我嘗試初始化類時的JavaFx調用目標異常

Question

我對javafx真的很陌生，我想我在這里缺少一些邏輯。 我的班級BankApplication包含實例變量Customer和SavingsAccount。 我想初始化它們，以便我的應用程序運行，客戶名稱和ID在頂部，以及余額。 因此，我想創建一個BankApplication實例，以向應用提供此信息。 它給了我一個錯誤。

這只是代碼的一部分。

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"); 
            }
        }

和javafx錯誤。

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

您可以根據傳遞給構造函數的客戶和帳戶初始化幾個實例變量（customerIDLabel，customerIDLabel）。 問題是，java在字段初始化塊之后調用構造函數。 這很容易解決：只需將字段初始化代碼移到構造函數中：

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");
}

這將修復您的NullPointerException，但是您是否注意到app1被忽略了？ 在JavaFX中，不應創建應用程序的實例。 相反，您必須讓JavaFx運行時為您執行此操作。 在這里一般認為使用靜態是可以接受的。

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");
            }
        }
    }
}//...

Answer 2

當啟動JavaFX應用程序時（通過調用Application.launch()或直接由系統啟動），許多事情會“自動”發生。

其中之一是通過調用無參數構造函數來創建Application子類的實例。

發生的另一件事是在創建的實例上調用了start()方法。 （這發生在FX Application線程上）。

其結果是：

1. 您的Application子類中必須有一個無參數的構造函數
2. 您應該將start()方法視為應用程序的入口點； 即發生的第一件事。 任何初始化都應在此處完成。 您通常應該避免在main()方法中除了調用launch() 。

因此，您可以將應用程序類重構為：

public class BankApplication extends Application implements EventHandler<ActionEvent>{

    public static void main(String[] args) {

        launch(args);

    }

    protected Customer customer;
    protected SavingsAccount bankAccount;


    private Button executeButton ;
    private Label customerNameLabel ;
    private Label customerIDLabel ;

    private Label balanceLabel ;
    private TextField depositTextField ;
    private TextField withdrawTextField ;



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

        Customer customer = new Customer("Amanda" , 1009);
        SavingsAccount bankAccount = new SavingsAccount(Amanda , 150);

        executeButton = new Button("Execute");
        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");


        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) {
        // ...
    }

}