如何使JTextArea的结尾可编辑

Question

Is there a way to make the end of a JTextArea editable and make anything that has already been printed to it not editable?

What I mean by this is if I've written "Hello World" for example to a JTextArea, how could I make it so that the user can type in whatever they want after "Hello World" but they cannot type before that or delete the already printed text?

Below is a small program to demonstrate my troubles...

public class Test {
    public static void main(String[] args) {
        //Here I create a simple JFrame with JTextArea
        JTextArea textArea = new JTextArea();
        JFrame frame = new JFrame();
        JFrame.setDefaultLookAndFeelDecorated(true);
        frame.setSize(250, 250);
        textArea.setEditable(true);
        textArea.setVisible(true);
        frame.add(textArea);
        frame.setVisible(true);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);


        /*Here I print "Hello World" onto the text area.. after the ">>" I want the
        the user to be able to type whatever they want.. however I don't want them
        to be able to edit the "Hello World"*/
        textArea.append("Hello World\n>>");
        textArea.setCaretPosition(textArea.getDocument().getLength());
    }
}

In the example the user is able to enter whatever text they want.. which is what I want.. however they are also able to edit the text that I printed using append.. which I don't want..

How can I solve this?

Answer 1

Yes, a DocumentFilter will work. Create one that only allows addition of text if the addition is at the end of the document -- that is if the offset equals the document's length. Also totally inactivate the remove method. Something like so:

import javax.swing.text.AttributeSet;
import javax.swing.text.BadLocationException;
import javax.swing.text.DocumentFilter;

public class MyFilter extends DocumentFilter {
    @Override
    public void insertString(FilterBypass fb, int offset, String string, AttributeSet attr)
            throws BadLocationException {
        // only insert text if at the end of the document
        // if offset == document length
        if (offset == fb.getDocument().getLength()) {
            super.insertString(fb, offset, string, attr);
        }
    }

    @Override
    public void replace(FilterBypass fb, int offset, int length, String text, AttributeSet attrs)
            throws BadLocationException {
        // only replace text if at the end of the document
        // if offset == document length
        if (offset == fb.getDocument().getLength()) {
            super.replace(fb, offset, length, text, attrs);
        }
    }

    @Override
    public void remove(FilterBypass fb, int offset, int length) throws BadLocationException {
        // do nothing. Totally inactivate this
    }
}

And you could test it like so:

import javax.swing.*;
import javax.swing.text.PlainDocument;

@SuppressWarnings("serial")
public class LimitedTextArea extends JPanel {
    private JTextArea textArea = new JTextArea(15, 50);

    public LimitedTextArea() {
        // get textArea's Document and cast to PlainDocument:
        PlainDocument document = (PlainDocument) textArea.getDocument();
        // set the document's filter with "MyFilter"
        document.setDocumentFilter(new MyFilter());

        textArea.setLineWrap(true);
        textArea.setWrapStyleWord(true);
        JScrollPane scrollPane = new JScrollPane(textArea);
        scrollPane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
        add(scrollPane);
    }

    private static void createAndShowGui() {
        LimitedTextArea mainPanel = new LimitedTextArea();

        JFrame frame = new JFrame("LimitedTextArea");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(mainPanel);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> createAndShowGui());
    }
}

Answer 2

You could also use a NavigationFilter :

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

public class NavigationFilterPrefix extends NavigationFilter
{
    private int prefixLength;
    private Action deletePrevious;

    public NavigationFilterPrefix(int prefixLength, JTextComponent component)
    {
        this.prefixLength = prefixLength;
        deletePrevious = component.getActionMap().get("delete-previous");
        component.getActionMap().put("delete-previous", new BackspaceAction());
        component.setCaretPosition(prefixLength);
    }

    @Override
    public void setDot(NavigationFilter.FilterBypass fb, int dot, Position.Bias bias)
    {
        fb.setDot(Math.max(dot, prefixLength), bias);
    }

    @Override
    public void moveDot(NavigationFilter.FilterBypass fb, int dot, Position.Bias bias)
    {
        fb.moveDot(Math.max(dot, prefixLength), bias);
    }

    class BackspaceAction extends AbstractAction
    {
        @Override
        public void actionPerformed(ActionEvent e)
        {
            JTextComponent component = (JTextComponent)e.getSource();

            if (component.getCaretPosition() > prefixLength)
            {
                deletePrevious.actionPerformed( null );
            }
        }
    }

    private static void createAndShowUI()
    {
        JTextField textField = new JTextField("Prefix_", 20);
        textField.setNavigationFilter( new NavigationFilterPrefix(7, textField) );

        JFrame frame = new JFrame("Navigation Filter Example");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(textField);
        frame.pack();
        frame.setLocationRelativeTo( null );
        frame.setVisible(true);
    }

    public static void main(String[] args)
    {
        SwingUtilities.invokeLater(new Runnable()
        {
            public void run()
            {
                createAndShowUI();
            }
        });
    }
}

This will allow the user to edit the text they add to the text field.

This will prevent the fixed text from being selected.

For more advanced features, check out the Protected Document which allows you to protect multiple areas of the Document from being changed.