java - JavaFX getText()，setText()，toString()提供空指针

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getText()，setText()和toString()变量后，调用方法usnm，pass和acc返回NullPointerException。知道为什么会这样吗？



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);
        }
    }
}

最佳答案

实际上，这里的设计存在很多问题。首先，您试图在txtField和passField控件的值实际存在于场景中之前对其进行访问。

使用JavaFX和FXML，在初始化场景之前，不会构建（或注入）这些控件并将其提供给控制器。

控件引用也应为private并使用@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。

在单击LOGIN按钮之后，您需要获取用户名和密码字段的值，因此更新您的onClick()方法以获取当时的值：

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，对于稍微更好的Controller.java，您可以使用以下代码：

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);
    }

}

关于java - JavaFX getText()，setText()，toString()提供空指针，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/50660863/