By the way, the model for a JList actually has another part in addition to the ListModel : An object of type ListSelectionModel stores information about which items in the list are currently selected. When the model is complex, it’s not uncommon to use several model objects to store di erent aspects of the state.

12.4.3Tables and TableModels

Like a JList, a JTable displays a collection of items to the user. However, tables are much more complicated than lists. Perhaps the most important di erence is that it is possible for the user to edit items in the table. Table items are arranged in a grid of rows and columns. Each grid position is called a cell of the table. Each column can have a header , which appears at the top of the column and contains a name for the column.

It is easy to create a JTable from an array that contains the names of the columns and a two-dimensional array that contains the items that go into the cells of the table. As an example, the sample program StatesAndCapitalsTableDemo.java creates a table with two columns named “State” and “Capital City.” The first column contains a list of the states of the United States and the second column contains the name of the capital city of each state. The table can be created as follows:

String[][] statesAndCapitals = new String[][] {

{"Alabama", "Montgomery" },

{"Alaska", "Juneau" },

{"Arizona", "Phoenix" },

.

.

.

{"Wisconsin", "Madison" },

{"Wyoming", "Cheyenne" }

};

String[] columnHeads = new String[] { "State", "Capital City" };

JTable table = new JTable(statesAndCapitals, columnHeads);

Since a table does not come with its own scroll bars, it is almost always placed in a JScrollPane to make it possible to scroll the table. In the example program this is done with:

add( new JScrollPane(table), BorderLayout.CENTER );

The column headers of a JTable are not actually part of the table; they are in a separate component. But when you add the table to a JScrolPane, the column headers are automatically placed at the top of the pane.

Using the default settings, the user can edit any cell in the table. (To select an item for editing, click it and start typing. The arrow keys can be used to move from one cell to another.) The user can change the order of the columns by dragging a column header to a new position. The user can also change the width of the columns by dragging the line that separates neighboring column headers. You can try all this in the sample program; there is an applet version in the on-line version of this section.

Allowing the user to edit all entries in the table is not always appropriate; certainly it’s not appropriate in the “states and capitals” example. A JTable uses an object of type TableModel to store information about the contents of the table. The model object is also responsible for deciding whether or not the user should be able to edit any given cell in the table. TableModel includes the method

public boolean isCellEditable(int rowNum, columnNum)

where rowNum and columnNum are the position of a cell in the grid of rows and columns that make up the table. When the controller wants to know whether a certain cell is editable, it calls this method in the table model. If the return value is true, the user is allowed to edit the cell.

The default model that is used when the table is created, as above, from an array of objects allows editing of all cells. For this model, the return value of isCellEditable() is true in all cases. To make some cells non-editable, you have to provide a di erent model for the table. One way to do this is to create a subclass of DefaultTableModel and override the isCellEditable() method. (DefaultTableModel and some other classes that are discussed in this section are defined in the package javax.swing.table.) Here is how this might be done in the “states and capitals” program to make all cells non-editable:

TableModel model = new DefaultTableModel(statesAndCapitals,columnHeads) { public boolean isCellEditable(int row, int col) {

return false;

}

};

JTable table = new JTable(model);

Here, an anonymous subclass of DefaultTableModel is created in which the isCellEditable() method returns false in all cases, and the model object that is created from that class is passed as a parameter to the JTable constructor.

The DefaultTableModel class defines many methods that can be used to modify the table, including for example: setValueAt(item,rowNum,colNum) to change the item in a given cell; removeRow(rowNum) to delete a row; and addRow(itemArray) to add a new row at the end of the table that contains items from the array itemArray. Note that if the item in a given cell is null, then that cell will be empty. Remember, again, that when you modify the model, the view is automatically changed to reflect the changes.

In addition to the isCellEditable() method, the table model method that you are most likely to want to override is getColumnClass(), which is defined as

public Class<?> getColumnClass(columnNum)

The purpose of this method is to specify what kind of values are allowed in the specified column. The return value from this method is of type Class. (The “<?>” is there for technical reasons having to do with generic programming. See Section 10.5, but don’t worry about understanding it here.) Although class objects have crept into this book in a few places— in the discussion of ClassLoaders in Subsection 12.1.3 for example—this is the first time we have directly encountered the class named Class. An object of type Class represents a class. A Class object is usually obtained from the name of the class using expressions of the form “Double.class” or “JTable.class”. If you want a three-column table in which the column types are String, Double, and Boolean, you can use a table model in which getColumnClass is defined as:

public Class<?> getColumnClass(columnNum) { if (columnNum == 0)

return String.class; else if (columnNum = 1) return Double.class;

else

return Boolean.class;