java - 多项式 GUI 不起作用

Question

我必须制作一个对多项式执行运算的 GUI，但我不断收到无法摆脱的 NullPointerExceptions。在输出上它没有显示任何内容。我尝试调试我的程序，据我所知，我从键盘插入的多项式在某种程度上没有被处理到系统中并被视为空，即使它们不应该如此。我怎样才能让我的程序运行？我需要尽快得到答复。

如果你向下滚动，你会发现我得到的异常，但这取决于我执行的操作。例如，加法不会返回异常，而是返回 0。

这里是所有 4 个类，供任何想要将它们放入编译器并测试它们的人使用:

类多项式

public class Polynomial {
    private int[] coef;
    private int degree;
    public boolean polyError = false;

    public Polynomial(int deg, int[] c)
    {
        degree = deg;
        coef = new int [deg +  1];
        for(int i = 0; i <= deg; i++)
        {
            coef[i] = c[i];
        }
    }

    public Polynomial() {

    }

    public Polynomial stringToPolynomial( String input)
    {   try {
            if(input.indexOf('x')>=0)
            {
                // first you need to configurate the string
                // write 1 for the powers where the coeff is 1
                input = input.replace("+x","+1x"); // positive
                input = input.replace("-x","-1x"); // negative
                // replace all - with +-
                input= input.replace("-","+-");
                // replace x with x^1
                input = input.replace("x+","x^1+"); // positive
                input = input.replace("x+-","x^1+-"); // negative
                // if the last element of the string is x then add ^1
                if(input.charAt(input.length()-1)=='x') input = input+"^1";
                //if the first element of the string is x add 1 before it
                if(input.charAt(0) == 'x') input = "1"+input;
                if(input.charAt(0) == '+') input = input.substring(1);
                // seconf you split the string and get the powers and coef
                // split the string
                String[] parts = input.split("\\+");
                //get the degree
                String[] dg = parts[0].split("x\\^");
                int degree = Integer.parseInt(dg[1]);
                int[] coeff = new int[degree+1];

                for (String part : parts)
                {
                    //delete all ^ characters
                    part=part.replace("^","");
                    //split the string c
                    String[] c= part.split("x");
                    try
                    {
                        coeff[Integer.parseInt(c[1])] = Integer.parseInt(c[0]);
                    }
                    catch(ArrayIndexOutOfBoundsException e)
                    {
                        coeff[0] = Integer.parseInt(c[0]);
                    }
                }
                return new Polynomial(degree,coeff);
            }
            else{
                int degree = 0;
                int[] coeff = new int[degree+1];
                coeff[0] = Integer.parseInt(input);
                return new Polynomial(degree,coeff);
            }
    }
    catch(NumberFormatException e)
    {
        polyError = true;
        return new Polynomial();
    }
    catch(ArrayIndexOutOfBoundsException e)
    {
        polyError = true;
        return new Polynomial();
    }
}
    // Extract from the string inserted by the user the coef and the degree of the polynomials
   /* public Polynomial (String input) {

        // Bring the input polynomial string to a convenient form:
        input = input.replaceAll("\\*","");
        input = input.replaceAll("-","+-");
// Split polynomial into "raw" coef (ceofficient + degree)
       String[] inputCoef = input.split("\\+");
// Split each "raw" coefficient and determine the coefficient
        for (String s: inputCoef) {
            int power = 0;
            if (s.length() > 0)
            {
                if (s.endsWith("x")) {
                    power = 1;
                }
                String[] temp = s.split("x");
                if (temp.length > 1)
                {
                    if (temp[1].indexOf('^') != -1) {
                        temp[1] = temp[1].replaceAll("\\^", "");
                    }
                    power = Integer.parseInt(temp[1]);
                    if (temp[0].length() == 0) {
                        this.coef[power] += 1;
                    } else {
                        this.coef[power] += Integer.parseInt(temp[0]);
                    }
                }
                else if (s.equals("x"))
                {
                    this.coef[1] += 1;
                }
                else if (temp[0].equals("-"))
                {
                    this.coef[power] -= 1;
                }
                else
                {
                    this.coef[power] += Integer.parseInt(temp[0]);
                }
            }
            this.degree = (power > this.degree ? power : this.degree);
        }*/


    // Write the string format of the polynomial at the output
    public String toString() {
        if (this.degree == 0)
            return " " + coef[0];
        if (this.degree == 1)
            return coef[1] + " x+ " + coef[0];
        String s = coef[this.degree] + " x^ " + this.degree;
        for (int i = this.degree - 1; i >= 0; i--) {
            if (coef[i] == 0)
                    continue;
            else if (coef[i] > 0)
                    s = s + " + " + (coef[i]);
            else if (coef[i] < 0)
                    s = s + " - " + (-coef[i]);
            if (i == 1)
                    s = s + "x";
            else if (i > 1)
                    s = s + " x^ " + i;
        }
        return s;
    }

    // Method that returns the degree of a polynomial
    public int getDegree() {
        int deg = 0;
        for (int i = 0; i < coef.length; i++)
            if (this.coef[i] != 0)
                deg = i;
        return deg;
    }

    // Addition
    public Polynomial add (Polynomial p) {
        int maxDegree = this.degree > p.degree ? this.degree : p.degree;
        int[] sumCoef = new int[maxDegree + 1];
        for (int i = 0; i < this.degree; i++)
            sumCoef[i] += this.coef[i];
        for (int i = (this.degree + 1); i < p.degree; i++)
            sumCoef[i] = p.coef[i];
        return new Polynomial(maxDegree, sumCoef);
        }

    // Differentiation
        public void differentiate()
        {
            this.coef[0] = 0;
            for (int i = 1; i <= this.degree; i++) {
                if (this.coef[i] != 0)
                {
                    this.coef[(i - 1)] = (this.coef[i] * i);
                    this.coef[i] = 0;
                }
            }
            this.degree = (this.degree > 0 ? this.degree - 1 : 0);
        }

    public boolean isEqual (Polynomial p) {
        if (this.degree != p.degree) return false;
        for (int i = this.degree; i >= 0; i--)
            if (this.coef[i] != p.coef[i]) return false;
        return true;
    }

    // Multiplication
    public Polynomial multiply (Polynomial p) {
        int[] mulCoef = new int[this.degree + p.degree + 1];
        int mulDegree = 0;
        for (int i = this.degree; i >= 0; i--) {
            for (int j = p.degree; j >= 0; j--) {
                if ((this.coef[i] != 0) && (p.coef[j] != 0))
                {
                    mulCoef[(i + j)] += this.coef[i] * p.coef[j];
                    mulDegree = i + j > mulDegree ? i + j : mulDegree;
                }
            }
        }
        this.degree = mulDegree;
        this.coef = mulCoef;
        return new Polynomial(mulDegree, mulCoef);
    }

    // Subtraction
    public void subtract(Polynomial p)
    {
        int subDegree = 0;
        int maxGrade = this.degree > p.degree ? this.degree : p.degree;
        this.degree = maxGrade;
        for (int i = maxGrade; i >= 0; i--) {
            this.coef[i] -= p.coef[i];
        }
        if (this.coef[maxGrade] == 0)
        {
            for (int j = maxGrade - 1; j >= 0; j--) {
                subDegree = this.coef[j] > 0 ? j : subDegree;
            }
            this.degree = subDegree;
        }
        else
        {
            this.degree = maxGrade;
        }
    }

    // Calculate value at a given point x
    public int calculateValue (int x) {
        int val = 0;
        for (int i = this.degree; i >= 0; i--)
            val += this.coef[i] * Math.pow(x, i);
        return val;
    }
}

类 View

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

public class View extends JFrame {
    private JTextField insertP1 = new JTextField(50);
    private JTextField insertP2 = new JTextField(50);
    private JTextField showResult = new JTextField(50);
    private JButton addBtn = new JButton("Add");
    private JButton subBtn = new JButton("Subtract");
    private JButton mulBtn = new JButton("Multiply");
    private JButton diffBtn = new JButton("Differentiate");
    private JButton calcBtn = new JButton("Calculate value");
    private JButton eqBtn = new JButton("Check for equality");
    private JButton clearBtn = new JButton("Clear");

    public View() {
        // Layout the components.
        JPanel polynomial1 = new JPanel();
        polynomial1.setLayout(new FlowLayout());
        polynomial1.add(new JLabel("Polynomial 1: "));
        polynomial1.add(insertP1);

        JPanel polynomial2 = new JPanel();
        polynomial2.setLayout(new FlowLayout());
        polynomial2.add(new JLabel("Polynomial 2: "));
        polynomial2.add(insertP2);

        JPanel operations = new JPanel();
        operations.setLayout(new FlowLayout());
        operations.add(addBtn);
        operations.add(subBtn);
        operations.add(mulBtn);
        operations.add(diffBtn);
        operations.add(calcBtn);
        operations.add(eqBtn);
        operations.setComponentOrientation(ComponentOrientation.LEFT_TO_RIGHT);

        JPanel result = new JPanel();
        result.setLayout(new FlowLayout());
        result.add(new JLabel("Result: "));
        result.add(showResult);
        result.add(clearBtn);

        // Finalize layout
        JPanel panel1 = new JPanel(); // panel where the user inserts the value of the first polynomial
        panel1.setLayout(new FlowLayout());
        panel1.add(polynomial1);

        JPanel panel2 = new JPanel(); // panel where the user insert the value of the second polynomial
        panel2.setLayout(new FlowLayout());
        panel2.add(polynomial2);

        JPanel panel3 = new JPanel(); // panel where the operations to be selected are placed
        panel3.setLayout(new FlowLayout());
        panel3.add(operations);

        JPanel panel4 = new JPanel(); // panel where the result will be displayed
        panel4.setLayout(new FlowLayout());
        panel4.add(result);

        final JPanel finalPanel = new JPanel(); // the final panel
        finalPanel.setLayout(new BoxLayout(finalPanel, BoxLayout.Y_AXIS));
        finalPanel.add(panel1);
        finalPanel.add(panel2);
        finalPanel.add(panel3);
        finalPanel.add(panel4);
        this.setContentPane(finalPanel);
        this.pack();
        this.setResizable(false);
        this.setTitle("Polynomial Processing");
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        showResult.setEditable(false);
    }

        public void clear() {
        insertP1.setText("");
        insertP2.setText("");
        showResult.setText("");
    }

    public void initialiseResult() {
        showResult.setText("");
    }

    public void displayResult(String result) {
        showResult.setText(result);
    }

    public String getP1() {
        return insertP1.getText();
    }

    public String getP2() {
        return insertP2.getText();
    }

    public void addClearListener(ActionListener clear) {
        clearBtn.addActionListener(clear);
    }

    public void addAddListener(ActionListener add)
    {
        addBtn.addActionListener(add);
    }

    public void addSubListener(ActionListener sub) {
        subBtn.addActionListener(sub);
    }

    public void addEqListener(ActionListener eq) {
        eqBtn.addActionListener(eq);
    }

    public void addMultListener(ActionListener mul) {
        mulBtn.addActionListener(mul);
    }

    public void addDiffListener(ActionListener diff) {
        diffBtn.addActionListener(diff);
    }

    public void addCalcListener(ActionListener calc) {
        calcBtn.addActionListener(calc);
    }
}

Controller 类

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

public class Controller {
    private Polynomial pol1 = new Polynomial();
    private Polynomial pol2 = new Polynomial();
    private View view;

    public Controller(View view) {
        this.view = view;
        view.addAddListener(new AddListener());
        view.addSubListener(new SubListener());
        view.addClearListener(new ClearListener());
        view.addEqListener(new EqualListener());
        view.addMultListener(new MulListener());
        view.addDiffListener(new DiffListener());
        view.addCalcListener(new CalcListener());
    }

    class AddListener implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            String p1 = view.getP1();
            String p2 = view.getP2();
            view.initialiseResult();
            pol1.stringToPolynomial(p1);
            pol2.stringToPolynomial(p2);
            view.displayResult((pol1.add(pol2)).toString());
        }
    }

    class SubListener implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            String p1 = view.getP1();
            String p2 = view.getP2();
            view.initialiseResult();
            pol1.stringToPolynomial(p1);
            pol2.stringToPolynomial(p2);
           // view.displayResult((pol1.subtract(pol2));
        }
    }

    class ClearListener implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            view.clear();
        }
    }

    class EqualListener implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            String p1 = view.getP1();
            String p2 = view.getP2();
            view.initialiseResult();
            pol1.stringToPolynomial(p1);
            pol2.stringToPolynomial(p2);
            if (pol1.isEqual(pol2)) {
                view.displayResult("true");
            } else view.displayResult("false");
            //  view.displayResult((pol1.isEqual(pol2)));
        }
    }

    class MulListener implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            String p1 = view.getP1();
            String p2 = view.getP2();
            view.initialiseResult();
            pol1.stringToPolynomial(p1);
            pol2.stringToPolynomial(p2);
            view.displayResult((pol1.multiply(pol2)).toString());
        }
    }

    class DiffListener implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            Object[] options = {"p1'", "p2'"};
            int choice = JOptionPane.showOptionDialog(view,
                    "Which polynomial do you want to differentiate?",
                    "Choose an option",
                    0,
                    3,
                    null,
                    options,
                    options[0]);
            if (choice == 0) { // if the user chose p1'
                String p1 = view.getP1();
                pol1.stringToPolynomial(p1);
                pol1.differentiate();
                view.displayResult(pol1.toString());
            } else if (choice == 1) { // if the user chose p2'
                String p2 = view.getP2();
                pol2.stringToPolynomial(p2);
                pol2.differentiate();
                view.displayResult(pol2.toString());
            }
        }
    }

    public class CalcListener implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            Object[] options = {"p1(x)", "p2(x)"};
            JPanel panel = new JPanel();
            panel.add(new JLabel("Enter a value for x and choose an operation"));
            JTextField textField = new JTextField(5);
            panel.add(textField);
            int choice = JOptionPane.showOptionDialog(view, panel, "Calculate value",
                    0, -1,
                    null, options, null);

            String input = textField.getText();
            boolean isInteger = true;
            if (input == null) {
                isInteger = false;
            } else {
                int length = input.length();
                if (length == 0) {
                    isInteger = false;
                } else {
                    int i = 0;
                    if (input.charAt(0) == '-') {
                        if (length == 1) {
                            isInteger = false;
                        }
                    }
                    for (i = 1; i < length; i++) {
                        char c = input.charAt(i);
                        if ((c <= '/') || (c >= ':')) {
                            isInteger = false;
                        }
                    }
                }
            }
            if (isInteger) {
                int x = Integer.parseInt(input);
                if (choice == 0) {
                    String p1 = view.getP1();
                    pol1.stringToPolynomial(p1);
                    int value = pol1.calculateValue(x);
                    System.out.println(value);
                } else if (choice == 1) {
                    String p2 = view.getP2();
                    pol2.stringToPolynomial(p2);
                    int value = pol2.calculateValue(x);
                    System.out.println(value);
                } else {
                    view.displayResult("The value you entered is not a number!");
                }
                String result = new String(view.toString());
                view.displayResult(result.toString());
            }
        }
    }
}

MVC 类

public class MVC {
    public static void main(String[] args) {
        View view = new View();
        Controller controller = new Controller(view);
        view.setVisible(true);
    }
}

异常示例

  Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at Polynomial.multiply(Polynomial.java:200)
        at Controller$MulListener.actionPerformed(Controller.java:69)
        at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
        at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2346)
        at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
        at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
        at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
        at java.awt.Component.processMouseEvent(Component.java:6525)
        at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
        at java.awt.Component.processEvent(Component.java:6290)
        at java.awt.Container.processEvent(Container.java:2234)
        at java.awt.Component.dispatchEventImpl(Component.java:4881)
        at java.awt.Container.dispatchEventImpl(Container.java:2292)
        at java.awt.Component.dispatchEvent(Component.java:4703)
        at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4898)
        at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4533)
        at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4462)
        at java.awt.Container.dispatchEventImpl(Container.java:2278)
        at java.awt.Window.dispatchEventImpl(Window.java:2750)
        at java.awt.Component.dispatchEvent(Component.java:4703)
        at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:751)
        at java.awt.EventQueue.access$500(EventQueue.java:97)
        at java.awt.EventQueue$3.run(EventQueue.java:702)
        at java.awt.EventQueue$3.run(EventQueue.java:696)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)
        at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:86)
        at java.awt.EventQueue$4.run(EventQueue.java:724)
        at java.awt.EventQueue$4.run(EventQueue.java:722)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:721)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

Answer 1

我在 Controller.MulListener 中看到错误。当你打电话时

pol1.stringToPolynomial(p1); 

您正在返回一个值。但是，您没有将值存储在任何地方。如果您将该值存储到 pol1，它将修复该错误。

pol1 = pol1.stringToPolynomial(p1); 

更一般地说，如果您想将 stringToPolynomial(String) 保留为类方法，那么您应该将其设置为调用它的实例。否则，将其编码为静态方法，并将其视为接受字符串并返回 Polynomial 实例的工厂方法，调用方式如下:

pol1 = Polynomial.stringToPolynomial(p1);