Java Swing：自己創建的可調整大小的GUI中的NullPointerException

Question

首先，我知道使用setLayout(null)並不是最好的方法，但該程序的目標是嘗試編寫一個可自行調整大小的GUI。

首先，這三個類（分裂GroundFrame.java和HomeScreen.java的原因是將來會有多個窗口）：

Start.java

public class Start {

    public static void main(String[] arg){
        new GroundFrame();  
    }
}

GroundFrame.java

public class GroundFrame extends JFrame{

    static final long       serialVersionUID = 0;
    Container               contentPane;
    HomeScreen              homeScreen;

    public GroundFrame(){
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
            }
        });
    }

    private void createAndShowGUI(){
        createBasicFrame();
        createPaneAndAddHome();
        setVisible(true);
    }

    private void createBasicFrame(){
        addComponentListener(new ComponentListener() {
            public void componentShown(ComponentEvent e) {}
            public void componentResized(ComponentEvent e) {
                homeScreen.resizeHome(getContentPane().getSize());
            }
            public void componentMoved(ComponentEvent e) {}
            public void componentHidden(ComponentEvent e) {}
        });

        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        contentPane = getContentPane();
        contentPane.setPreferredSize(new Dimension(600, 400));
        contentPane.setLayout(null);
        pack();
    }

    private void createPaneAndAddHome(){
        homeScreen = new HomeScreen(contentPane.getSize());
        contentPane.add(homeScreen);
    }
}

HomeScreen.java

public class HomeScreen extends JPanel{

    static final long       serialVersionUID = 0;
    private JLabel          labelHeader, labelFooter;
    private JTextField      textField;
    private int             x, y;

    public HomeScreen(final Dimension dimension){
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowPanel(dimension);
            }
        });
    }

    private void createAndShowPanel(Dimension dimension){
        labelHeader = new JLabel("HEADER");
        textField = new JTextField();
        labelFooter = new JLabel("FOOTER");
        setupPanel(dimension);
    }

    private void setupPanel(Dimension dimension){
        x = dimension.width;
        y = dimension.height;
        setBounds(0,0,x,y);
        setLayout(null);
        setupTitle(new Dimension(x,y));
    }

    private void setupTitle(Dimension dimension){
        x = dimension.width;
        y = dimension.height;
        labelHeader.setBounds((int)(x*0.1), (int)(y*0.1), (int)(x*0.8), (int)(y*0.3));
        labelHeader.setOpaque(true);
        labelHeader.setBackground(Color.cyan);
        labelHeader.setFont(new Font("Arial", Font.PLAIN, (int)(y*0.25)));

        textField.setBounds((int)(x*0.25), (int)(y*0.45), (int)(x*0.5), (int)(y*0.1));
        textField.setFont(new Font("Arial", Font.PLAIN, (int)(y*0.08)));

        labelFooter.setBounds((int)(x*0.1), (int)(y*0.6), (int)(x*0.8), (int)(y*0.3));
        labelFooter.setOpaque(true);
        labelFooter.setBackground(Color.cyan);
        labelFooter.setFont(new Font("Arial", Font.PLAIN, (int)(y*0.25)));

        add(labelHeader);
        add(labelFooter);
        add(textField);
    }

    public void resizeHome(Dimension dimension){
        setupPanel(dimension);
    }
}

現在一切都按照我的想法運行，但是我在HomeScreen.java后續行中得到NullpointerException

labelHeader.setBounds((int)(x*0.1), (int)(y*0.1), (int)(x*0.8), (int)(y*0.3));

這是錯誤消息

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at gui.HomeScreen.setupTitle(HomeScreen.java:45)
    at gui.HomeScreen.setupPanel(HomeScreen.java:39)
    at gui.HomeScreen.resizeHome(HomeScreen.java:64)
    at gui.GroundFrame$2.componentResized(GroundFrame.java:43)
    at java.awt.AWTEventMulticaster.componentResized(AWTEventMulticaster.java:159)
    at java.awt.Component.processComponentEvent(Component.java:6331)
    at java.awt.Component.processEvent(Component.java:6285)
    at java.awt.Container.processEvent(Container.java:2229)
    at java.awt.Window.processEvent(Window.java:2022)
    at java.awt.Component.dispatchEventImpl(Component.java:4861)
    at java.awt.Container.dispatchEventImpl(Container.java:2287)
    at java.awt.Window.dispatchEventImpl(Window.java:2719)
    at java.awt.Component.dispatchEvent(Component.java:4687)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:703)
    at java.awt.EventQueue.access$000(EventQueue.java:102)
    at java.awt.EventQueue$3.run(EventQueue.java:662)
    at java.awt.EventQueue$3.run(EventQueue.java:660)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
    at java.awt.EventQueue$4.run(EventQueue.java:676)
    at java.awt.EventQueue$4.run(EventQueue.java:674)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:673)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:244)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:163)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:151)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:147)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:139)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:97)

盡管如此，該計划仍然沒有發生任何事情。 畢竟，程序啟動時會出現錯誤，因此在沒有執行手動調整大小的位置（錯誤消息指示從HomeScreen的resizeHome()調用錯誤）。 為什么會這樣？ 為什么程序繼續運行？ 是否有可能構建GUI本身是ComponentListener的componentResized()的第一次調用？

Answer 1

這是因為您在構造函數中使用了SwingUtilities#invokeLater

SwingUtilities.invokeLater(new Runnable() {
    public void run() {
        createAndShowGUI();
    }
});

你創建的是你UI中的競爭條件。

基本上， homeScreen.resizeHome被調用（來自componentResized ）BEFORE HomeScreen#createAndShowPanel被調用，因此意味着lablelHeader還沒有被初始化...

您當前的事件序列看起來像......

• 主要
• new GrounFrame()
• invokeLater GroundFrame#createAndShowGUI
• ...（返回主要）
• GroundFrame#createBasicFrame （添加ComponentListener ， pack ）
• GroundFrame#createPaneAndAddHome
• new HomeScreen
• invokeLater HomeScreen#createAndShowPanel ...立即返回...
• GrounFrame#setVisible(true)
• GrounFrame#ComponentListener#componentResized
• HomeScreen#resizeHome
• HomeScreen#setupPanel
• Null Pointer Exception
• HomeScreen#createAndShowPanel

您應該將UI與main的EDT同步

public static void main(String[] args) {
    SwingUtilities.invokeLater(new Runnable() {
        public void run() {
            new GroundFrame();
        }
    });
}

並刪除對SwingUtilities#invokeLater的其他引用