Adding a Custom Data Model

The JTable supports the replacement of its data model in order to improve performance or to help reduce the size of the code required for a given application.

The custom data model feature used in JTable is actually simpler than the one used for (for example) the JTree class:

• JTable only has to manage a simple 2D matrix of data

Let us now develop a simple data model example which basically reengineers the code from the previous example, AdvancedTableExample.java. This example, CustomDataTableExample.java:

• build a new application that uses a custom data model but
• produces exactly the same results the previous example.
• Thoughthe output looks the same as the one shown previously, it offers much better performance, particularly as the size of the data set grows.

Two classes are implemented in this example:

• CustomDataTableExample.java - contains the code for the main frame class, which is a modified version of the previous example:
  • the CreateData() method has been removed -- it is no longer required because the custom data model generates the data set.
  • the CreateColumns() method has been changed -- the code no longer uses an instance of the DefaultDataModel class (instantiated automatically by JTable in the previous example).
  • the application is now required to create its own columns -- the CreateColumns() method reverts to first principles by creating an instance of a TableColumn object and populating it with the appropriate text before adding it to the table.
  • the example now generates its own column data therefore it notifies the table not to attempt to determine this information from the data. It does this by calling JTable's setAutoCreateColumnsFromModel() method.

• CustomDataModel.java -- holds the code for the custom data model, which extends the AbstractTableModel class and provides four methods called by the JTable code when data is accessed.
  • Since the data model is now synthesized, the getValueAt() method simply determines the row and column being accessed and generates a string representing the cell data.
  • The getColumnCount() method returns a value of zero, which may not be what you expected. Remember, though, that this method returns the number of columns managed by the table code. In the CreateColumns() method in the main CustomDataTableExample class, we informed the table that it should not attempt to generate any of its own columns. As a result, the value of getColumnCount() is zero.

The full code listing for the CustomDataTableExample.java is as follows:

// Imports
import java.awt.*;
import java.util.*;
import javax.swing.*;
import javax.swing.table.*;

class CustomDataTableExample
		extends 	JFrame
 {
	// Instance attributes used in this example
	private	JPanel		topPanel;
	private	JTable		table;
	private	JScrollPane scrollPane;

	private	String		columnNames[];
	private	String		dataValues[][];


	// Constructor of main frame
	public CustomDataTableExample()
	{
		// Set the frame characteristics
		setTitle( "Custom Table Data Model Application" );
		setSize( 300, 200 );
		setBackground( Color.gray );

		// Create a panel to hold all other components
		topPanel = new JPanel();
		topPanel.setLayout( new BorderLayout() );
		getContentPane().add( topPanel );

		// Create the custom data model
		CustomDataModel customDataModel = new CustomDataModel();

		// Create a new table instance
		table = new JTable( customDataModel ); //dataValues, columnNames );

		// Create columns
		CreateColumns();

		// Configure some of JTable's paramters
		table.setShowHorizontalLines( false );
		table.setRowSelectionAllowed( true );
		table.setColumnSelectionAllowed( true );

		// Change the selection colour
		table.setSelectionForeground( Color.white );
		table.setSelectionBackground( Color.red );

		// Add the table to a scrolling pane
		scrollPane = new JScrollPane( table );
		topPanel.add( scrollPane, BorderLayout.CENTER );
}

	public void CreateColumns()
	{
		// Say that we are manually creating the columns
		table.setAutoCreateColumnsFromModel( false );

		for( int iCtr = 0; iCtr < 8; iCtr++ )
		{
			// Manually create a new column
			TableColumn column = new TableColumn( iCtr );
			column.setHeaderValue( (Object)("Col:" + iCtr) );

			// Add the column to the table
			table.addColumn( column );
		}
	}

	// Main entry point for this example
	public static void main( String args[] )
	{
		// Create an instance of the test application
		CustomDataTableExample mainFrame	= new CustomDataTableExample();
		mainFrame.setVisible( true );
	}
}

The full code listing for the CustomDataModel.java is as follows:

// Imports
import javax.swing.*;
import javax.swing.table.*;

class CustomDataModel
			extends		AbstractTableModel  
{

	public Object getValueAt( int iRowIndex, int iColumnIndex )
	{
		return "" + iColumnIndex + "," + iRowIndex;
	}

	public void setValueAt( Object aValue, int iRowIndex, int iColumnIndex ) 
	{
		// All data is manufactured - nothing to do here
	}
	
	public int getColumnCount()
	{
		// Return 0 because we handle our own columns
		return 0;
	}	

	public int getRowCount()
	{
		return 500;
	}	
	
}