mjhjmtu

Can someone explain to me why I am getting a NullPointerException in the getRowCount() method? The variable is initialized with an empty ArrayList...

public class BeschriftungssetTableModel extends DefaultTableModel 

 private static final long serialVersionUID = -4980235976337188354L;

 private List<BeschriftungssetBean> data = new ArrayList<>();


 public void setData(List<BeschriftungssetBean> data) 
 this.data = data;
 

 @Override
 public int getColumnCount() 
 return 1;
 

 @Override
 public int getRowCount() 
 return data.size();
 

 @Override
 public Object getValueAt(int row, int column) 
 return data.get(row).getBezeichnung();
 

 @Override
 public String getColumnName(int column) 
 return "Bezeichnung";
 

 public static void main(String args) 
 BeschriftungssetTableModel beschriftungssetTableModel = new BeschriftungssetTableModel();
 beschriftungssetTableModel.getRowCount();
 




public class BeschriftungssetBean 
 private String objId;
 private String bezeichnung;

 public String getBezeichnung() 
 return bezeichnung;
 

 public void setBezeichnung(String bezeichnung) 
 this.bezeichnung = bezeichnung;
 

 public String getObjId() 
 return objId;
 

 public void setObjId(String objId) 
 this.objId = objId;
 

Exception in thread "main" java.lang.NullPointerException
at ch.aaa.xxx.yyy.gruppen.plugin.anzeige.beschriftungseinstellungen.BeschriftungssetTableModel.getRowCount(BeschriftungssetTableModel.java:36)
at javax.swing.table.DefaultTableModel.setDataVector(DefaultTableModel.java:224)
at javax.swing.table.DefaultTableModel.<init>(DefaultTableModel.java:124)
at javax.swing.table.DefaultTableModel.<init>(DefaultTableModel.java:106)
at javax.swing.table.DefaultTableModel.<init>(DefaultTableModel.java:86)
at ch.aaa.xxx.yyy.gruppen.plugin.anzeige.beschriftungseinstellungen.BeschriftungssetTableModel.<init>(BeschriftungssetTableModel.java:18)
at ch.aaa.xxx.yyy.gruppen.plugin.anzeige.beschriftungseinstellungen.BeschriftungssetTableModel.main(BeschriftungssetTableModel.java:50)

The constructor of DefaultTableModel calls getRowCount before the subclass had a chance to initialize its content, thus leading to NPE with your implem. This is a bad design of the base class, as calling overridable methods from within a constructor is considered a bad practice, but hey, Swing API has quite a few of these :)

Cf. What's wrong with overridable method calls in constructors?

Do not extend DefaultTableModel, extend AbstractTableModel instead.

DefaultTableModel uses a Vector to maintain the data and does not expect you having your own data in a subclass. As @spi wrote, it is calling getRowCount which was overriden to return the size of data that is not yet initialized.

The AbstractTableModel is easy to extend (DefaultTableModel also does), just these 3 methods need to be implemented (already done in questions code):

public int getRowCount();
public int getColumnCount();
public Object getValueAt(int row, int column);

Others methods can be overridden to adjust its behavior like isCellEditable or getColumnClass. If the data is changed the appropriate fireXXX method should be called so the registered listeners are invoked (e.g. JTable using that model).

An alternative is to implement TableModel, but AbstractTableModel takes care of some standard (nasty?) functionality like maintaining the listeners.

^{(bad alternative, just check for null, but this probably will mess up with the DefaultTableModel code - do not do this)}