Question

我的编码问题在这里，我无法弄清楚为什么我有它。

我正确地将ArrayList声明为private ArrayList，并在声明时初始化它。现在我遇到的问题是向它添加对象。每当我尝试向其添加一个对象时，我会在程序应该按照应有的方式工作时不断获得NullPointerException。

我声明的地方

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

public class AdditionFrame implements ActionListener{

private JFrame frame;
private JPanel panel1, panel2,panel3;
private JMenuBar menuBar;
private JMenu mOpt1, mOpt2;
private JMenuItem mI1, mI2, mI3, mI4, mI5, mI6;
private JLabel label1,label2,label3;
private JTextField box1;
private JButton button1, button2, button3;
private int a, b, lives = 3, score = 0, max = 50, min = 35, c;
public ArrayList<HighScoreObject> hsoArray = new ArrayList<HighScoreObject>();

public AdditionFrame(JFrame f) {
    // TODO Auto-generated constructor stub

    this.frame = f;
    initialize();
    frame.setVisible(true);
}

private void initialize(){
    generateRandomNumbers();

    frame.setTitle("Mathematical Sums - Addition");

    panel1 = new JPanel();
    panel1.setLayout(new GridLayout());
    frame.getContentPane().add(panel1, BorderLayout.NORTH);

        label2 = new JLabel("Score: " + score);
        label2.setFont(new Font("SimSun", Font.BOLD, 22));
        label2.setLocation(0,0);
        label2.setSize(117,37);
        panel1.add(label2);

        label3 = new JLabel("Lives: " + lives);
        label3.setFont(new Font("SimSun", Font.BOLD, 22));
        label3.setLocation(0,70);
        label3.setSize(105,49);
        panel1.add(label3);

    panel2 = new JPanel();
    panel2.setLayout(new BorderLayout());
    frame.getContentPane().add(panel2, BorderLayout.CENTER);

        label1 = new JLabel(a +" + " +b);
        label1.setFont(new Font("SimSun", Font.BOLD, 22));
        label1.setHorizontalAlignment(SwingConstants.CENTER);
        panel2.add(label1, BorderLayout.NORTH);

        box1 = new JTextField();
        box1.setLocation(249,176);
        box1.setSize(96,25);
        panel2.add(box1);
        box1.setEditable(true);

    panel3 = new JPanel();
    panel3.setLayout(new GridLayout());
    frame.getContentPane().add(panel3, BorderLayout.SOUTH);

        button1 = new JButton("Answer");
        panel3.add(button1);
        button1.addActionListener(this);
        button1.setLocation(187,236);
        button1.setSize(105,50);

        button2 = new JButton("Save Score");
        panel3.add(button2);
        button2.addActionListener(this);
        button2.setVisible(false);
        button2.setLocation(302,236);
        button2.setSize(105,50);

        button3 = new JButton("Reset");
        panel3.add(button3);
        button3.addActionListener(this);
        button3.setVisible(false);
        button3.setLocation(532,236);
        button3.setSize(105,50);

    menuBar = new JMenuBar();

    mOpt1 = new JMenu("File");
    menuBar.add(mOpt1);
    mI1 = new JMenuItem("Return to Main");
    mI1.addActionListener(this);
    mOpt1.add(mI1);
    mI2 = new JMenuItem("Quit");
    mI2.addActionListener(this);
    mOpt1.add(mI2);

    mOpt2 = new JMenu("Help");
    menuBar.add(mOpt2);
    mI3 = new JMenuItem("Display Help");
    mI3.addActionListener(this);
    mOpt2.add(mI3);
    mI4 = new JMenuItem("Gameplay Help");
    mI4.addActionListener(this);
    mOpt2.add(mI4);

    frame.setJMenuBar(menuBar);
}

public void actionPerformed(ActionEvent e){
    String action = e.getActionCommand();

    if(action.equals("Display Help")){
        JOptionPane.showMessageDialog(null, "In This Mode, Random numbers are generated and added together" +
                " \nAnd you have to correctly guess the right answer without losing lives " +
                "\nGame is over when you run out of lives");
    }
    if(action.equals("Gameplay Help")){
        JOptionPane.showMessageDialog(null, "In Gameplay: " +
                "\n- Click on 'Answer' to check if your answer is valid" +
                " \n- Green = Correct Answer " +
                "\n- Red = Wrong Answer");
    }
    if(action.equals("Return to Main")){
        frame.setVisible(false);
        frame.getContentPane().removeAll();
        Assignment2A g = new Assignment2A(frame);
        g.getArray(hsoArray);
    }
    if(action.equals("Answer")){
        String getAnswer = box1.getText();
        int check = Integer.parseInt(getAnswer);
        box1.setText("");
        if(c == check){
            box1.setBackground(Color.GREEN);
            score++;
            generateRandomNumbers();
            label1.setText(String.valueOf(a) + " + " +String.valueOf(b));
            label2.setText("Score: "+String.valueOf(score));
        }
        else if(lives <= 0){
            button1.setVisible(false);
            button2.setVisible(true);
            button3.setVisible(true);
            box1.setEditable(false);
            JOptionPane.showMessageDialog(null, "Game Over, Final Score: " + String.valueOf(score));
        }
        else if(c != check){
            box1.setBackground(Color.RED);
            lives--;
            label3.setText("Lives: " + String.valueOf(lives));
        }
    }
    if(action.equals("Save Score")){
        String name = JOptionPane.showInputDialog("Please Enter your name");
        HighScoreObject hso = new HighScoreObject(name, "Addition", score);
        System.out.println(hso.getName()+" "+hso.getType()+" "+hso.getScore());
        hsoArray.add(hso);
    }
    if(action.equals("Reset")){
        box1.setEditable(true);
        box1.setBackground(Color.WHITE);
        button1.setVisible(true);
        button2.setVisible(false);
        button3.setVisible(false);
        score = 0; lives = 3;
        generateRandomNumbers();
        label1.setText(String.valueOf(a) + " + " +String.valueOf(b));
        label2.setText("Score: "+String.valueOf(score));
        label3.setText("Lives: " + String.valueOf(lives));
    }
    if(action.equals("Quit")){
        System.exit(0);
    }
}

private void generateRandomNumbers(){
    Random r = new Random();
    a = r.nextInt(max);
    b = r.nextInt(min);

    c = a + b;
    System.out.println(c);
}

public void getArray(ArrayList<HighScoreObject> hsoArray) {
    // TODO Auto-generated method stub
    this.hsoArray = hsoArray;
}
}

这是我得到的例外

MELVCIN Addition 5
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at AdditionFrame.actionPerformed(AdditionFrame.java:160)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$200(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

Answer 1

创建AdditionFrame对象时，会创建HighScoreObject的arrayList，但在执行if(action.equals("Save Score")){之前，会调用getArray方法，并且它会收到null。

Answer 2

看看这个存根代码：

public void getArray(ArrayList<HighScoreObject> hsoArray) { // TODO Auto-generated method stub this.hsoArray = hsoArray; }

使用null参数调用getArray的可能性很大。此外，这看起来你实际上希望它的行为类似于setArray方法而不是getArray方法。

Answer 3

正如DavMac在评论中建议的那样，我从主要的java代码

接收了一个数组

if(action.equals("Addition")){
    frame.setVisible(false);
    frame.getContentPane().removeAll();
    AdditionFrame p2 = new AdditionFrame(frame);
     p2.getArray(array);
    }

这个方法来自我的主类，并将其数组传递给我的添加类，然后当我试图将一个对象放到我的hsoArray时，程序不知道该对象放在哪里，所以它给了我异常

我已经这么久了，但是我找到了它，感谢DavMac

初始化ArrayList上的Java.lang.nullpointerexception

3 个答案: