在写成员变量的时候，为什么super调用的父类成员变量只能写在等号右边

在子类中，我们可以通过 `super` 关键字来调用父类中的成员变量。但是，我们只能在等号右边给父类成员变量赋值，而不能在等号左边。

这是因为在 Java 中，成员变量的初始化是在构造方法之前完成的。在子类的构造方法中，如果我们试图给父类成员变量赋值，这些成员变量实际上已经被初始化了。因此，我们只能在等号右边给父类成员变量赋值，而不能在等号左边。

例如，假设我们有一个父类 `Animal` 和一个子类 `Cat`：

public class Animal {
    protected int age;
}

public class Cat extends Animal {
    public Cat(int age) {
        super.age = age; // 合法
        // age = super.age; // 非法，会导致编译错误
    }
}

在 `Cat` 的构造方法中，我们可以使用 `super.age = age` 来给父类成员变量赋值。但是，如果我们尝试使用 `age = super.age`，就会导致编译错误。

相关问题

1）所有类省略成员变量的修改器和读取器； （2）GeometricObject类的color、isFilled、createdDate属性分别表示图形的颜色、是否填充和创建日期； （3）GeometricObject类的toString()方法，返回描述图形颜色、是否填充信息的字符串（不需要返回创建时间）； （4）Rectangle类width和heigth属性分别表示矩形的宽和高； （5）Rectangle类的toString()方法返回描述矩形的颜色、是否填充、半径、面积信息（getArea()方法实现），注意调用父类的toString()方法获取矩形的颜色和是否填充信息； （6）Rectangle类的equals方法用于判断两个矩形对象是否相等，判断依据：如果两个矩形的面积相等，则二者相等； （7）Circle类中的toString()方法返回描述圆的颜色、是否填充、半径、面积信息（getArea()方法实现），注意调用父类的toString()方法获取圆的颜色和是否填充信息； （8）Circle类的equals方法用于判断两个圆对象是否相等，判断依据：如果两个圆的半径相等，则二者相等； （9）Color类的三个成员变量red、green、blue，分别表示颜色中红绿蓝三原色的透明度值，三者的取值范围为[0.0 - 1.0]，带下划线部分表示静态常量，等号右侧给出了三原色的透明度值； （10）Color类的equals()方法用于判断两个颜色对象是否相等，判断依据：如果两个颜色的三原色透明度值相等，则二者相等； （11）Color类的toString()方法，用于返回描述颜色的字符串。

下面是根据题目要求编写的GeometricObject、Rectangle、Circle和Color类的代码：

GeometricObject类：

import java.util.Date;

public class GeometricObject {
    private String color;
    private boolean isFilled;
    private Date createdDate;

    public GeometricObject() {
        createdDate = new Date();
    }

    public GeometricObject(String color, boolean isFilled) {
        this.color = color;
        this.isFilled = isFilled;
        createdDate = new Date();
    }

    public String getColor() {
        return color;
    }

    public void setColor(String color) {
        this.color = color;
    }

    public boolean isFilled() {
        return isFilled;
    }

    public void setFilled(boolean filled) {
        isFilled = filled;
    }

    public Date getCreatedDate() {
        return createdDate;
    }

    public void setCreatedDate(Date createdDate) {
        this.createdDate = createdDate;
    }

    public String toString() {
        return "color: " + color + ", filled: " + isFilled;
    }
}

Rectangle类：

public class Rectangle extends GeometricObject {
    private double width;
    private double height;

    public Rectangle() {
    }

    public Rectangle(double width, double height) {
        this.width = width;
        this.height = height;
    }

    public Rectangle(double width, double height, String color, boolean isFilled) {
        super(color, isFilled);
        this.width = width;
        this.height = height;
    }

    public double getWidth() {
        return width;
    }

    public void setWidth(double width) {
        this.width = width;
    }

    public double getHeight() {
        return height;
    }

    public void setHeight(double height) {
        this.height = height;
    }

    public double getArea() {
        return width * height;
    }

    public String toString() {
        return super.toString() + ", width: " + width + ", height: " + height + ", area: " + getArea();
    }

    public boolean equals(Rectangle rectangle) {
        return getArea() == rectangle.getArea();
    }
}

Circle类：

public class Circle extends GeometricObject {
    private double radius;

    public Circle() {
    }

    public Circle(double radius) {
        this.radius = radius;
    }

    public Circle(double radius, String color, boolean isFilled) {
        super(color, isFilled);
        this.radius = radius;
    }

    public double getRadius() {
        return radius;
    }

    public void setRadius(double radius) {
        this.radius = radius;
    }

    public double getArea() {
        return Math.PI * radius * radius;
    }

    public String toString() {
        return super.toString() + ", radius: " + radius + ", area: " + getArea();
    }

    public boolean equals(Circle circle) {
        return radius == circle.getRadius();
    }
}

Color类：

public class Color {
    public static final double RED = 1.0;
    public static final double GREEN = 1.0;
    public static final double BLUE = 1.0;

    private double red;
    private double green;
    private double blue;

    public Color() {
        red = RED;
        green = GREEN;
        blue = BLUE;
    }

    public Color(double red, double green, double blue) {
        this.red = red;
        this.green = green;
        this.blue = blue;
    }

    public double getRed() {
        return red;
    }

    public void setRed(double red) {
        this.red = red;
    }

    public double getGreen() {
        return green;
    }

    public void setGreen(double green) {
        this.green = green;
    }

    public double getBlue() {
        return blue;
    }

    public void setBlue(double blue) {
        this.blue = blue;
    }

    public boolean equals(Color color) {
        return red == color.getRed() && green == color.getGreen() && blue == color.getBlue();
    }

    public String toString() {
        return "Red: " + red + ", Green: " + green + ", Blue: " + blue;
    }
}

帮我用Java语言写一份实现一个简易计算器的代码，要求如下： （1）输入：不含变量的数学表达式的中缀形式，可以接受的操作符包括+、-、*、/、%和（）以及小数点。 （2）输出：如果表达式正确，则输出表达式的结果，如果表达式非法，则输出错误信息。 （3）能够正确运算带括号的数学表达式的中缀形式。 （4）可以借助栈来实现，使用图形用户界面。

以下是一个简单的Java代码实现一个基础计算器，支持加、减、乘、除、取模和括号运算：

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

public class Calculator extends JFrame implements ActionListener {

    private JTextField displayTextField;

    public Calculator() {
        super("简易计算器");

        JPanel mainPanel = new JPanel();
        mainPanel.setLayout(new BorderLayout());

        displayTextField = new JTextField();
        displayTextField.setEditable(false);
        mainPanel.add(displayTextField, BorderLayout.NORTH);

        JPanel buttonPanel = new JPanel();
        buttonPanel.setLayout(new GridLayout(4, 4));

        String[] buttonLabels = {"7", "8", "9", "/", "4", "5", "6", "*", "1", "2", "3", "-", "0", ".", "=", "+"};

        for (int i = 0; i < buttonLabels.length; i++) {
            JButton button = new JButton(buttonLabels[i]);
            button.addActionListener(this);
            buttonPanel.add(button);
        }

        mainPanel.add(buttonPanel, BorderLayout.CENTER);

        setContentPane(mainPanel);
    }

    private String calculate(String expression) {
        // Check if expression is null or empty
        if (expression == null || expression.isEmpty()) {
            return "错误：表达式为空";
        }

        // Check if expression contains only valid characters
        if (!expression.matches("[0-9+\\-*/%.()]+")) {
            return "错误：表达式包含无效字符";
        }

        // Check if expression has balanced parentheses
        int parenthesesCount = 0;
        for (int i = 0; i < expression.length(); i++) {
            char c = expression.charAt(i);
            if (c == '(') {
                parenthesesCount++;
            } else if (c == ')') {
                parenthesesCount--;
            }
            if (parenthesesCount < 0) {
                return "错误：表达式包含未匹配的右括号";
            }
        }
        if (parenthesesCount > 0) {
            return "错误：表达式包含未匹配的左括号";
        }

        // Evaluate expression
        try {
            double result = evaluate(expression);
            return Double.toString(result);
        } catch (IllegalArgumentException e) {
            return "错误：" + e.getMessage();
        }
    }

    private double evaluate(String expression) {
        Stack<Double> operands = new Stack<>();
        Stack<Character> operators = new Stack<>();

        int i = 0;
        while (i < expression.length()) {
            char c = expression.charAt(i);
            if (Character.isDigit(c) || c == '.') {
                StringBuilder operandBuilder = new StringBuilder();
                while (i < expression.length() && (Character.isDigit(expression.charAt(i)) || expression.charAt(i) == '.')) {
                    operandBuilder.append(expression.charAt(i));
                    i++;
                }
                double operand = Double.parseDouble(operandBuilder.toString());
                operands.push(operand);
            } else if (c == '+' || c == '-' || c == '*' || c == '/' || c == '%') {
                while (!operators.isEmpty() && hasHigherOrEqualPrecedence(c, operators.peek())) {
                    evaluateTop(operands, operators);
                }
                operators.push(c);
                i++;
            } else if (c == '(') {
                operators.push(c);
                i++;
            } else if (c == ')') {
                while (!operators.isEmpty() && operators.peek() != '(') {
                    evaluateTop(operands, operators);
                }
                if (operators.isEmpty()) {
                    throw new IllegalArgumentException("表达式包含未匹配的右括号");
                }
                operators.pop();
                i++;
            } else {
                throw new IllegalArgumentException("表达式包含无效字符 " + c);
            }
        }

        while (!operators.isEmpty()) {
            evaluateTop(operands, operators);
        }

        if (operands.size() != 1 || !operators.isEmpty()) {
            throw new IllegalArgumentException("表达式无效");
        }

        return operands.pop();
    }

    private boolean hasHigherOrEqualPrecedence(char operator1, char operator2) {
        return (operator1 == '*' || operator1 == '/' || operator1 == '%') || (operator1 == '+' || operator1 == '-') && (operator2 == '+' || operator2 == '-');
    }

    private void evaluateTop(Stack<Double> operands, Stack<Character> operators) {
        double operand2 = operands.pop();
        double operand1 = operands.pop();
        char operator = operators.pop();
        double result = applyOperator(operator, operand1, operand2);
        operands.push(result);
    }

    private double applyOperator(char operator, double operand1, double operand2) {
        switch (operator) {
            case '+':
                return operand1 + operand2;
            case '-':
                return operand1 - operand2;
            case '*':
                return operand1 * operand2;
            case '/':
                if (operand2 == 0) {
                    throw new IllegalArgumentException("除数不能为零");
                }
                return operand1 / operand2;
            case '%':
                if (operand2 == 0) {
                    throw new IllegalArgumentException("除数不能为零");
                }
                return operand1 % operand2;
            default:
                throw new IllegalArgumentException("无效操作符 " + operator);
        }
    }

    public void actionPerformed(ActionEvent e) {
        String command = e.getActionCommand();
        if (command.equals("=")) {
            String expression = displayTextField.getText();
            String result = calculate(expression);
            displayTextField.setText(result);
        } else {
            displayTextField.setText(displayTextField.getText() + command);
        }
    }

    public static void main(String[] args) {
        Calculator calculator = new Calculator();
        calculator.setSize(300, 400);
        calculator.setLocationRelativeTo(null);
        calculator.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        calculator.setVisible(true);
    }
}

该代码中使用了图形用户界面，包含数字、操作符和等号按钮，以及一个文本框来显示表达式和结果。当用户点击等号按钮时，程序会调用 `calculate` 方法来计算表达式的值，然后将结果显示在文本框中。 `calculate` 方法使用栈来处理表达式，首先检查表达式的合法性，然后使用 Dijkstra's Shunting Yard Algorithm 将中缀表达式转换为后缀表达式，并计算结果。