设计界面并完成事件。界面包括2个jlabel,2个textfield,实现在一个文本框中输入1个

数字，点击按钮后在另一个文本框中显示这个数字的平方。

以下是示例代码：

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

public class SquareCalculator implements ActionListener {
    private JFrame frame;
    private JLabel inputLabel, outputLabel;
    private JTextField inputField, outputField;
    private JButton calculateButton;

    public SquareCalculator() {
        // Create the JFrame and set its properties
        frame = new JFrame("Square Calculator");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(300, 150);
        frame.setLayout(new GridLayout(3, 2));

        // Create the input label and field
        inputLabel = new JLabel("Enter a number:");
        inputField = new JTextField();
        frame.add(inputLabel);
        frame.add(inputField);

        // Create the output label and field
        outputLabel = new JLabel("Result:");
        outputField = new JTextField();
        outputField.setEditable(false);
        frame.add(outputLabel);
        frame.add(outputField);

        // Create the calculate button and add an action listener to it
        calculateButton = new JButton("Calculate");
        calculateButton.addActionListener(this);
        frame.add(calculateButton);

        // Show the JFrame
        frame.setVisible(true);
    }

    public void actionPerformed(ActionEvent e) {
        // Get the input value from the input field
        String inputString = inputField.getText();

        try {
            // Convert the input value to a double and calculate its square
            double input = Double.parseDouble(inputString);
            double output = input * input;

            // Set the output value in the output field
            outputField.setText(String.valueOf(output));
        } catch (NumberFormatException ex) {
            // Show an error message if the input value is not a valid number
            JOptionPane.showMessageDialog(frame, "Please enter a valid number.", "Error", JOptionPane.ERROR_MESSAGE);
        }
    }

    public static void main(String[] args) {
        // Create a new SquareCalculator object
        new SquareCalculator();
    }
}

运行上述代码后，将会弹出一个界面，其中包含一个输入框、一个输出框和一个按钮。在输入框中输入一个数字，然后点击按钮，就会在输出框中显示这个数字的平方。如果输入的不是数字，则会弹出一个错误对话框提示用户输入一个有效的数字。