[Java] 이자계산기 만들기_ swing 구현

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.FlowLayout;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

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

public class Answer3 extends JFrame implements ActionListener {

    private JLabel lblMoney;
    private JTextField tfMoney;
    private JLabel lblRate;
    private JTextField tfRate;

    private JButton btnCalc;

    private JTextField tfResult;

    public Answer3() {
        init();
        setDisplay();
        addListeners();
        showFrame();
    }


    private void init() {
        lblMoney = new JLabel("원금을 입력하시오", JLabel.LEFT);
        lblRate = new JLabel("이자율을 입력하시오", JLabel.LEFT);

        tfMoney = new JTextField(5);
        tfRate = new JTextField(5);

        btnCalc = new JButton("변환");

        tfResult = new JTextField(15);
        tfResult.setEditable(false);
        tfResult.setBackground(Color.WHITE);
    }

    private void setDisplay() {
        JPanel pnlLeft = new JPanel(new GridLayout(2, 1));
        JPanel pnlMoneyInfo = new JPanel(new FlowLayout(FlowLayout.LEFT));
        pnlMoneyInfo.add(lblMoney);
        JPanel pnlRateInfo = new JPanel(new FlowLayout(FlowLayout.LEFT));
        pnlRateInfo.add(lblRate);

        pnlLeft.add(pnlMoneyInfo);
        pnlLeft.add(pnlRateInfo);


        JPanel pnlRight = new JPanel(new GridLayout(2, 1));
        JPanel pnlMoney = new JPanel();
        pnlMoney.add(tfMoney);
        JPanel pnlRate = new JPanel();
        pnlRate.add(tfRate);
        pnlRight.add(pnlMoney);
        pnlRight.add(pnlRate);

        JPanel pnlSouth = new JPanel(new BorderLayout());

        JPanel pnlBtn = new JPanel();
        pnlBtn.add(btnCalc);
        JPanel pnlResult = new JPanel();
        pnlResult.add(tfResult);

        pnlSouth.add(pnlBtn, BorderLayout.NORTH);
        pnlSouth.add(pnlResult, BorderLayout.CENTER);

        add(pnlLeft, BorderLayout.WEST);
        add(pnlRight, BorderLayout.CENTER);
        add(pnlSouth, BorderLayout.SOUTH);
    }


    private void showFrame() {
        setTitle("이자 계산기");
        pack();
        setLocationRelativeTo(null);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setResizable(false);
        setVisible(true);
    }
    public void addListeners() {
        btnCalc.addActionListener(this);
    }

    @Override
    public void actionPerformed(ActionEvent ae) {
        JTextField input = tfMoney;

        try {
            double money = Double.parseDouble(tfMoney.getText());
            input = tfRate;
            double rate = Double.parseDouble(tfRate.getText());
            money += rate * money;
            tfResult.setText(String.valueOf(money));
        } catch (NumberFormatException e) {
            String target = "이자율";
            if(input == tfMoney) {
                target = "원금";
            }
            JOptionPane.showMessageDialog(
                    this,
                    target + "은 숫자로 입력해주세요",
                    "경고",
                    JOptionPane.WARNING_MESSAGE);
            input.requestFocus();
            input.selectAll();
        }

    }

    public static void main(String[] args) {
        new Answer3();
    }
}