当我尝试初始化类时 JavaFx 调用目标异常

Question

我真的是 javafx 的新手，我想我在这里遗漏了一些逻辑。我的 BankApplication 类包含实例变量 Customer 和 SavingsAccount。我想初始化这些，以便我的应用程序运行，并在顶部显示客户名称和 ID，以及余额。所以我想创建一个 BankApplication 的实例，为应用程序提供这些信息。它给我一个错误。

这只是代码的一部分..

public class BankApplication extends Application implements 
EventHandler<ActionEvent>{

public static void main(String[] args) {

    Customer Amanda = new Customer("Amanda" , 1009);
    SavingsAccount AmandaBANK = new SavingsAccount(Amanda , 150);
    BankApplication app1 = new BankApplication(Amanda,AmandaBANK);

    launch(args);

}

protected Customer customer;
protected SavingsAccount bankAccount;

public BankApplication(Customer customer, SavingsAccount bankAccount) {
    this.customer = customer;
    this.bankAccount = bankAccount;
}

private Button executeButton = new Button("Execute");
Label customerNameLabel = new Label("Customer name: " + customer.getName());
Label customerIDLabel = new Label("Customer ID: "+ customer.getID());

Label balanceLabel = new Label("Current balance: $" + 
bankAccount.getBalance() + ".");
TextField depositTextField = new TextField("Amt to deposit");
TextField withdrawTextField = new TextField("Amt to withdraw");



@Override
public void start(Stage primaryStage) throws Exception {

    FlowPane root = new FlowPane();
    Scene scene = new Scene(root,400,300);
    primaryStage.setScene(scene);
    primaryStage.setTitle("Bank application");

    executeButton.setOnAction(this);

    HBox middle = new HBox(depositTextField,withdrawTextField);
    middle.setSpacing(8);
    middle.setPadding(new Insets(8));

    VBox top = new VBox(customerNameLabel,customerIDLabel);
    VBox bottom = new VBox(executeButton,balanceLabel);
    top.setSpacing(8);
    bottom.setSpacing(8);
    top.setPadding(new Insets(8));
    bottom.setPadding(new Insets(8));

    root.getChildren().add(top);
    root.getChildren().add(middle);
    root.getChildren().add(bottom);


    primaryStage.show();


}

@Override
public void handle(ActionEvent event) {
    if (event.getSource() == executeButton) {

        String depositAmount = depositTextField.getText();
        String withdrawAmount = withdrawTextField.getText();

        if (isDouble(depositAmount) == true) {

            double old_balance = bankAccount.getBalance();

            bankAccount.deposit(Double.parseDouble(depositAmount));

            double new_balance = bankAccount.getBalance();

            if (new_balance != old_balance) { 
                updateBalanceLabel();
                depositTextField.clear();
                depositTextField.setText("Amt to deposit"); 
            }

            else if (new_balance == old_balance) { 
                depositTextField.clear();
                depositTextField.setText("Amt to deposit"); 
            }
        }

还有 javafx 错误..

java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(LauncherImpl.java:389)
at com.sun.javafx.application.LauncherImpl.launchApplication(LauncherImpl.java:328)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at sun.launcher.LauncherHelper$FXHelper.main(Unknown Source)


Caused by: java.lang.NullPointerException
at BankAccount.BankApplication.<init>(BankApplication.java:37)
at BankAccount.BankApplication.main(BankApplication.java:22)
... 11 more
Exception running application BankAccount.BankApplication

Answer 1

您根据传递给构造函数的客户和帐户初始化几个实例变量(customerIDLabel、customerIDLabel)。问题是，java 在字段初始化 block 之后 调用您的构造函数。这很容易修复:只需将字段初始化代码移动到您的构造函数中:

protected Customer customer;
protected SavingsAccount bankAccount;
private Button executeButton = new Button("Execute");
private Label customerIDLabel;
private Label balanceLabel;
private TextField depositTextField;
private TextField withdrawTextField;
private Label customerNameLabel;

public BankApplication(Customer customer, SavingsAccount bankAccount) {
    this.customer = customer;
    this.bankAccount = bankAccount;
    customerNameLabel = new Label("Customer name: " + customer.getName());
    customerIDLabel = new Label("Customer ID: " + customer.getID());

    balanceLabel = new Label("Current balance: $" +
            bankAccount.getBalance() + ".");
    depositTextField = new TextField("Amt to deposit");
    withdrawTextField = new TextField("Amt to withdraw");
}

这会修复您的 NullPointerException，但是您是否注意到 app1 被忽略了？在 JavaFX 中，您不应该创建应用程序的实例。相反，您必须让 JavaFx 运行时为您完成。这里通常认为使用静力学是可以接受的。

public class BankApplication extends Application implements
    EventHandler<ActionEvent> {
private static Customer customer;
private static SavingsAccount bankAccount;

private Button executeButton = new Button("Execute");
private Label customerIDLabel;
private Label balanceLabel;
private TextField depositTextField;
private TextField withdrawTextField;
private Label customerNameLabel;

public static void main(String[] args) {
    customer = new Customer("Amanda", 1009);
    bankAccount = new SavingsAccount("Amanda", 150);

    Application.launch(args);

}

public BankApplication() {
    customerNameLabel = new Label("Customer name: " + customer.getName());
    customerIDLabel = new Label("Customer ID: " + customer.getID());

    balanceLabel = new Label("Current balance: $" +
            bankAccount.getBalance() + ".");
    depositTextField = new TextField("Amt to deposit");
    withdrawTextField = new TextField("Amt to withdraw");
}


@Override
public void start(Stage primaryStage) throws Exception {

    FlowPane root = new FlowPane();
    Scene scene = new Scene(root, 400, 300);
    primaryStage.setScene(scene);
    primaryStage.setTitle("Bank application");

    executeButton.setOnAction(this);

    HBox middle = new HBox(depositTextField, withdrawTextField);
    middle.setSpacing(8);
    middle.setPadding(new Insets(8));

    VBox top = new VBox(customerNameLabel, customerIDLabel);
    VBox bottom = new VBox(executeButton, balanceLabel);
    top.setSpacing(8);
    bottom.setSpacing(8);
    top.setPadding(new Insets(8));
    bottom.setPadding(new Insets(8));

    root.getChildren().add(top);
    root.getChildren().add(middle);
    root.getChildren().add(bottom);


    primaryStage.show();


}

@Override
public void handle(ActionEvent event) {
    if (event.getSource() == executeButton) {

        String depositAmount = depositTextField.getText();
        String withdrawAmount = withdrawTextField.getText();

        if (isDouble(depositAmount) == true) {

            double old_balance = bankAccount.getBalance();

            bankAccount.deposit(Double.parseDouble(depositAmount));

            double new_balance = bankAccount.getBalance();

            if (new_balance != old_balance) {
                updateBalanceLabel();
                depositTextField.clear();
                depositTextField.setText("Amt to deposit");
            } else if (new_balance == old_balance) {
                depositTextField.clear();
                depositTextField.setText("Amt to deposit");
            }
        }
    }
}//...