当我尝试初始化时 JavaFx 调用目标异常 class

Question

我是 javafx 的新手，我想我在这里遗漏了一些逻辑。我的 class BankApplication 包含实例变量 Customer 和 SavingsAccount。我想初始化这些，这样我的应用程序就会运行，顶部有客户名称和 ID，还有余额。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了几个实例变量（customerIDLabel、customerIDLabel）。问题是，java 在字段初始化块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。这里一般认为可以使用静力学。

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

启动 JavaFX 应用程序时（通过调用 Application.launch()，或直接由系统启动），会发生许多事情 "automatically"。

其中之一是您的 Application subclass 的实例是通过调用 no-argument 构造函数创建的。

发生的另一件事是在创建的实例上调用 start() 方法。（这发生在 FX 应用程序线程上）。

这样做的结果是：

您的 Application 子class
您应该将 start() 方法视为应用程序的入口点；即发生的第一件事。任何初始化都应该在这里完成。你应该（通常）避免在 main() 方法中有任何东西，除了调用 launch().

因此您可以将您的应用程序 class 重组为：

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) {
        // ...
    }

}

当我尝试初始化时 JavaFx 调用目标异常 class

JavaFx invocation target exception when I try to initialize class

java

javafx

invocationtargetexception