從兩個textfield獲取兩個數據，然后點擊不同的button計算加減乘除，結果顯示在textarea，
但是textField.getText()只能獲取String，而且TextArea.setText（）只能給String
代碼如下

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyEvent;
import java.awt.image.Kernel;

public class e4_1 {


    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                caculateGUI();
            }
        });
    }
    private static void caculateGUI(){
        JFrame.setDefaultLookAndFeelDecorated(true);
        JFrame frame = new JFrame();
        frame.setTitle("caculator");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setTitle("caculator");
        frame.setLayout(new GridLayout(5,2));

        //add label
        frame.add(new JLabel("1st num"));
        frame.add(new JLabel("2nd num"));

        //creat and add textfield
        JTextField textField1 = new JTextField();
        JTextField textField2 = new JTextField();
        frame.add(textField1);
        frame.add(textField2);

        //transfer String to int for caulating
        String m = textField1.getText();
        String n = textField2.getText();
        int a = Integer.parseInt(m);
        int b = Integer.parseInt(n);

        //add result_label and result_textarea
        frame.add(new JLabel("result"));
        JTextArea jTextArea = new JTextArea();
        frame.add(jTextArea);


        //creat buttons
        JButton jButton1 = new JButton("+");
        JButton jButton2 = new JButton("-");
        JButton jButton3 = new JButton("*");
        JButton jButton4 = new JButton("/");


        //buttonActions
        frame.add(jButton1);
        jButton1.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                jTextArea.setText(Integer.toString(a+b));
            }
        });

        frame.add(jButton2);
        jButton2.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                jTextArea.setText(Integer.toString(a-b));
            }
        });

        frame.add(jButton3);
        jButton3.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                jTextArea.setText(Integer.toString(a*b));
            }
        });

        frame.add(jButton4);
        jButton4.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                jTextArea.setText(Integer.toString(a/b));
            }
        });

        int frameWidth = 400;
        int frameHeight = 800;
        Dimension screenSize =
                Toolkit.getDefaultToolkit().getScreenSize();
        frame.setBounds((int)screenSize.getWidth() - frameWidth,0,frameWidth,frameHeight);
        frame.setVisible(true);

    }

}