JAVA父类多态指向子类对象，然后用一个子类对象来恢复原子类对象，为什么会失败？

[love277369986]

/* 可以显示图标的ListCell绘制器 */
public class IconListItemRenderer extends JLabel implements ListCellRenderer
{
private Border
selectedBorder = BorderFactory.createLineBorder(Color.blue,1),
emptyBorder = BorderFactory.createEmptyBorder(1,1,1,1);
IconListItem item ;

public Component getListCellRendererComponent(
JList list,
Object value,
int index,
boolean isSelected,
boolean cellHasFocus)
{
try
{
if(value instanceof IconListItem){
item = (IconListItem)value;
}
}catch(Exception e){System.out.println("转换失败！");}

this.setIcon(item.getIcon());
this.setText(item.getText());



if ( isSelected ) setBorder (selectedBorder);
else setBorder(emptyBorder);
return this;
}
}

this.setIcon(item.getIcon());this.setText(item.getText());导致运行时失败，知道错什么的告诉下，谢谢！