JList中通过defaultListCard插入列表，产生一个可供选择的列表，但是运行时某些时候会出现空指针错误。
代码如下：
public void showCard(String[] cardsList) {
cardListModel.clear();
System.out.println(cardListModel.capacity());
for (int i = 0; i < cardsList.length; i++) {
String str = CardType.cardShowParse(cardsList[i]);
System.out.println(str);
cardListModel.addElement(str);
}
}
控制台输出：
20
4
4
4
5
Exception in thread "Thread-2" java.lang.NullPointerException
at javax.swing.plaf.basic.BasicListUI.updateLayoutState(BasicListUI.java:1368)
at javax.swing.plaf.basic.BasicListUI.maybeUpdateLayoutState(BasicListUI.java:1311)
at javax.swing.plaf.basic.BasicListUI$Handler.valueChanged(BasicListUI.java:2623)
at javax.swing.DefaultListSelectionModel.fireValueChanged(DefaultListSelectionModel.java:184)
at javax.swing.DefaultListSelectionModel.fireValueChanged(DefaultListSelectionModel.java:164)
at javax.swing.DefaultListSelectionModel.fireValueChanged(DefaultListSelectionModel.java:211)
at javax.swing.DefaultListSelectionModel.insertIndexInterval(DefaultListSelectionModel.java:632)
at javax.swing.plaf.basic.BasicListUI$Handler.intervalAdded(BasicListUI.java:2581)
at javax.swing.AbstractListModel.fireIntervalAdded(AbstractListModel.java:148)
at javax.swing.DefaultListModel.addElement(DefaultListModel.java:367)
at douDiZhu.Table$SouthPanel.showCard(Table.java:279)
at douDiZhu.Table.showCard(Table.java:186)
at douDiZhu.Client$ReceiveThread.handle(Client.java:147)
at douDiZhu.Client$ReceiveThread.receive(Client.java:129)
at douDiZhu.Client$ReceiveThread.run(Client.java:115)
at java.lang.Thread.run(Thread.java:745)
可见容量为20，所以应该不是容量不够，str也是有值的，错误信息指向defaultlistmodel，最终指向下面这个不知道干什么用的代码的
cellHeights[index] = cellSize.height;
这一行。
/**
* Recompute the value of cellHeight or cellHeights based
* and cellWidth, based on the current font and the current
* values of fixedCellWidth, fixedCellHeight, and prototypeCellValue.
*
* @see #maybeUpdateLayoutState
*/
protected void updateLayoutState()
{
/* If both JList fixedCellWidth and fixedCellHeight have been
* set, then initialize cellWidth and cellHeight, and set
* cellHeights to null.
*/
int fixedCellHeight = list.getFixedCellHeight();
int fixedCellWidth = list.getFixedCellWidth();
cellWidth = (fixedCellWidth != -1) ? fixedCellWidth : -1;
if (fixedCellHeight != -1) {
cellHeight = fixedCellHeight;
cellHeights = null;
}
else {
cellHeight = -1;
cellHeights = new int[list.getModel().getSize()];
}
/* If either of JList fixedCellWidth and fixedCellHeight haven't
* been set, then initialize cellWidth and cellHeights by
* scanning through the entire model. Note: if the renderer is
* null, we just set cellWidth and cellHeights[*] to zero,
* if they're not set already.
*/
if ((fixedCellWidth == -1) || (fixedCellHeight == -1)) {
ListModel dataModel = list.getModel();
int dataModelSize = dataModel.getSize();
ListCellRenderer renderer = list.getCellRenderer();
if (renderer != null) {
for(int index = 0; index < dataModelSize; index++) {
Object value = dataModel.getElementAt(index);
Component c = renderer.getListCellRendererComponent(list, value, index, false, false);
rendererPane.add(c);
Dimension cellSize = c.getPreferredSize();
if (fixedCellWidth == -1) {
cellWidth = Math.max(cellSize.width, cellWidth);
}
if (fixedCellHeight == -1) {
cellHeights[index] = cellSize.height;
}
}
}
else {
if (cellWidth == -1) {
cellWidth = 0;
}
if (cellHeights == null) {
cellHeights = new int[dataModelSize];
}
for(int index = 0; index < dataModelSize; index++) {
cellHeights[index] = 0;
}
}
}
columnCount = 1;
if (layoutOrientation != JList.VERTICAL) {
updateHorizontalLayoutState(fixedCellWidth, fixedCellHeight);
}
}
这个错误有时出现有时不出现，我只在网上找到了这个http://www.tc5u.com/java/1862834.htm ，但是没有解决方法，stackoverflow上也没找到。
请问这个错误是什么原因，怎么解决，或者有没有替代方法。