我必须制作一个对多项式执行运算的GUI，但我不断收到我无法摆脱的NullPointerExceptions。在输出上，它不显示任何内容。我试图调试我的程序，据我所知，我从键盘插入的多项式不知何故没有被处理到系统中，即使它们不应该被处理，也被认为是空的。如何使我的程序正常工作？我需要尽快得到答案。
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，加法不返回异常，而是返回 0。
以下是所有 4 个类，适合任何想要将它们放入编译器并测试它们的人：
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;
}
}
类视图
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);
}
}
类控制器
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());
}
}
}
}
类 MVC
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)
我在Controller.MulListener中看到一个错误。当您致电时
pol1.stringToPolynomial(p1);
您正在返回一个值。但是，您不会将值存储在任何地方。如果将该值存储到 pol1，它将修复错误。
pol1 = pol1.stringToPolynomial(p1);
更一般地说，如果要保留 stringToPolynomial（String） 作为类方法，则应让它将值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多项式实例的工厂方法，按如下方式调用它：
pol1 = Polynomial.stringToPolynomial(p1);