带有 JButton 数组的空指针

Question

到目前为止，我已经在添加我的 ControlPanel 类时创建了一个框架，该类采用一个 int，并且该 int 用于用添加到面板并显示的 JButton 对象填充数组。

但是，当我尝试将 JButtons 添加到数组时出现空指针错误。我不知道为什么（因为我认为空指针来自当您尝试引用数组中不存在的东西时？）

对此的任何帮助将不胜感激。

错误信息：

Exception in thread "main" java.lang.NullPointerException
    at elevator.ControlPanel.<init>(ControlPanel.java:22)
    at elevator.Elevator_Simulator.main(Elevator_Simulator.java:27)

主类

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


public class Elevator_Simulator extends JFrame {




    public static void main(String[] args) {


        JFrame frame = new JFrame ("Elevator Simulation");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(new ControlPanel(8));
        frame.pack();
        frame.setVisible(true);


    }

}

控制面板类：

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

public class ControlPanel extends JPanel implements ActionListener {

    public JButton[] floorButton; // an array of floor buttons.
    public int[] floorIn;          // state of button. 0 == not click >= 1 means clicked (how many times).

    public ControlPanel(int x) {

        JPanel floors = new JPanel();

        for (int i = 0; i < x; i++) { // interates through user input and creates that many JButtons. 
            floorButton[i].add(new JButton("F" + Integer.toString(i))); // adds a button to button array. Sets text to F(floorNo).
            floors.add(floorButton[i]); // adds buttons to the floor panel.
            floorButton[i].addActionListener(this); // adds action listener to the buttons.
        }
    }

    public void actionPerformed(ActionEvent e) {
        for (int i = 0; i < floorButton.length; i++) {
            if (e.getSource() == floorButton[i]) { // checks which button the event occured on.

                floorButton[i].setBackground(Color.red); // sets button background to red.
                floorButton[i].setText(floorButton[i].getText() + "(1)"); // marks button as clicked.

            }
        }
    }

}

Answer 1

floorButton 没有初始化为任何东西...

public JButton[] floorButton; // I'm null

public ControlPanel(int x) {
    //...
    for (int i = 0; i < x; i++) { // in
        // Still null
        floorButton[i].add(new JButton("F" + Integer.toString(i))); 

初始化数组以反映您需要的内容，另外，不要使用floorButton[i].add，您不想将按钮添加到（当前是null 元素）按钮，您想将其分配给数组的位置...

public ControlPanel(int x) {
    //...
    floorButton = new JButton[x];
    for (int i = 0; i < x; i++) { // in
        floorButton[i] = new JButton("F" + Integer.toString(i)); 

我猜你会想对floorIn做同样的事情...

Answer 2

您创建了 JButton 数组，但并未创建数组中的每个元素。为此：

替换这个：

 floorButton[i].add(new JButton("F" + Integer.toString(i)));

通过这个：

 floorButton[i] = new JButton("F" + Integer.toString(i)); 

Answer 3

你必须初始化你的floorButton。

    floorButton = new JButton [yourCount];