Question

I have one class extending JScrollPane, that’s creating an object of another class that extends JTable. Basically it looks like this:

class CustomScrollPane{
   private CustomTable table

   public CustomScrollPane(..){
   table = new CustomTable(this);
   ..
   }
   public void scrollToBottom(){
      ...
   }
}

In the CustomTable class I override tableChanged:

public class CustomTable extends JTable{

private CustomScrollPane scrollPane;

public CustomTable(CustomScrollPane scrollPane){
    super();
    this.scrollPane = scrollPane;
}

@Override
public void tableChanged(TableModelEvent e) {
    super.tableChanged(e);
    scrollPane.scrollToBottom();
}

When I run this I get a NullPointerException on scrollPane in tableChanged(), how is that possible? How can scrollPane be null when it was set in the constructor? Running it in the debugger shows that tableChanged() is called before the constructor. Adding the condition

     if (scrollPane != null)

actually fixes the problem, because later on the constructor is called. Also, defining the JTable as its constructed, like:

        table = new JTable(){
        @Override
        public Component prepareRenderer(TableCellRenderer renderer, int row, int column) {
            final Component c = super.prepareRenderer(new CustomTableCellRenderer(), row, column);
            if (c instanceof JComponent){
                ((JComponent) c).setOpaque(true);
            }
            return c;
        }

        @Override
        public void paint(Graphics g) {
            int scrolling = scrollPane.getViewport().getViewPosition().y;
            super.paint(g);
            g.drawImage(image.getImage(), -30, -50 + scrolling, null, null);
        }

        @Override
        public void tableChanged(TableModelEvent e) {
            super.tableChanged(e);
            scrollPane.scrollToBottom();
        }
    };

directly in the CustomScrollPane constructor also works. Why can’t a break it out into a seperate class?

Answer 1

It looks like the JTable constructor invokes method tableChanged(...) – which means it is invoked before you’re able to initialize the scrollPane instance variable.

First, I suggest you take a look at some puzzlers in the book Java Puzzlers – specifically puzzle 51: What’s the Point and maybe puzzle 53: Do Your Thing. They should help you understand what’s going on. Basically, the first line of your CustomTable constructor invokes the JTable constructor (via super()). The JTable constructor is trying to invoke tableChanged – which has been overriden. The overriden tableChanged tries to manipulate scrollPane… but all this is happening on line 1 of your constructor (super()) – before the line this.scrollPane = scrollPane has executed, so scrollPane is still null.

Next, I suggest using the observer pattern. Here you have two objects – your scroll pane and the custom table – and one needs to be notified when the other is changed. That is textbook observer pattern. Here’s the rough idea:

File CustomTable.java

public class CustomTable extends JTable {

    // No more scroll pane; only observers
    private List<ChangeListener> listeners = [];

    // no more scroll pagne
    public CustomTable(){
        super();
    }

    @Override
    public void tableChanged(TableModelEvent e) {
        super.tableChanged(e);
        this.fireChangeEvent();
    }

    /* new methods */

    public void addChangeListener(ChangeListener listener) {
        listeners.add(listener);
    }

    public void removeChangeListener(ChangeListener listener) {
        // ...
    }

    private void fireChangeEvent() {
        for(String l : listeners ){
            l.onChange();
        }
    }
}

File CustomScrollPane.java

class CustomScrollPane implements ChangeListener{
   private CustomTable table

   public CustomScrollPane(/*...*/){
       table = new CustomTable();
       table.addChangeListener(this);
       //...
   }

   public void scrollToBottom(){
      //...
   }

   /* new methods */

  @Override
  public void onChange() {
      scrollToBottom();
  }
}