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

Question

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!

Answer 1

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'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'd be empty as the component hasn'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's a bunch of other things as well, including, your layouts all over the place; there's no validation; you're not reporting back the speed to the user when it changes.

I could go on for quite some time, but instead, I'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("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
        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("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());

        // Don't want to use these until AFTER you've created a instance of Car
        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();
                // 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'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.
        }
    }
}

I would suggest having a look at:

• Laying Out Components Within a Container
• How to Write an Action Listener
• How to Use Buttons, Check Boxes, and Radio Buttons

It's really important that you take some time to better understand how an event driven environment works. Create some buttons which don't do anything, attach some ActionListener s to them and use System.out.println to print out what's going on, this will help you get your head around it

Answer 2

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

Answer 3

You can call an method from another class by

File: A.java

public class A {  
  private void message(){System.out.println("hello java"); }  
}  

File: MethodCall.java

import java.lang.reflect.Method;  
public class MethodCall{  
public static void main(String[] args)throws Exception{  
  
    Class c = Class.forName("A");  
    Object o= c.newInstance();  
    Method m =c.getDeclaredMethod("message", null);  
    m.setAccessible(true);  
    m.invoke(o, null);  
}  
}  

Output: hello java

Another example to call a parameterized private method from another class Let's see the example to call a parameterized private method from another class

File: A.java

class A{  
private void cube(int n){System.out.println(n*n*n);}  
}  

File: M.java

import java.lang.reflect.*;  
class M{  
public static void main(String args[])throws Exception{  
Class c=A.class;  
Object obj=c.newInstance();  
  
Method m=c.getDeclaredMethod("cube",new Class[]{int.class});  
m.setAccessible(true);  
m.invoke(obj,4);  
}}  

Output:64