JavaFX getText()、setText()、toString() 给出空指针

Question

Controller.java

import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.control.PasswordField;
import javafx.scene.control.TextField;

public class Controller {

    public TextField txtField;
    public PasswordField passField;
    public Button btn;
    public Label access;

    private String USNM = "crimsoncoder";
    private String PASS = "toor";

▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬问题从这里开始▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬

    private String usnm = txtField.getText();
    private String pass = passField.getText();
    private String acc = access.getText();

    private final String ACCESS_GRANTED = "/ACCESS_GRANTED/";
    private final String ACCESS_DENIED = "|/?ACCESS_DENIED?/|";

    public void onClick() {
        if (usnm.equals(USNM)) {
            if (pass.equals(PASS)) {
                acc = ACCESS_GRANTED;
            } else {
                acc = ACCESS_DENIED;
            }
        } else {
            acc = ACCESS_DENIED;
        }
    }
}

在 Controller.java 中声明 usnm、pass 和 acc 变量后调用方法 getText()、setText() 和 toString() 会返回 NullPointerException。知道为什么会发生这种情况吗？

<小时/>

新代码:

public class Controller {

    @FXML
    private TextField txtField;
    @FXML
    private PasswordField passField;
    @FXML
    private Button btn;
    @FXML
    private Label access;

    private String USNM = "crimsoncoder";
    private String PASS = "toor";

    private static final String ACCESS_GRANTED = "/ACCESS_GRANTED/";
    private static final String ACCESS_DENIED = "|/?ACCESS_DENIED?/|";

    public void onClick() {
        String usnm = txtField.getText();
        String pass = passField.getText();

        if (usnm.equals(USNM)) {
            if (pass.equals(PASS)) {
                access.setText(ACCESS_GRANTED);
            } else {
                access.setText(ACCESS_DENIED);
            }
        } else {
            access.setText(ACCESS_DENIED);
        }
    }
}

Answer 1

您的设计实际上存在很多问题。首先，您尝试在 txtField 和 passField 控件实际存在于场景中之前访问这些控件的值。

使用 JavaFX 和 FXML，在场景初始化之前，这些控件不会构建(或注入(inject))并可供 Controller 使用。

控件引用也应该是私有(private)并用@FXML关键字进行注释，如下所示:

@FXML
private TextField txtField;

这实际上告诉 Java TextField 已在关联的 FXML 文档中定义。

并且您需要在单击 LOGIN 按钮之后获取用户名和密码字段的值，因此请更新您的 onClick() 方法以检索当时的值:

public void onClick() {

        String usnm = txtField.getText();
        String pass = passField.getText();

        String acc;

        if (usnm.equals(USNM)) {
            if (pass.equals(PASS)) {
                acc = ACCESS_GRANTED;
            } else {
                acc = ACCESS_DENIED;
            }
        } else {
            acc = ACCESS_DENIED;
        }

        System.out.println(acc);
    }

因此，对于稍微好一点的 Controller.java，您可以使用如下内容:

public class Controller {

    @FXML
    public TextField txtField;
    @FXML
    public PasswordField passField;
    @FXML
    public Button btn;
    @FXML
    public Label access;

    private String USNM = "crimsoncoder";
    private String PASS = "toor";

    private static final String ACCESS_GRANTED = "/ACCESS_GRANTED/";
    private static final String ACCESS_DENIED = "|/?ACCESS_DENIED?/|";

    public void onClick() {

        String usnm = txtField.getText();
        String pass = passField.getText();

        String acc;

        if (usnm.equals(USNM)) {
            if (pass.equals(PASS)) {
                acc = ACCESS_GRANTED;
            } else {
                acc = ACCESS_DENIED;
            }
        } else {
            acc = ACCESS_DENIED;
        }

        System.out.println(acc);
    }

}