我的计算器有问题；一切运行良好。但是当输入操作员时，我需要之前输入的号码以及操作员留在屏幕上。我还需要一种方法来修复在输入运算符后按下等于时发生的错误。
任何帮助，将不胜感激。谢谢
import java.awt.*;
import javax.swing.*;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
import java.awt.event.*;
import javax.swing.JLabel;
public class Chelculator extends JPanel{
//creates buttons
private JTextField screen; //creates the screen
private JButton delete, clear; //creates clear and delete buttons
private JButton zero, one, two, three, four, five, six, seven, eight, nine; //creates number buttons
private JButton plus, minus, square, divide, multiply, squareroot, point, equals; //creates operation buttons
private JButton mclear, mplus, mminus, mrecall; //creates memory buttons
double Mem;
double Subtract;
double number1, number2, result;
char choice;
static String lastCommand = null;
public Chelculator(){
JPanel buttons = new JPanel();
buttons.setBackground(Color.pink); //changes background to pink
plus = new JButton("+");
minus = new JButton("-");
square = new JButton("^");
clear = new JButton("CE");
divide = new JButton("÷");
multiply = new JButton("x");
squareroot = new JButton("√");
delete = new JButton("DE");
mclear = new JButton("MC");
mplus = new JButton("M+");
mminus = new JButton("M-");
mrecall = new JButton("MR");
one = new JButton("1");
two = new JButton("2");
three = new JButton("3");
four = new JButton("4");
five = new JButton("5");
six = new JButton("6");
seven = new JButton("7");
eight = new JButton("8");
nine = new JButton("9");
zero = new JButton("0");
equals = new JButton("=");
point = new JButton(".");
buttons.setLayout(new GridLayout(6,4)); //creates a grid for buttons
//adds buttons into calculator
buttons.add(mclear);
buttons.add(mrecall);
buttons.add(mplus);
buttons.add(mminus);
buttons.add(clear);
clear.setForeground(Color.red);
buttons.add(delete);
buttons.add(square);
buttons.add(squareroot);
buttons.add(plus);
buttons.add(minus);
buttons.add(divide);
buttons.add(multiply);
buttons.add(seven);
buttons.add(eight);
buttons.add(nine);
buttons.add(zero);
buttons.add(four);
buttons.add(five);
buttons.add(six);
buttons.add(point);
buttons.add(one);
buttons.add(two);
buttons.add(three);
buttons.add(equals);
EventListener Listener = new EventListener();
plus.addActionListener(Listener);
minus.addActionListener(Listener);
square.addActionListener(Listener);
clear.addActionListener(Listener);
divide.addActionListener(Listener);
multiply.addActionListener(Listener);
squareroot.addActionListener(Listener);
delete.addActionListener(Listener);
mclear.addActionListener(Listener);
mplus.addActionListener(Listener);
mminus.addActionListener(Listener);
mrecall.addActionListener(Listener);
one.addActionListener(Listener);
two.addActionListener(Listener);
three.addActionListener(Listener);
four.addActionListener(Listener);
five.addActionListener(Listener);
six.addActionListener(Listener);
seven.addActionListener(Listener);
eight.addActionListener(Listener);
nine.addActionListener(Listener);
zero.addActionListener(Listener);
equals.addActionListener(Listener);
point.addActionListener(Listener);
setPreferredSize(new Dimension(270,280));
screen = new JTextField();
screen.setEditable(false);
JPanel screens = new JPanel();
screens.setLayout(new BorderLayout());
screens.setBackground(Color.pink);
screens.setPreferredSize(new Dimension(270,40)); //creates a panel and puts screen in it. This allows the size of the screen to be changed.
screens.add(screen);
setLayout(new BorderLayout());
add(screens, BorderLayout.NORTH);
add(buttons, BorderLayout.CENTER);
}
private class EventListener implements ActionListener{
public void actionPerformed(ActionEvent e){
String input = e.getActionCommand();
if(Character.isDigit(input.charAt(0))){
screen.setText(screen.getText() + input);
}
if(e.getSource() == equals){
equal();
}
if(e.getSource() == plus){
number1 = Double.parseDouble(screen.getText());
screen.setText("");
choice = '+';
}
if(e.getSource() == minus){
number1 = Double.parseDouble(screen.getText());
screen.setText("");
choice = '-';
}
if(e.getSource() == square){
screen.setText("^");
}
if(e.getSource() == squareroot){
screen.setText("√");
}
if(e.getSource() == divide){
number1 = Double.parseDouble(screen.getText());
screen.setText("");
choice = '/';
}
if(e.getSource() == multiply){
number1 = Double.parseDouble(screen.getText());
screen.setText("");
choice = 'x';
}
if(e.getSource() == clear){
screen.setText(null);
number1 = 0;
number2 = 0;
}
if(e.getSource() == delete){
screen.getText().substring(0, (screen.getText().length() - 1));
}
if(e.getSource() == point){
screen.setText(screen.getText() + ".");
}
//MEMORY
if(e.getSource() == mplus){
Mem = Mem + Double.parseDouble(screen.getText());
}
if(e.getSource() == mrecall){
screen.setText(Double.toString(Mem));
}
if(e.getSource() == mminus){
Subtract = Double.parseDouble(screen.getText());
Mem = Mem - Subtract;
}
if(e.getSource() == mclear){
Mem = 0;
}
}
public void equal(){
number2 = Double.parseDouble(screen.getText());
if (choice == '+'){
result = number1 + number2;
number1 = 0;
number2 = 0;
}
if (choice == '-'){
result = number1 - number2;
number1 = 0;
number2 = 0;
}
if (choice == '/'){
result = number1 / number2;
number1 = 0;
number2 = 0;
}
if (choice == '*'){
result = number1 * number2;
number1 = 0;
number2 = 0;
}
screen.setText(Double.toString(result));
}
private void assign(String no){
if(screen.getText().equals("0"))
screen.setText(no);
else if(lastCommand == "="){
screen.setText(no);
lastCommand = null;
}
else
screen.setText(screen.getText()+no);
}
}
}