如何从另一个类中调用一个方法以在我的ActionListener类中使用

huangapple
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英文:

How do I call a method from another class to use in my ActionListener class

问题

import javax.swing.*;
import java.awt.event.*;

public class CarView extends JFrame {
    // ... (other member variables)

    // ... (constructor and other methods)

    //AccelerateButtonListener is an action listener private inner class for the Accelerate Button.
    private class AccelerateButtonListener implements ActionListener {
        //The actionPerformed method executes when the user clicks on the Accelerate Button
        public void actionPerformed(ActionEvent e) {
            vehicle.accelerate(); // Using the instance variable we made of Car earlier we can call the accelerate method from Car class.
            speedTextField.setText(String.valueOf(vehicle.getSpeed())); // Update speedTextField with the new speed value
        }
    }

    //BrakeButtonListener is an action listener private inner class for the Brake Button.
    private class BrakeButtonListener implements ActionListener {
        //The actionPerformed method executes when the user clicks on the Brake Button
        public void actionPerformed(ActionEvent e) {
            vehicle.brake(); // Using the instance variable we made of Car earlier we can call the brake method from Car class.
            speedTextField.setText(String.valueOf(vehicle.getSpeed())); // Update speedTextField with the new speed value
        }
    }

    // ... (rest of the code)
}

Note: I've added lines to update the speedTextField with the new speed value after calling the accelerate() and brake() methods to ensure the displayed speed is updated correctly.

英文:

I am trying to use the accelerate and brake methods from my Car class to use them in my CarView class whenever a specific button is pressed. I keep getting an error with my current code when it comes to the ActionListeners and I am not sure where to go from here. Here is my code.

import javax.swing.*; //Needed for Swing classes
import java.awt.event.*; // Needed for ActionListener Interface
public class CarView extends JFrame
{
    private JPanel panel; //To reference a panel
    private JLabel modelYearLable; //To reference a model year Label
    private JLabel makeLable; //To reference a make Label
    private JLabel speedLable; //To reference a speed Label
    private JTextField modelTextField; // To reference a model yeartext field
    private JTextField makeTextField; // To reference a make text field
    private JTextField speedTextField; // To reference a speed text field
    private JButton accelerateButton; // To reference an accelerate button
    private JButton brakeButton; // To reference a brake button
    private final int WINDOW_WIDTH = 310; // Window width
    private final int WINDOW_HEIGHT = 100; // Window heigh
    private final int carYear = Integer.parseInt(modelTextField.getText()); 
    private final String type = makeTextField.getText(); 
    Car vehicle = new Car(this.carYear, this.type); //Create an instance variable of the Car class!
    //Constructor 
    public CarView()
    {
        //Set the window titile.
        setTitle("Car referencer!");
        
        //Set the size of the window.
        setSize(WINDOW_WIDTH, WINDOW_HEIGHT);
        
        // Specify what happens when the close button is clicked
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        
        //Build thhe panel and add it to the frame.
        buildPanel();
        
        //Add the panel to the frame's content pane.
        add(panel);
        
        //Display the window
        setVisible(true);
    }
        
    //buildPanel()
    //Responisibilities: Adds labels, text fields, and buttons to the panel
    private void buildPanel()
    {
       //Create labels to display instructions 
       modelYearLable = new JLabel("Enter the year of your model");
       makeLable = new JLabel("Enter the make of your car");
       speedLable = new JLabel("Enter the current speed of your car");
            
       //Create text fields as well
       modelTextField = new JTextField(5);
       makeTextField = new JTextField(5);
       speedTextField = new JTextField(5);
            
       //Create the buttons
       accelerateButton = new JButton("Accelerate");
       brakeButton = new JButton("Brake");
            
       //Add an action listener to the buttons
       accelerateButton.addActionListener(new AccelerateButtonListener());
       brakeButton.addActionListener(new BrakeButtonListener());
            
       //Create a JPanel object and let the panel field reference it. 
       panel = new JPanel();
            
       //Add the labels,text fields, and buttons to the panel
       panel.add(modelYearLable);
       panel.add(makeLable);
       panel.add(speedLable);
       panel.add(modelTextField);
       panel.add(makeTextField);
       panel.add(speedTextField);
       panel.add(accelerateButton);
       panel.add(brakeButton);
    }
        
    //AccelerateButtonListener is an action listener private inner class for the Accelerate Button.
    private class AccelerateButtonListener implements ActionListener
    {
        //The acitonPerformed method executes when the user clicks on the Accelerate Button
        public void actionPerformed(ActionEvent e)
        {
           vehicle.accelerate();//Using the instance variable we made of Car earlier we can call the accelerate method from Car class.
        }
           
    }
    
    
    //BrakeButtonListener is an action listener private inner class for the Brake Button.
    private class BrakeButtonListener implements ActionListener
    {
        //The actton Performed method executes when the user clicks on the Brake Button
        public void actionPerformed(ActionEvent e)
        {
           vehicle.brake();//Using the instance variable we made of Car earlier we can call the brake method from Car class.
        }
    }
    //The main method creates an instance of the CarView, which causes it to display its window
    public static void main(String[] args)
    {
        CarView cv = new CarView();
    }

This is the Car Class

public class Car
{
    private int yearModel; //The yearModel is an int that holds the car's modcel year
    private String make; //The make references a String object that holds the make of the car.
    private double speed; //The speed field is a double that hold's thhe car's current speed.
    
    //Construtor that accepts the car's year model and make as arguments. These values should be assigned to the obkect's modelYear and make fields.
    //Also assign 0 to the speed
    public Car(int model, String type)
    {
        this.yearModel = model;
        this.make = type;
        speed = 0.0; //Set the speed to 0.
    }
    
    //Get and Set methods for modelYear, make and speed fields.
    
    //getModel
    //Responsibilities: gets the model of the car 
    public int getModel() 
    {
        return yearModel;
    }
    
    //getMake
    //Responsibilities: gets the make of the car 
    public String getMake() 
    {
        return make;
    }
    
    //getSpeed
    //Responsibilities: gets the speed of the car 
    public double getSpeedl() 
    {
        return speed;
    }
    
    //accelerate()
    //Responsibilities: Shouyld add 8 to the speed each time it is called 
    public void accelerate() 
    {
        speed = speed + 8; //Everytime this method is called, add 8 to the speed each time
    }
    
    //brake()
    //Responsibilities: Should  subtract 6 from the speed each time it is called.
    public void brake()
    {
        speed = speed - 6; //Everytime this method is called subtract 6 from speeed.
    }
}

Error

> I am getting a NullPointerException when I run my current code but I
> do not know exactly how to counter this. I want to just use my Car
> class methods accelerate and brake in my action listeners but I do not
> know-how.

Any help is appreciated thank you!

答案1

得分: 1

当我运行你的代码时，我收到一个NullPointerException。仔细查看代码后，我可以看到两个可能导致此问题的问题...

public class CarView extends JFrame
{
    //...
    private JTextField modelTextField; // 引用模型年份文本字段
    private JTextField makeTextField; // 引用制造商文本字段
    //...
    private final int carYear = Integer.parseInt(modelTextField.getText()); 
    private final String type = makeTextField.getText();

在类的初始化阶段，你不能从modelTextField或makeTextField获取值，因为这些变量为null，即使它们不是null，它们也是空的，因为组件还没有显示在屏幕上。

相反，你需要在其他时间点获取值 - 请记住GUI是事件驱动的，而不是过程化或线性的。

还有很多其他问题，包括布局问题，没有验证，当速度发生变化时没有向用户报告。

我可以继续讲很长时间，但我要给你一点启发：

import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.text.NumberFormat;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;

public class CarView extends JFrame {

    private JPanel panel; // 引用面板
    private JLabel modelYearLabel; // 引用车型年份标签
    private JLabel makeLabel; // 引用制造商标签
    private JLabel speedLabel; // 引用速度标签
    private JTextField modelTextField; // 引用车型年份文本字段
    private JTextField makeTextField; // 引用制造商文本字段
    private JTextField speedTextField; // 引用速度文本字段
    private JButton accelerateButton; // 引用加速按钮
    private JButton brakeButton; // 引用刹车按钮
    private JButton makeCarButton;

    private Car vehicle;

    // 构造函数
    public CarView() {
        // 设置窗口标题。
        setTitle("Car referencer!");

        // 设置窗口大小。
        //setSize(WINDOW_WIDTH, WINDOW_HEIGHT);

        // 指定当单击关闭按钮时发生的情况
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        // 构建面板并将其添加到窗体中。
        buildPanel();

        // 将面板添加到窗体的内容面板。
        add(panel);

        // 显示窗口
        pack();
        setVisible(true);
    }

    // buildPanel()
    // 负责：向面板添加标签、文本字段和按钮
    private void buildPanel() {
        // 创建显示说明的标签
        modelYearLabel = new JLabel("Enter the year of your model");
        makeLabel = new JLabel("Enter the make of your car");
        speedLabel = new JLabel("Enter the current speed of your car");

        // 创建文本字段
        modelTextField = new JTextField(5);
        makeTextField = new JTextField(5);
        speedTextField = new JTextField(5);

        // 创建按钮
        accelerateButton = new JButton("Accelerate");
        brakeButton = new JButton("Brake");

        // 为按钮添加动作侦听器
        accelerateButton.addActionListener(new AccelerateButtonListener());
        brakeButton.addActionListener(new BrakeButtonListener());

        // 不要在创建Car实例之前使用这些按钮
        accelerateButton.setEnabled(false);
        brakeButton.setEnabled(false);
        speedTextField.setEnabled(false);

        makeCarButton = new JButton("Make car");
        makeCarButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                String make = makeTextField.getText();
                String model = modelTextField.getText();
                // 一些奇怪的验证
                if (make == null || make.isBlank() || model == null || model.isBlank()) {
                    // 添加错误消息
                    return;
                }
                int year = Integer.parseInt(model);

                vehicle = new Car(year, make);

                makeCarButton.setEnabled(false);
                accelerateButton.setEnabled(true);
                brakeButton.setEnabled(true);
                speedTextField.setEnabled(true);
            }
        });

        // 创建一个带有GridLayout的JPanel对象，并使面板字段引用它。
        panel = new JPanel(new GridLayout(-1, 2));

        // 将标签、文本字段和按钮添加到面板
        panel.add(modelYearLabel);
        panel.add(modelTextField);
        panel.add(makeLabel);
        panel.add(makeTextField);

        panel.add(new JPanel());
        panel.add(makeCarButton);

        panel.add(speedLabel);
        panel.add(speedTextField);

        panel.add(accelerateButton);
        panel.add(brakeButton);
    }

    // AccelerateButtonListener是用于“加速”按钮的动作侦听器的私有内部类。
    private class AccelerateButtonListener implements ActionListener {

        // 当用户单击“加速”按钮时，actionPerformed方法执行。
        public void actionPerformed(ActionEvent e) {
            vehicle.accelerate();//使用我们之前创建的Car类的实例变量，可以调用Car类中的accelerate方法。
            speedTextField.setText(NumberFormat.getInstance().format(vehicle.speed));
        }
    }

    // BrakeButtonListener是用于“刹车”按钮的动作侦听器的私有内部类。
    private class BrakeButtonListener implements ActionListener {

        // 当用户单击“刹车”按钮时，actionPerformed方法执行。
        public void actionPerformed(ActionEvent e) {
            vehicle.brake();//使用我们之前创建的Car类的实例变量，可以调用Car类中的brake方法。
            speedTextField.setText(NumberFormat.getInstance().format(vehicle.speed));
        }
    }

    // main方法创建CarView的实例，这会导致窗口显示出来。
    public static void main(String[] args) {
        CarView cv = new CarView();
    }

    public class Car {

        private int yearModel; // yearModel是一个int，保存汽车的型号年份
        private String make; // make引用一个String对象，保存汽车的制造商。
        private double speed; // speed字段是一个double，保存汽车的当前速度。

        // 构造函数接受汽车的型号年份和制造商作为参数。应将这些值分配给对象的yearModel和make字段。
        // 速度也应分配为0。
        public Car(int model, String type) {
            this.yearModel = model;
            this.make = type;
            speed = 0.0; // 将速度设置为0。
        }

        // getModel方法
        // 负责：获取汽车的型号
        public int getModel() {


<details>
<summary>英文:</summary>

So, when I run you code, I get a `NullPointerException`.  Taking a closer look at the code I can see two issues which would cause this...

    public class CarView extends JFrame
    {
        //...
        private JTextField modelTextField; // To reference a model yeartext field
        private JTextField makeTextField; // To reference a make text field
        //...
        private final int carYear = Integer.parseInt(modelTextField.getText()); 
        private final String type = makeTextField.getText(); 

You can&#39;t get the values from `modelTextField` or `makeTextField` during the initialisation phase of the class, as the variables are `null` and even if they were, they&#39;d be empty as the component hasn&#39;t even been shown on the screen yet

Instead, you need to get the values at some other point in time - remember a GUI is event driven, not procedural or linear.

There&#39;s a bunch of other things as well, including, your layouts all over the place; there&#39;s no validation; you&#39;re not reporting back the speed to the user when it changes.

I could go on for quite some time, but instead, I&#39;m going to give you a little push

    import java.awt.GridLayout;
    import java.awt.event.ActionEvent;
    import java.awt.event.ActionListener;
    import java.text.NumberFormat;
    import javax.swing.JButton;
    import javax.swing.JFrame;
    import javax.swing.JLabel;
    import javax.swing.JPanel;
    import javax.swing.JTextField;

    public class CarView extends JFrame {

        private JPanel panel; //To reference a panel
        private JLabel modelYearLable; //To reference a model year Label
        private JLabel makeLable; //To reference a make Label
        private JLabel speedLable; //To reference a speed Label
        private JTextField modelTextField; // To reference a model yeartext field
        private JTextField makeTextField; // To reference a make text field
        private JTextField speedTextField; // To reference a speed text field
        private JButton accelerateButton; // To reference an accelerate button
        private JButton brakeButton; // To reference a brake button

        private JButton makeCarButton;

        //private final int carYear;
        //private final String type;

        private Car vehicle;

        //Constructor 
        public CarView() {
            //Set the window titile.
            setTitle(&quot;Car referencer!&quot;);

            //Set the size of the window.
            //setSize(WINDOW_WIDTH, WINDOW_HEIGHT);

            // Specify what happens when the close button is clicked
            setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

            //Build thhe panel and add it to the frame.
            buildPanel();

            //Add the panel to the frame&#39;s content pane.
            add(panel);

            //Display the window
            pack();
            setVisible(true);
        }

        //buildPanel()
        //Responisibilities: Adds labels, text fields, and buttons to the panel
        private void buildPanel() {
            //Create labels to display instructions 
            modelYearLable = new JLabel(&quot;Enter the year of your model&quot;);
            makeLable = new JLabel(&quot;Enter the make of your car&quot;);
            speedLable = new JLabel(&quot;Enter the current speed of your car&quot;);

            //Create text fields as well
            modelTextField = new JTextField(5);
            makeTextField = new JTextField(5);
            speedTextField = new JTextField(5);

            //Create the buttons
            accelerateButton = new JButton(&quot;Accelerate&quot;);
            brakeButton = new JButton(&quot;Brake&quot;);

            //Add an action listener to the buttons
            accelerateButton.addActionListener(new AccelerateButtonListener());
            brakeButton.addActionListener(new BrakeButtonListener());

            // Don&#39;t want to use these until AFTER you&#39;ve created a instance of Car
            accelerateButton.setEnabled(false);
            brakeButton.setEnabled(false);
            speedTextField.setEnabled(false);

            makeCarButton = new JButton(&quot;Make car&quot;);
            makeCarButton.addActionListener(new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    String make = makeTextField.getText();
                    String model = modelTextField.getText();
                    // Some funky validation
                    if (make == null || make.isBlank() || model == null || model.isBlank()) {
                        // Add an error message
                        return;
                    }
                    int year = Integer.parseInt(model);

                    vehicle = new Car(year, make);

                    makeCarButton.setEnabled(false);
                    accelerateButton.setEnabled(true);
                    brakeButton.setEnabled(true);
                    speedTextField.setEnabled(true);
                }
            });

            //Create a JPanel object and let the panel field reference it. 
            panel = new JPanel(new GridLayout(-1, 2));

            //Add the labels,text fields, and buttons to the panel
            panel.add(modelYearLable);
            panel.add(modelTextField);
            panel.add(makeLable);
            panel.add(makeTextField);

            panel.add(new JPanel());
            panel.add(makeCarButton);

            panel.add(speedLable);
            panel.add(speedTextField);

            panel.add(accelerateButton);
            panel.add(brakeButton);
        }

        //AccelerateButtonListener is an action listener private inner class for the Accelerate Button.
        private class AccelerateButtonListener implements ActionListener {

            //The acitonPerformed method executes when the user clicks on the Accelerate Button
            public void actionPerformed(ActionEvent e) {
                vehicle.accelerate();//Using the instance variable we made of Car earlier we can call the accelerate method from Car class.
                speedTextField.setText(NumberFormat.getInstance().format(vehicle.speed));
            }

        }

        //BrakeButtonListener is an action listener private inner class for the Brake Button.
        private class BrakeButtonListener implements ActionListener {

            //The actton Performed method executes when the user clicks on the Brake Button
            public void actionPerformed(ActionEvent e) {
                vehicle.brake();//Using the instance variable we made of Car earlier we can call the brake method from Car class.
                speedTextField.setText(NumberFormat.getInstance().format(vehicle.speed));
            }
        }

        //The main method creates an instance of the CarView, which causes it to display its window
        public static void main(String[] args) {
            CarView cv = new CarView();

        }

        public class Car {

            private int yearModel; //The yearModel is an int that holds the car&#39;s modcel year
            private String make; //The make references a String object that holds the make of the car.
            private double speed; //The speed field is a double that hold&#39;s thhe car&#39;s current speed.

            //Construtor that accepts the car&#39;s year model and make as arguments. These values should be assigned to the obkect&#39;s modelYear and make fields.
            //Also assign 0 to the speed
            public Car(int model, String type) {
                this.yearModel = model;
                this.make = type;
                speed = 0.0; //Set the speed to 0.
            }

            //Get and Set methods for modelYear, make and speed fields.
            //getModel
            //Responsibilities: gets the model of the car 
            public int getModel() {
                return yearModel;
            }

            //getMake
            //Responsibilities: gets the make of the car 
            public String getMake() {
                return make;
            }

            //getSpeed
            //Responsibilities: gets the speed of the car 
            public double getSpeedl() {
                return speed;
            }

            //accelerate()
            //Responsibilities: Shouyld add 8 to the speed each time it is called 
            public void accelerate() {
                speed = speed + 8; //Everytime this method is called, add 8 to the speed each time
            }

            //brake()
            //Responsibilities: Should  subtract 6 from the speed each time it is called.
            public void brake() {
                speed = speed - 6; //Everytime this method is called subtract 6 from speeed.
            }
        }
    }

I would suggest having a look at:

* [Laying Out Components Within a Container](https://docs.oracle.com/javase/tutorial/uiswing/layout/index.html)
* [How to Write an Action Listener](https://docs.oracle.com/javase/tutorial/uiswing/events/actionlistener.html)
* [How to Use Buttons, Check Boxes, and Radio Buttons](https://docs.oracle.com/javase/tutorial/uiswing/components/button.html)

It&#39;s really important that you take some time to better understand how an event driven environment works.  Create some buttons which don&#39;t do anything, attach some `ActionListener`s to them and use `System.out.println` to print out what&#39;s going on, this will help you get your head around it

</details>



# 答案2
**得分**: 0

包含两个类在同一个包中，这样你就可以在第二个类中创建第一个类的对象。
希望你明白！

<details>
<summary>英文:</summary>

include both class in same package so that you can create object of the first class in your second class.
hope you understand !


</details>

huangapple
  • 本文由 发表于 2020年8月8日 12:02:07
  • 转载请务必保留本文链接：https://go.coder-hub.com/63311682.html
  • java
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