Java Swing : Error when dynamically adding JScrollpane to JPanel

Question

更新:如果我将代码移动到 mouseclick 事件中(即，我不调用 initMarksScreen()，而是直接将代码放入其中)，它就会按预期工作。所以我的问题是在它自己的方法中调用代码。这是否意味着如果我想在另一个点或在不同的按钮上执行相同的步骤，我必须将代码直接放在那里，而不是在我可以调用的方法中？

<小时/>

我对 Java 相当陌生，正在尝试创建一个添加包含 JTable 的 JScrollPane 的按钮。

调用者:

    public void mouseClicked(java.awt.event.MouseEvent evt) {
            initMarksScreen();
        }

代码是:

public final void initMarksScreen() {
    String[] columnNames = {"Student ID",
                            "Last Name",
                            "Firstname",
                            "Status",
                            "Degree",
                            "Candidate No.",
                            "Stage",
                            "Year",
                            "Code",
                            "Title",
                            "Grade Mode",
                            "Mark",
                            "Result"};
    Object[][] data = {
            {"100123456", "Cooper","Sheldon", "Signed Up", "BSc Physics","1201234","1","12","PH1001","Blackholes and Revelations","D",new Integer(99),"P"},
            {"100123456", "Cooper","Sheldon", "Signed Up", "BSc Physics","1201234","1","12","PH1025","Astrophysics","D",new Integer(95),"P"}
        };
    JTable tMarks = new JTable(data, columnNames);
    JScrollPane scrollPane = new JScrollPane(tMarks);
    tMarks.setFillsViewportHeight(true);
    panelCentral.add(scrollPane); --!!ERROR AT THIS LINE!!
    panelCentral.revalidate();
}   

面板声明为:

private static final Container panelCentral = null;

由于我对 Java 很陌生，我想有一个简单的修复方法，但如果我需要添加 SSCCE。

一切都显示正常，直到我单击按钮，然后我得到的错误是:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at GUI.mainScreen.initMarksScreen(mainScreen.java:312)
at GUI.mainScreen$2.mouseClicked(mainScreen.java:183)
at java.awt.AWTEventMulticaster.mouseClicked(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

如果panelCentral是JPanel且未初始化，则在添加之前初始化

 panelCentral = new JPanel();
 panelCentral.add(scrollPane);