Java Swing - JSlider 和 JCombobox 导致运行时错误

Question

好的，我将发布这三个类的代码，因为它不太长。

package guiDemonstration;

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

public class TabbedGUI {
    //create fields
    private JFrame frame;
    private JTabbedPane tabbedPane;

    //make the following three labels public for access in the GUIEngine class
    private JLabel comboLabel; 
    private JLabel sliderLabel; 
    private JLabel radioLabel; 
    private JSlider slider;
    private JComboBox combo;
    private JPanel comboPanel, sliderPanel, radioPanel;
    private String []comboArray;
    private JRadioButton radio, radio1, radio2;
    private ButtonGroup buttonGroup;

    private String comboText = "Please Make a Choice";
    private String sliderText = "Move the Slider";
    private String radioText = "Choose a Radio Button";

    //Create field to hold GUIEngine Class
    GUIEngine engine;

    // empty constructor
    public TabbedGUI(){
    }

    //method used to construct the gui
    private void makeFrame(){
        //create the Frame
        frame = new JFrame("Example of a Tabbed GUI");

        //set the initial size of the frame in pixels
        frame.setSize(500, 200);
        //add the frame to the contentPane
        Container contentPane = frame.getContentPane();

        //create an instance of the GUIEngine class
        engine = new GUIEngine();

       //create an array of size 3 to be used as dropdown values in the combobox
        comboArray = new String[3];

        //initialise the array
        comboArray[0] = "First Choice";
        comboArray[1] = "Second Choice";
        comboArray[2] = "Third Choice";

        //create instance of JComboBox and add array
        combo = new JComboBox(comboArray);

        //create instance of JSlider
        slider = new JSlider();

        //creat instance of Button Group
        // this ButtonGroup will hold the individual radio buttons
        buttonGroup = new ButtonGroup();

        radio = new JRadioButton();
        radio1 = new JRadioButton();
        radio2 = new JRadioButton();

        //create instances of JPanel
        comboPanel = new JPanel();
        sliderPanel= new JPanel();
        radioPanel = new JPanel();

        //create flowlayout for comboPanel
        comboPanel.setLayout(new FlowLayout());

        //create instances of labels
        comboLabel = new JLabel(comboText);
        sliderLabel=new JLabel(sliderText);
        radioLabel = new JLabel(radioText);

        //add radio buttons to the group
        buttonGroup.add(radio);
        buttonGroup.add(radio1);
        buttonGroup.add(radio2);

        //add a border to the button group


        //begin creation of the tabbed GUI and add to contentPane
        tabbedPane = new JTabbedPane();
        contentPane.add(tabbedPane);

        frame.add(tabbedPane);

        //add instances of JPanel to each tab
        tabbedPane.addTab("Combo Box", comboPanel);
        tabbedPane.addTab("Slider", sliderPanel);
        tabbedPane.addTab("Radio", radioPanel);

        //add components to each JPanel of each tab
        comboPanel.add(combo);
        comboPanel.add(comboLabel);
        sliderPanel.add(sliderLabel);
        radioPanel.add(radioLabel);
        sliderPanel.add(slider);
        radioPanel.add(radio);
        radioPanel.add(radio1);
        radioPanel.add(radio2);

        //set a border around the Slider
        slider.setBorder(
                BorderFactory.createMatteBorder(1, 1, 1, 1, Color.BLUE));

        // call the method to add the listeners to each component
        addListeners();

        //by default the frame is set to invisible. Set the frame to visible
        frame.setVisible(true);
    }

    /**
     * This method adds listeners to each component
     */
    public void addListeners(){

        //add actionListeners to each component

        combo.addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent e){
                engine.useCombo();
            }
        });

        slider.addChangeListener(new ChangeListener(){
            public void stateChanged(ChangeEvent e){
                engine.useSlider();
            }
        });

        radio.addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent e){
                engine.useRadioButtons();
            }
        });

        radio1.addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent e){
                engine.useRadioButtons();
            }
        });

        radio2.addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent e){
                engine.useRadioButtons();
            }
        });
    }

    /*
     * The following three methods set the text for each
     * label on the three individual tabs.
     * These methods are called from the GUIEngine Class.
     */

    //set the text on the comboLabel
    public void setComboLabel(){
        String updatedComboText = (String)combo.getSelectedItem();
        comboLabel.setText(updatedComboText);
        //System.out.println("You selected" + comboText);
    }

    //set the text on the sliderLabel
    public void setSliderLabel(){
        sliderLabel.setText("You've moved the slider!");
    }

    //set the text on the radioLabel
    public void setRadioLabel(){
        System.out.println("You've selected a radio button!");
    }

    /**
     * This method is used to begin execution of the program
     */
    public void runProgram(){
        makeFrame();
    }
}

类 2：

package guiDemonstration;

public class GUIEngine {

    TabbedGUI tg;

    //constructor
    public GUIEngine(){
        tg = new TabbedGUI();
    }

    public void useCombo(){
        //System.out.println("You Used the Combo Box");
        tg.setComboLabel();
    }

    public void useSlider(){
        tg.setSliderLabel();
    }

    public void useRadioButtons(){
        //System.out.println("You clicked a radio button");
        tg.setRadioLabel();
    }
}

只有 main 方法：

package guiDemonstration;

public class Controller {
    /**
     * #This is the main method where program execution begins
     * @param args
     */
    public static void main(String[] args) {
        TabbedGUI tg = new TabbedGUI();
        tg.runProgram();
    }
}

JCombobox 和 JSlider 会导致运行时错误。代码编译正常，但是当我移动 JSlider 或选择 JComboBox 上的项目时，程序崩溃。

有什么想法吗？

GF

Answer 1

您的 GUIEngine 正在创建自己的 TabbedGUI 实例，而不是使用您在 main 方法中创建的实例。

如果您需要保留 GUIEngine 类，我建议在 makeFrame 中执行此操作

engine = new GUIEngine(this);

然后更改 GUIEngine 构造函数以接受 TabbedGUI 作为参数。不妨也将实例变量设为最终变量。

Answer 2

sliderLabel.setText("You've moved the slider!");

这会导致 NullPointerException，这意味着 sliderLabel 尚未创建。确保执行以下操作：

sliderLabel = new JLabel()

在调用 setSliderLabel 之前