具加減功能的簡易Java GUI計算機

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

public class calculate extends JFrame {

 private JPanel contentPane;
 private JTextField textFieldForexpression; 
 private JButton btnEnter = new JButton();
 private JLabel lblInput = new JLabel("Please input the math expression");
 
 public calculate()
 {
  setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  setBounds(100, 100, 700, 500);
  contentPane = new JPanel();
  contentPane.setBackground(new Color(238, 238, 209 ));
  contentPane.setBorder(BorderFactory.createEtchedBorder(Color.red, Color.darkGray));
  contentPane.setLayout(null);
  setContentPane(contentPane);
  
  textFieldForexpression = new JTextField();
  textFieldForexpression.setBounds(200, 200, 300, 30);
  textFieldForexpression.setVisible(true);
  contentPane.add(textFieldForexpression);
  
  btnEnter.setHorizontalAlignment(SwingConstants.CENTER);
  btnEnter.addActionListener(new ActionListener() {
   public void actionPerformed(ActionEvent arg0) 
   {
    if (!hasInputError())
     JOptionPane.showMessageDialog(null, "請輸入正確的算式");
    else
    { 
     if (getAnswer() == Float.MAX_VALUE)
      return;
     int i = (int) getAnswer();
     if ( getAnswer() == (int) getAnswer())
      JOptionPane.showMessageDialog(null, "答案是： "  + i);
     else
      JOptionPane.showMessageDialog(null, "答案是： "  + getAnswer());
    }
   }
  });
  btnEnter.setBounds(300, 300, 100, 40);
  btnEnter.setText("Enter");
  btnEnter.setVisible(true);
  btnEnter.setForeground(Color.blue);
  btnEnter.setFont(new Font("Vrinda", Font.BOLD, 20));
  contentPane.add(btnEnter);
  
  lblInput.setFont(new Font("Aparajita", Font.BOLD, 24));
  lblInput.setBounds(150, 80, 400, 35);
  lblInput.setHorizontalAlignment(JLabel.CENTER);
  lblInput.setVisible(true);
  lblInput.setForeground(new Color(255, 71, 0));
  contentPane.add(lblInput);
 } //end of calculate()
 
 public boolean isFloat(String str)
 {
  try {
   Float.parseFloat(str);
   return true;
  }   catch (NumberFormatException e) {
   return false;
  }
 } //end of boolean isFloat(String str)
 
 public boolean hasInputError()
 {
  String s = textFieldForexpression.getText();
  if (s.length() == 0)
   return false;
  else
  for (int i = 0; i < s.length() - 1; i ++ )
  {
   if ((s.charAt(i) == '+' || s.charAt(i) == '-') && (s.charAt(i + 1) == '+' || s.charAt(i + 1) == '-'))
    return false;
   else if( (s.charAt(i) >= '0' && s.charAt(i) <= '9') ||
     s.charAt(i) == '+' || s.charAt(i) == '-' || s.charAt(i) == '.' );
   else
    return false;
  }
  if ( !(s.charAt(s.length() - 1) >= '0' && s.charAt(s.length() - 1) <= '9'))
   return false;
  return true;
 }  //end of boolean hasInputError()
 
 public float getAnswer()
 {
  int previousJiaJianIndex = 0;
  String s = textFieldForexpression.getText();
  float result = 0;
  
  int i;
  for ( i = 1; i < s.length(); i ++)
  {
   if (s.charAt(i) == '+' || s.charAt(i) == '-')
   { 
    if (isFloat(s.substring(previousJiaJianIndex, i)))
     result += Float.parseFloat(s.substring(previousJiaJianIndex, i));
    else
    { 
     JOptionPane.showMessageDialog(null, "請輸入正確的算式");
     return Float.MAX_VALUE;
    }
    previousJiaJianIndex = i; 
   }
  }
  
  if (previousJiaJianIndex == 0)
  { 
   JOptionPane.showMessageDialog(null, "請輸入正確的算式");
   return Float.MAX_VALUE;
  }
  else
   return result + Float.parseFloat(s.substring(previousJiaJianIndex, i));
 } //end of float getAnswer()

 public static void main(String args[]) {

  EventQueue.invokeLater(new Runnable() {
   public void run() {
    try {
     calculate frame = new calculate();
     frame.setVisible(true);
    } catch (Exception e) {
     e.printStackTrace();
    }
   }
  });
 } //end of static void main
 
}