java - 无法在 JscrollPane 中查看 LastItem

Question

我无法滚动到上面滚动条中的最后一项。请帮忙......提前致谢

代码:

JList myAvailableList = new JList();
myAvailableList.setName("AvailableList");
myAvailableList.addListSelectionListener(this);

JScrollPane myAvailableScroll = new JScrollPane(myAvailableList);
myAvailableScroll.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_AS_NEEDED);
myAvailableScroll.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_AS_NEEDED);
myAvailableScroll.setPreferredSize(new Dimension(LIST_PREF_WIDTH, LIST_PREF_HEIGHT));

Answer 1

对于JList您可以设置JList#ensureIndexIsVisible(int index)用于滚动到最后添加的 Item

来自 JList tutorial 的代码示例

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;

/* ListDemo.java requires no other files. */
public class ListDemo extends JPanel implements ListSelectionListener {

    private static final long serialVersionUID = 1L;
    private JList list;
    private DefaultListModel listModel;
    private static final String hireString = "Hire";
    private static final String fireString = "Fire";
    private JButton fireButton;
    private JTextField employeeName;

    public ListDemo() {
        super(new BorderLayout());
        listModel = new DefaultListModel();
        listModel.addElement("Jane Doe");
        listModel.addElement("John Smith");
        listModel.addElement("Kathy Green");

        //Create the list and put it in a scroll pane.
        list = new JList(listModel);
        list.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
        list.setSelectedIndex(0);
        list.addListSelectionListener(this);
        list.setVisibleRowCount(5);
        JScrollPane listScrollPane = new JScrollPane(list);

        JButton hireButton = new JButton(hireString);
        HireListener hireListener = new HireListener(hireButton);
        hireButton.setActionCommand(hireString);
        hireButton.addActionListener(hireListener);
        hireButton.setEnabled(false);

        fireButton = new JButton(fireString);
        fireButton.setActionCommand(fireString);
        fireButton.addActionListener(new FireListener());

        employeeName = new JTextField(10);
        employeeName.addActionListener(hireListener);
        employeeName.getDocument().addDocumentListener(hireListener);
        String name = listModel.getElementAt(list.getSelectedIndex()).toString();

        //Create a panel that uses BoxLayout.
        JPanel buttonPane = new JPanel();
        buttonPane.setLayout(new BoxLayout(buttonPane, BoxLayout.LINE_AXIS));
        buttonPane.add(fireButton);
        buttonPane.add(Box.createHorizontalStrut(5));
        buttonPane.add(new JSeparator(SwingConstants.VERTICAL));
        buttonPane.add(Box.createHorizontalStrut(5));
        buttonPane.add(employeeName);
        buttonPane.add(hireButton);
        buttonPane.setBorder(BorderFactory.createEmptyBorder(5, 5, 5, 5));

        add(listScrollPane, BorderLayout.CENTER);
        add(buttonPane, BorderLayout.PAGE_END);
    }

    class FireListener implements ActionListener {

        public void actionPerformed(ActionEvent e) {
            //This method can be called only if
            //there's a valid selection
            //so go ahead and remove whatever's selected.
            int index = list.getSelectedIndex();
            listModel.remove(index);
            int size = listModel.getSize();
            if (size == 0) { //Nobody's left, disable firing.
                fireButton.setEnabled(false);
            } else { //Select an index.
                if (index == listModel.getSize()) {
                    //removed item in last position
                    index--;
                }
                list.setSelectedIndex(index);
                list.ensureIndexIsVisible(index);
            }
        }
    }

    //This listener is shared by the text field and the hire button.
    class HireListener implements ActionListener, DocumentListener {

        private boolean alreadyEnabled = false;
        private JButton button;

        public HireListener(JButton button) {
            this.button = button;
        }

        //Required by ActionListener.
        public void actionPerformed(ActionEvent e) {
            String name = employeeName.getText();

            //User didn't type in a unique name...
            if (name.equals("") || alreadyInList(name)) {
                Toolkit.getDefaultToolkit().beep();
                employeeName.requestFocusInWindow();
                employeeName.selectAll();
                return;
            }

            int index = list.getSelectedIndex(); //get selected index
            if (index == -1) { //no selection, so insert at beginning
                index = 0;
            } else { //add after the selected item
                index++;
            }

            listModel.insertElementAt(employeeName.getText(), index);
            //If we just wanted to add to the end, we'd do this:
            //listModel.addElement(employeeName.getText());
            //Reset the text field.
            employeeName.requestFocusInWindow();
            employeeName.setText("");
            //Select the new item and make it visible.
            list.setSelectedIndex(index);
            list.ensureIndexIsVisible(index);
        }

        //This method tests for string equality. You could certainly
        //get more sophisticated about the algorithm.  For example,
        //you might want to ignore white space and capitalization.
        protected boolean alreadyInList(String name) {
            return listModel.contains(name);
        }

        //Required by DocumentListener.
        public void insertUpdate(DocumentEvent e) {
            enableButton();
        }

        //Required by DocumentListener.
        public void removeUpdate(DocumentEvent e) {
            handleEmptyTextField(e);
        }

        //Required by DocumentListener.
        public void changedUpdate(DocumentEvent e) {
            if (!handleEmptyTextField(e)) {
                enableButton();
            }
        }

        private void enableButton() {
            if (!alreadyEnabled) {
                button.setEnabled(true);
            }
        }

        private boolean handleEmptyTextField(DocumentEvent e) {
            if (e.getDocument().getLength() <= 0) {
                button.setEnabled(false);
                alreadyEnabled = false;
                return true;
            }
            return false;
        }
    }

    //This method is required by ListSelectionListener.
    public void valueChanged(ListSelectionEvent e) {
        if (e.getValueIsAdjusting() == false) {

            if (list.getSelectedIndex() == -1) {
                //No selection, disable fire button.
                fireButton.setEnabled(false);

            } else {
                //Selection, enable the fire button.
                fireButton.setEnabled(true);
            }
        }
    }

    /**
     * Create the GUI and show it.  For thread safety,
     * this method should be invoked from the
     * event-dispatching thread.
     */
    private static void createAndShowGUI() {
        //Create and set up the window.
        JFrame frame = new JFrame("ListDemo");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        //Create and set up the content pane.
        JComponent newContentPane = new ListDemo();
        newContentPane.setOpaque(true); //content panes must be opaque
        frame.setContentPane(newContentPane);

        //Display the window.
        frame.pack();
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        //Schedule a job for the event-dispatching thread:
        //creating and showing this application's GUI.
        javax.swing.SwingUtilities.invokeLater(new Runnable() {

            public void run() {
                createAndShowGUI();
            }
        });
    }
}