java jlabel 图片的更新及清空解决方法

import java.awt.GridLayout;import java.awt.event.ActionEvent;import java.awt.event.ActionListener;import java.net.URL;import javax.swing.BorderFactory;import javax.swing.ImageIcon;import javax.swing.JButton;import javax.swing.JComboBox;import javax.swing.JFrame;import javax.swing.JLabel;import javax.swing.JPanel;import javax.swing.UIManager;public class ImageRandomAccess2 implements ActionListener {    final static int NUM_IMAGES = 8;//显示的图片总数    final static int START_INDEX = 3;//初始化的时候默认显示的图片    ImageIcon[] images = new ImageIcon[NUM_IMAGES];//初始化数组    JPanel mainPanel, selectPanel, displayPanel;//三个面板        JLabel selectLabel=null;        JComboBox combobox=null;//控制显示有哪些图片，非随即滚动    JButton controlBtn=null;    JLabel imageIconLabel = null;    // Constructor    public ImageRandomAccess2() {        // Create the phase selection and display panels.        selectPanel = new JPanel();        displayPanel = new JPanel();        // Add various widgets to the sub panels.        addWidgets();        // Create the main panel to contain the two sub panels.        mainPanel = new JPanel();        mainPanel.setLayout(new GridLayout(2, 1, 5, 5));        mainPanel.setBorder(BorderFactory.createEmptyBorder(5, 5, 5, 5));        // Add the select and display panels to the main panel.        mainPanel.add(selectPanel);        mainPanel.add(displayPanel);    }    // Create and the widgets to select and display the images of peoples.    private void addWidgets() {        // Get the images and put them into an array of ImageIcon.        for (int i = 0; i < NUM_IMAGES; i++) {            String imageName = "images/" + (i + 1) + ".jpg";            System.out.println("getting image: " + imageName);            // URL iconURL = ClassLoader.getSystemResource(imageName);            ImageIcon icon = new ImageIcon(imageName);            images[i] = icon;        }        // Create label for displaying  images and put a border around        // it.        imageIconLabel = new JLabel();        imageIconLabel.setHorizontalAlignment(JLabel.CENTER);        imageIconLabel.setVerticalAlignment(JLabel.CENTER);        imageIconLabel.setVerticalTextPosition(JLabel.CENTER);        imageIconLabel.setHorizontalTextPosition(JLabel.CENTER);        imageIconLabel.setBorder(BorderFactory.createCompoundBorder(                BorderFactory.createLoweredBevelBorder(),                BorderFactory.createEmptyBorder(5, 5, 5, 5)));        imageIconLabel.setBorder(BorderFactory.createCompoundBorder(                BorderFactory.createEmptyBorder(0, 0, 10, 0),                imageIconLabel.getBorder()));        controlBtn=new JButton("无图片");        selectLabel=new JLabel("选择图片");                combobox=new JComboBox();                for(int i=0;i<NUM_IMAGES;i++){            combobox.addItem(i);        }        selectLabel.setLabelFor(combobox);        // Display the first image.        imageIconLabel.setIcon(images[START_INDEX]);        imageIconLabel.setText("");                        // Add border around the select panel.        selectPanel.setBorder(BorderFactory.createCompoundBorder(                BorderFactory.createTitledBorder("Select Phase"),                BorderFactory.createEmptyBorder(5, 5, 5, 5)));        // Add border around the display panel.        displayPanel.setBorder(BorderFactory.createCompoundBorder(                BorderFactory.createTitledBorder("Display Phase"),                BorderFactory.createEmptyBorder(5, 5, 5, 5)));        // Add control button to select panel and image label to        // displayPanel.        selectPanel.add(selectLabel);        selectPanel.add(combobox);        selectPanel.add(controlBtn);        displayPanel.add(imageIconLabel);        // Listen to events from control button.//        controlBtn.addActionListener(this);        combobox.addActionListener(this);        controlBtn.addActionListener(this);    }    boolean run = false;    // Implementation of ActionListener interface.    public void actionPerformed(ActionEvent event) {        if(event.getSource()==combobox){            int imageIndex=combobox.getSelectedIndex();            imageIconLabel.setIcon(images[imageIndex]);        }        if(event.getSource()==controlBtn){            System.out.println("------------");            imageIconLabel.setIcon(null);//            imageIconLabel.setIcon(new ImageIcon());        }    }    // main method    public static void main(String[] args) {        // create a new instance of ImageRandomAccess        ImageRandomAccess2 phases = new ImageRandomAccess2();        // Create a frame and container for the panels.        JFrame mainFrame = new JFrame("ImangeRandomAccess");        // Set the look and feel.        try {            UIManager.setLookAndFeel(UIManager                    .getCrossPlatformLookAndFeelClassName());        } catch (Exception e) {        }        mainFrame.setContentPane(phases.mainPanel);        // Exit when the window is closed.        mainFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);        // Show the converter.        mainFrame.pack();        mainFrame.setLocationRelativeTo(null);        mainFrame.setVisible(true);    }}