {
      return this.username;
  }
}

This is my loginView.java under package com.hello.view

import com.hello.model.loginModel;

public class loginView extends javax.swing.JFrame {
  loginModel login = new loginModel();

  public loginView() {
      initComponents();
      this.setLocationRelativeTo(null);
      loginFunction();
  }

  private void loginFunction(){
    String username = usernameText.getText();
    String password = passwdText.getText();
    String query = "select * from access where username = '" +username+ "' AND password = '" +password+"'";
    databaseConnect db = new databaseConnect();

    try (Connection con = DriverManager.getConnection(db.url, db.user, db.password);
        Statement st = con.createStatement();
        ResultSet rs = st.executeQuery(query)) {

        if(rs.next()) {
            if(username.equals(rs.getString("username")) && password.equals(rs.getString("password"))){
                JOptionPane.showMessageDialog(null, "login Success");
                String name = rs.getString("name");
                String privilege = rs.getString("privilege");
                login.setUsername(name);

                menu = new menuView();
                menu.setVisible(true);
                this.setVisible(false);
            }
        } else {
                JOptionPane.showMessageDialog(null, "username or password incorrect");
            }
    } catch (SQLException e) {
        System.err.format("SQL State: %s\n%s", e.getSQLState(), e.getMessage());

    } catch (Exception e) {
        e.printStackTrace();
    }
  }
}

I want call my username from menuView.java under package com.hello.view after login success

import com.hello.model.loginModel;
import com.hello.view.loginView;

public class menuView extends javax.swing.JFrame {    
  private String username;
  loginModel login = new loginModel();

  public menuView() {
    initComponents();
    this.setLocationRelativeTo(null);
    initMenu();
  }

  private void initMenu(){
    username = login.getUsername();
    JOptionPane.showMessageDialog(this, username);
  }
}

As per my question when I call Get method from loginModel, messagebox return null.

I've tried:

1. Put system.out.println directly in loginModel.java , value return and call system.out.println in menuView.java at the same time but value return null. How?
2. Send string between jframe with menu = menuView(username) in loginView.java and retrieve in menuView.java , value return null
3. Using no model and create set string in loginView and call it in menuView , value return null

I need values that I want to use in another class/package/jframe. Am I doing wrong?

Answer 1

I am not well versed in Swing but I can see the problem, just not the exact solution.

Your code creates an instance of loginModel in both the menuView and in loginView. Then in loginView is sets the name in the instance it has, in in menuView it gets the name from its own instance.

You need to create a single instance of the model and share it between the two views.

In a pojo way I would pass the loginModel to both "views" in a constructor.

menu = new menuView(login);

And in menuView

public menuView(loginModel login) {
    this.login = login;
}

Answer 2

Your menuView instance isn't using the loginModel class that you instantiate in loginView , it's using the new one you created using new menuView() when you initialized the login variable in the menuView class. You just need to add a setter method for the loginModel attribute in the menuView class like this:

import com.hello.model.loginModel;
import com.hello.view.loginView;

public class menuView extends javax.swing.JFrame {    
  private String username;
  loginModel login = new loginModel();

  public menuView() {
    initComponents();
    this.setLocationRelativeTo(null);
    initMenu();
  }

  private void initMenu(){
    username = login.getUsername();
    JOptionPane.showMessageDialog(this, username);
  }

  public void setLogin(loginModel loginModel) {
    this.login = loginModel;
  }
}

Then call the setter in loginView.loginFunction like this:

... code before
 login.setUsername(name);
 menu = new menuView();
 menu.setLogin(login);
 menu.setVisible(true);
 this.setVisible(false);
... code after

Notice the only changes to your code are the added setLogin method on the menuView class and the call to menu.setLogin(login) in loginView.loginFunction .

Answer 3

You need to think in stages/steps. Login is a single step, it has one of two outcomes, success or failure.

Your app needs to perform this step and take appropriate action based on the outcome of the result.

You also need to think about "separation of responsibility" - in this case, it's not really the responsibility of the loginView to perform the login operation, it just coordinates the user input.

The responsibility actually falls to the LoginModel

// Just a custom exception to make it easier to determine 
// what actually went wrong
public class LoginException extends Exception {

    public LoginException(String message) {
        super(message);
    }

}

// LoginModel ... that "does" stuff
public class LoginModel {

    private String username;
    DatabaseConnect db;

    public LoginModel(DatabaseConnect db) {
        this.db = db;
    }

    // I would consider not doing this.  You need to ask what reasons would
    // the app need this information and expose it only if there is really a 
    // reason to do so
    public String getUsername() {
        return username;
    }

    public boolean isLogedIn() {
        return username != null;
    }

    public void validate(String username, String password) throws SQLException, LoginException {

        String query = "select * from access where username = ? AND password = ?";

        try ( Connection con = DriverManager.getConnection(db.url, db.user, db.password);  PreparedStatement st = con.prepareStatement(query)) {
            st.setString(1, username);
            st.setString(2, password);

            try ( ResultSet rs = st.executeQuery()) {
                if (rs.next()) {
                    this.username = username;
                } else {
                    throw new LoginException("Invalid user credentials");
                }
            }
        }
    }

}

This is an overly simplified example, as the actual responsibility for performing the login should fall to the controller, which would then generate the model, but I'm getting ahead of myself.

Because the flow of the app shouldn't be controlled/determined by the login view, the LoginView should itself be a dialog. This way, it can be shown when you need it, it can perform what ever operations it needs and then go away, leaving the rest of the decision making up to who ever called it

public class LoginView extends javax.swing.JDialog {

    private LoginModel model;

    public LoginView(LoginModel model) {
        initComponents();
        setModal(true);
        this.model = model;
        this.setLocationRelativeTo(null);
    }

    // This will get executed when the user taps some kind of "perform login button"
    private void loginFunction() {
        String username = usernameText.getText();
        String password = passwdText.getText();

        try {
            model.validate(username, password);
            dispose()
        } catch (SQLException ex) {
            // This should probably be considered a fatal error
            model = null;
            dispose();
        } catch (LoginException ex) {
            JOptionPane.showMessageDialog(this, "Login vaild");
        }
    }
}

This then means you might put it together something like this...

DatabaseConnect db = new DatabaseConnect();
LoginModel model = new LoginModel(db);

LoginView loginView = new LoginView(model);
// Because we're using a modal dialog, the code execution will wait here
// till the window is disposed/closed
loginView.setVisible(true);

if (loginView.model != null) {
    // model is now valid and can continue to be used
    // in what ever fashion you need
} else {
    JOptionPane.showMessageDialog(null, "Fatal Error");
}

This takes you a step closer to a more decoupled solution, where you feed information to the classes when they need it, rather than the classes making decisions about what they should create/use.

It also moves you a step closer to re-usable classes, as they do their specific job and nothing more.

You might find taking the time to read up on "model-view-controller" will help you better understand this approach