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Java Swing O Reilly This sole method of the TreeExpansionEvent returns the path that was collapsed or expanded. You can query the path to see whether or not it is currently expanded, but the listener interface allows you to easily distinguish which event occurred. 17.6.6 The TreeExpansionListener Interface To catch these events yourself, you can implement the TreeExpansionListener interface, which provides the following two methods to easily separate tasks for collapsing and expanding paths: public void treeExpanded(TreeExpansionEvent e) public void treeCollapsed(TreeExpansionEvent e) Called when a path is collapsed or expanded. 17.6.7 Pending Expansion Events The JDK1.2 beta4 release introduced two new classes that help you listen and react to expansion events before they occur. The TreeWillExpandListener interface allows you to register interest in pending expansion events. Implementations of that interface throw an ExpandVetoException if they decide that the expansion or collapse should not be allowed. 17.6.7.1 The TreeWillExpandListener Interface This interface gives you access to the expansion events (both expanding and collapsing) before the event takes place in the tree itself. The only reason you would want to hear about such an event rather than listening for the real expansion event is if you want to do something with the tree before it changes. The interface provides the following two methods: one for expand events, the other for collapse events. public void treeWillExpand(TreeExpansionEvent event) throws ExpandVetoException public void treeWillCollapse(TreeExpansionEvent event) throws ExpandVetoException Implement these methods to react to pending expansion events. 17.6.7.2 The ExpandVetoException Class The most common reason for listening to pending expansion events is that you may want to stop one of those events from occurring. If the user did not have permission to expand a folder in a filesystem, for example, you could have a listener checking each expand event. If you find such a case where an expansion or collapse should not occur, your listener can throw an ExpandVetoException. Each of the listener methods mentioned above can throw this exception. The JTree setExpandedState() method catches these exceptions, and if one is thrown, the fireTreeExpanded() or fireTreeCollapsed() methods are never called. 17.6.7.3 Constructors public ExpandVetoException(TreeExpansionEvent event) public ExpandVetoException(TreeExpansionEvent event, String message) - 515 -
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Java Swing O Reilly public TreePath getOldLeadSelctionPath() These methods give you access to the old and new lead selection path objects respectively. The old lead selection path may be empty (null). public TreePath getPath() public TreePaths[] getPaths() These methods allow you to retrieve the currently selected path or paths. If the selection has multiple paths and you call getPath(), you’ll receive the first path in the array that would be returned by getPaths(). public boolean isAddedPath() Determines whether a particular selection event reflects an addition to an ongoing selection, such as a range or discontiguous selection. Only the first entry in the selection event is checked, even if multiple selections are part of the event. public boolean isAddedPath(TreePath path) This version of isAddedPath() checks the specified path to see if it was added to the current selection. 17.6.4 The TreeSelectionListener Interface The TreeSelectionListener interface carries only one method: public void valueChanged(TreeSelectionEvent e) Called whenever the selection on a tree changes. The DefaultTreeModel puts this method to use even for selections caused programmatically. 17.6.5 The TreeExpansionEvent Class Normally, expanding and collapsing elements of a tree is handled for you by the tree UI. However, if you want to listen and react to these events, you can do so. The TreeExpansionEvent class covers both expanding and collapsing a tree node. 17.6.5.1 Fields protected TreePath path Stores the path associated with this event. 17.6.5.2 Constructors public TreeExpansionEvent(Object source, TreePath path) The source for this constructor is most often the tree itself, but it’s certainly possible to imagine a GUI trigger being considered the source for a collapse or expand call. 17.6.5.3 Methods public TreePath getPath() - 514 -
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Java Swing O Reilly 17.6.3 The TreeSelectionEvent Class Selection events occur whenever a user (or program for that matter) changes the selection on a tree. For example, if you went through a directory tree and manually selected 12 discontiguous files, that would generate 12 selection events, each building on the last. If you were to pick 12 contiguous files by selecting the first file and using a modifier to pick the last, that would generate only two selection events. (Both of these examples assume that nothing was originally selected.) As with list selections, unselecting something also counts as a selection event. In many cases where more than one file can be selected, you don’t want to listen for selection events directly, but rather, you should provide an “OK” button or some other means for the user to indicate the current selection is the final, desired selection. 17.6.3.1 Fields protected boolean[] areNew For all the paths in this event, this array stores a corresponding true value if the path is new to the selection. protected TreePath newLeadSelectionPath protected TreePath oldLeadSelectionPath Store the lead path and previous lead path associated with the event. protected TreePath[] paths Stores all of the paths associated with this event. 17.6.3.2 Constructors public TreeSelectionEvent(Object source, TreePath path, boolean isNew, TreePath oldLeadSelectionPath, TreePath newLeadSelectionPath) Builds a TreeSelectionEvent centered on one path. The isNew argument determines whether the selection is an addition to (true) or a removal from (false) the current selection. public TreeSelectionEvent(Object source, TreePath paths[], boolean areNew[], TreePath oldLeadSelectionPath, TreePath newLeadSelectionPath) This version of the constructor allows you to build a selection event that starts off with multiple selections in place. This would be useful in a filesystem tree for selecting things that matched a filter, like all the .java files. 17.6.3.3 Methods public Object cloneWithSource(Object newSource) This clever method allows you to clone an event and modify the source component that ostensibly generated the event. This is great for a component that delegates some or all of its visual presence to a tree. You can use this method in an event adapter to pass on the event to some other listener, with the new component listed as the source. public TreePath getNewLeadSelectionPath() - 513 -
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Java Swing O Reilly double value = evaluate(node); setText(”Current expression value: ” + value); repaint(); } protected double evaluate(Object n) { if (n instanceof Integer) { return ((Integer)n).doubleValue(); } // must be an OpNode . . . OpNode node = (OpNode) n; double leftSide = evaluate(node.getChild(0)); double rightSide = evaluate(node.getChild(1)); String op = node.getOperator(); // Ok, do the correct calculation of leftside OP rightside if (op.equals( OpNode.ADD )) return leftSide + rightSide; if (op.equals( OpNode.SUBTRACT )) return leftSide - rightSide; if (op.equals( OpNode.MULTIPLY )) return leftSide * rightSide; if (op.equals( OpNode.DIVIDE )) return leftSide / rightSide; // Shouldn’t get here, but just in case return Double.NaN; } // Implement the TreeModelListener methods. Regardless of the change, we just// need to recalculate and redisplay the tree’s value from the root down. public void treeNodesChanged(TreeModelEvent tme) { Object source = tme.getSource(); if (source instanceof JTree) { showEvaluation(((JTree)source).getModel().getRoot()); } else if (tme.getSource() instanceof TreeModel) { showEvaluation(((TreeModel)source).getRoot()); } } public void treeNodesInserted(TreeModelEvent tme) { treeNodesChanged(tme); } public void treeNodesRemoved(TreeModelEvent tme) { treeNodesChanged(tme); } public void treeStructureChanged(TreeModelEvent tme) { treeNodesChanged(tme); } } The modifications to the ExprTree class init() method are fairly minimal. To make sure that the label follows changes to the tree, we have to add it as a listener to the model events: // Add the Evaluator stuff . . . EvaluatorLabel el = new EvaluatorLabel(); getContentPane().add(el, BorderLayout.SOUTH); el.showEvaluation(operators[0]); treeModel.addTreeModelListener(el); - 512 -
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Java Swing O Reilly Indicate that nodes have changed, have been inserted, or have been removed, respectively. public void treeStructureChanged(TreeModelEvent e) Indicates that the tree structure has changed significantly (such as several subtrees being deleted) and may require more analysis than can be performed on the nodes and children retrievable through the event object e. We can use this class of events to monitor the state of the tree. For example, with our expression tree, we could write a simple expression evaluator class that displays the current calculated value of the expression. Any time we change the tree, we’ll want to update the calculated value. For this simple example, we react the same to the various subtypes of events, since we have to recalculate the entire tree to get the correct response. In a large model tree, we might decide to cache the results of a subtree in the OpNode class, and then we would only have to evaluate portions of the tree to update the total calculated value. We’ll bundle this expression evaluator into a JLabel object so we can monitor it. Figure 17.8 shows the results. Figure 17.8. A tree event listener label that updates the current value of our expression The code for the EvaluatorLabel is listed below. The evaluate() method does the recursive work of calculating the answer to the expression. The various TreeModelListener methods ensure that any time the tree changes, we re-evaluate the expression. // EvaluatorLabel.java // An extension of the JLabel class that evaluates the value of an expression // tree. This class is specifically designed to work with OpNodes and Integer // objects, but it returns a double value so that odd expressions will show an // interesting value instead of 0 . . . // import javax.swing.*; import javax.swing.event.*; import javax.swing.tree.*; public class EvaluatorLabel extends JLabel implements TreeModelListener { public EvaluatorLabel() { super(”No tree specified”); } public void showEvaluation(Object node) { - 511 -
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Java Swing O Reilly 17.6.1 The TreeModelEvent Class The TreeModelEvent class encapsulates model changes in the form of the path that has changed, as well as information on the children of that path. 17.6.1.1 Fields protected int[] childIndices protected Object[] children Hold the children and their indices in the parent node associated with this event. protected TreePath path Holds the path to the parent of the children associated with this event. 17.6.1.2 Constructors public TreeModelEvent(Object source, Object path[], int childIndices[], Object children[]) public TreeModelEvent(Object source, TreePath path, int childIndices[], Object children[]) These constructors allow you to build an event that encompasses the children of a modified node. This type of event is useful if the references to the node’s children have changed, or if the number of children has changed. public TreeModelEvent(Object source, Object path[]) public TreeModelEvent(Object source TreePath path) If the modified node is the only interesting node for this event (if its value changed, but nothing happened to its children, for example) then you can use these constructors. 17.6.1.3 Methods public int[] getChildIndices() public Object[] getChildren() If the event contains information about the affected children of a node, you can retrieve those indices and the children themselves with these methods. public Object[] getPath() public TreePath getTreePath() These methods supply access to the main node of the event. Whether you look at that node through a TreePath object or through an Object array depends on your program; both methods lead to the same node. 17.6.2 The TreeModelListener Interface The TreeModelListener requires that listeners implement the following methods: public void treeNodesChanged(TreeModelEvent e) public void treeNodesInserted(TreeModelEvent e) public void treeNodesRemoved(TreeModelEvent e) - 510 -
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Java Swing O Reilly protected boolean arePathsContiguous(TreePath paths[]) Returns true if the entries in paths are contiguous. The paths do not need to be sorted in any order to be contiguous. protected boolean canPathsBeAdded(TreePath paths[]) Returns true if the entries in paths can be added to the current selection. For example, entries might not be “addable” if they are not part of a contiguous block, and the selection mode is set for contiguous. protected boolean canPathsBeRemoved(TreePath paths[]) Returns true if the entries in paths can be removed without disrupting the selection with respect to the current selection mode. protected void fireValueChanged(TreeSelectionEvent event) Notifies all of the registered TreeSelectionListener objects associated with this selection that the selection has changed. protected void insureRowContinuity() Ensures that a selection is made up of continuous rows. To do this, it starts with the minimum selected row (min), and works its way up to the maximum selected row (max). If any intermediate row, say r, is not selected, the selection is reset to contain the rows from min to r - 1. protected void insureUniqueness() Ensures that all entries listed in the selection are unique. Any duplicate entries are removed. A null selection is unique. protected void notifyPathChange(Vector paths, TreePath oldLeadSelection) Indicates that a change in the selection has occurred. It builds a new TreeSelectionEvent and then calls fireValueChanged(). The paths argument is a vector of PathPlaceHolder objects. The PathPlaceHolder class is a non-public class that holds one TreePath object and a boolean variable to indicate whether or not the tree path is new. protected void updateLeadIndex() Updates the leadIndex property to match the current lead path in the selection. If the selection is null, the leadIndex property is reset to -1. 17.6 Tree Events Trees generate three new types of events that are worth mentioning. Apart from the obvious selection and expansion events from the graphical side, you can also catch structural changes with model events. - 509 -
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Java Swing O Reilly 17.5.3.3 Constant The DefaultTreeSelectionModel contains one constant, shown in Table 17.14. Table 17.14, DefaultTreeSelectionModel Constant Constant Type Description SELECTION_MODE_PROPERTY String The name of the selection mode property used in property change events 17.5.3.4 Fields protected SwingPropertyChangeSupport changeSupport protected EventListenerList listenerList Support the event listeners for property change and selection events, respectively. protected TreePath[] selection Stores the current selection for the tree. You can access it through the getSelectionPaths() or getSelectionRows() methods. protected transient RowMapper rowMapper Stores the value for the rowMapper property. protected int leadIndex protected TreePath leadPath protected int leadRow Store the lead path information associated with the leadSelectionPath and leadSelectionRow properties. The leadIndex is the index of the leadPath. The leadRow is the row number of the leadPath. protected DefaultListSelectionModel listSelectionModel Stores the list selection model that this DefaultListSelectionModel class is based on. protected int selectionMode Stores the value for the selectionMode property. 17.5.3.5 Constructors public DefaultTreeSelectionModel() Creates an instance of the default selection model with all of the properties initialized to the default values listed in Table 17.13. 17.5.3.6 Additional Methods The following protected methods support some of the types of selections available and support the events for the TreeSelectionModel. These might be useful if you subclass DefaultTreeSelectionModel. - 508 -
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Java Swing O Reilly public void removeSelectionPath(TreePath path) public void removeSelectionPaths(TreePath paths[])) Remove the listed path or paths from the current selection. Any selected paths not specified remain selected. public void resetRowSelection() Updates the set of currently selected rows. This would be done if the row mapper changes, for example. 17.5.3 The DefaultTreeSelectionModel Class Swing provides a default implementation of the tree selection model that supports all three modes of selection. You can see this model in use in an example near the beginning of this chapter. 17.5.3.1 Properties The DefaultTreeSelectionModel inherits all of its properties from the TreeSelectionModel interface and supplies the default values listed in Table 17.13. Table 17.13, DefaultTreeSelectionModel Properties Property Data Type get is set bound Default Value leadSelectionPath* TreePath null leadSelectionRow* int -1 maxSelectionRow* int -1 minSelectionRow* int -1 rowMapper* RowMapper null selectionCount* int 0 selectionMode* int DISCONTIGUOUS_TREE_SELECTION selectionPath* TreePath null selectionPaths* TreePath[] null selectionRows* int[] null selectionEmpty* boolean true 17.5.3.2 Events The DefaultTreeSelectionModel supports the same property change and tree selection events as the TreeSelectionModel interface. public void addPropertyChangeListener(PropertyChangeListener l) public void removePropertyChangeListener(PropertyChangeListener l) Add or remove listeners interested in receiving property change events. public void addTreeSelectionListener(TreeSelectionListener l) public void removeTreeSelectionListener(TreeSelectionListener l) Add or remove listeners interested in receiving tree selection events. These events come from the DefaultTreeSelectionModel, rather than the JTree itself. - 507 -
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Java Swing O Reilly The value of the selectionMode property must be one of the constants listed in Table 17.12 and defined in the TreeSelectionModel interface. Table 17.12, TreeSelectionModel Constants Constant Type Description Allows only one path in the tree to be selected at any one time. SINGLE_TREE_SELECTION int Choosing a new path deselects the previous choice. Allows several paths to be selected; they must be in a continuous CONTIGUOUS_TREE_SELECTION int block. The block ranges from the minSelectionRow to the maxSelectionRow. Allows several paths to be selected; they can be any set of nodes, DISCONTIGUOUS_TREE_SELECTION int contiguous or otherwise. 17.5.2.3 Events The TreeSelectionModel requires classes that implement the model to implement methods for registering listeners for property change events and tree selection events. TreeSelectionEvent is discussed in greater detail in “Tree Events,” later in this chapter. public void addPropertyChangeListener(PropertyChangeListener l) public void removePropertyChangeListener(PropertyChangeListener l) Add and remove listeners interested in receiving property change events. public void addTreeSelectionListener(TreeSelectionListener l) public void removeTreeSelectionListener(TreeSelectionListener l) Add and remove listeners interested in receiving tree selection events. These methods differ from the JTree methods for adding or removing listeners, in that the event source sent to these listeners is the selection model, whereas the event source sent to the JTree selection listeners is the tree itself. 17.5.2.4 Selection Methods Many of these will look familiar if you read through the section on the JTree class. Trees pass along many of the selection calls to the selection model that supports it, so that you can deal primarily with the tree itself. public void addSelectionPath(TreePath path) public void addSelectionPaths(TreePath paths[]) These methods augment the current selection with the supplied path or paths. public void clearSelection() Clears the current selection, leaving the selection empty. If nothing is selected before calling clearSelection(), this method should have no effect. public boolean isPathSelected(TreePath path) public boolean isRowSelected(int row) Return true if the path or row specified is in the current selection. - 506 -
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