将JTextField中输入的值传递给ActionListener

Question

GUI Purpose:

User should be able to input a number into the number field and click on the Square button. Once the button is clicked the program then calculates the square of whatever entered number and displays it in the Square field.

My Question: How exactly do I pass the number entered to use in the ActionListener class?

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

public class ButtonDemo extends JFrame {
    private JButton SquareButton;
    private JButton ResetButton;
    private JLabel NumberLabel;
    private JLabel SquareLabel;

    private JTextField NumberField;
    private  JTextField SquareField;


    public ButtonDemo()
    {

        Container container=getContentPane();
        container.setLayout(new FlowLayout());

        Panel panel1= new Panel();
        Panel panel2= new Panel();
        Panel panel3 = new Panel();
        panel1.setLayout(new FlowLayout());

        SquareButton = new  JButton("Square");
        ResetButton = new  JButton("Reset");

        NumberLabel = new JLabel("Number");
        SquareLabel = new JLabel("Square");

        NumberField = new JTextField();
        NumberField.setColumns(8);

        SquareField = new JTextField();
        SquareField.setColumns(8);

        panel3.add(panel1);
        panel3.add(panel2);

        panel1.add(NumberLabel);
        panel1.add(SquareLabel);
        panel1.add(NumberField);
        panel1.add(SquareField);

        panel2.add(SquareButton);
        panel2.add(ResetButton);



        container.add(panel1);
        container.add(panel2);
        container.add(panel3);

        bhandler bhandler1 = new bhandler();

        SquareButton.addActionListener(bhandler1);
        ResetButton.addActionListener(bhandler1);

        SquareField.addActionListener(bhandler1);
        NumberField.addActionListener(bhandler1);

        setSize(500,500);
        setVisible(true);
    }

    public static void main(String args[])
    {
        ButtonDemo application = new ButtonDemo();
        application.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
    private class bhandler implements ActionListener
    {

        public void actionPerformed(ActionEvent event)
        {
            if(event.getSource() == SquareButton)
            {

            }

        }

    }
}//end of  class Buttondemo

Answer 1

You have several options here:

• If you want the action class be a separate class then pass both the NumberField and the Square fields into the bhandler's class constructor;
• You can also implement the ActionListener as an inner class of the ButtonDemo class. In this case you'll have the access to the parent class private fields;
• My favorite way of defining action listeners is using Java 8 Lambdas. Here's how you add such an action listener:

SquareButton.addActionListener(e -> this::handleSquareButtonClicked);

Then add the following method to your ButtonDemo class:

private void handleSquareButtonClicked(ActionEvent event)
{
       <your event handling coded here.>
}

And here's the working example with your code using lambdas:

package examples;

import java.awt.Container;
import java.awt.FlowLayout;
import java.awt.Panel;
import java.awt.event.ActionEvent;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JTextField;

public class ButtonDemo extends JFrame 
{
    private JButton squareButton;
    private JButton resetButton;
    private JLabel numberLabel;
    private JLabel squareLabel;

    private JTextField numberField;
    private  JTextField squareField;


    public ButtonDemo()
    {

        Container container=getContentPane();
        container.setLayout(new FlowLayout());

        Panel panel1= new Panel();
        Panel panel2= new Panel();
        Panel panel3 = new Panel();
        panel1.setLayout(new FlowLayout());

        squareButton = new  JButton("Square");
        resetButton = new  JButton("Reset");

        numberLabel = new JLabel("Number");
        squareLabel = new JLabel("Square");

        numberField = new JTextField();
        numberField.setColumns(8);

        squareField = new JTextField();
        squareField.setColumns(8);

        panel3.add(panel1);
        panel3.add(panel2);

        panel1.add(numberLabel);
        panel1.add(squareLabel);
        panel1.add(numberField);
        panel1.add(squareField);

        panel2.add(squareButton);
        panel2.add(resetButton);

        container.add(panel1);
        container.add(panel2);
        container.add(panel3);

        squareButton.addActionListener(e -> handleSquareButtonClick(e));
        resetButton.addActionListener(e -> handleresetButtonClick(e));

        setSize(500,500);
        setVisible(true);
    }

    private void handleresetButtonClick(ActionEvent e) 
    {
        this.numberField.setText(null);
        this.squareField.setText(null);
    }

    private void handleSquareButtonClick(ActionEvent e)
    {
        try
        {
            double number = Double.parseDouble(this.numberField.getText());
            this.squareField.setText(String.valueOf(number * number));
        }
        catch (NumberFormatException ex)
        {
            System.out.println(ex.getMessage());
        }

    }

    public static void main(String args[])
    {
        ButtonDemo application = new ButtonDemo();
        application.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
}//end of  class Buttondemo

Please note that I've renamed some variables to adhere the Java naming convention.

Answer 2

In ButtonDemo constructor for attaching the listener , there is no need to attach the listener to the text fields , just for buttons like this :

SquareButton.addActionListener(new bhandler1(Integer.valueOf(NumberField.getText())));

In the handler class add a constructor like this

private class bhandler implements ActionListener
    {
        int userIput ;
        public bhandler(int userInput){
             this.userInput = userInput ;
        }

        public void actionPerformed(ActionEvent event)
        {
            if(event.getSource() == SquareButton)
            {
                // Do what ever you want with userInput 
            }

        }

    }