Java - Changing background color of a nonstatic JTextArea that needs to appear in multiple windows simultaneously

问题

If I have a static JTextArea object, I am able to change its color using name.setBackground(Color c);. However, if the JTextArea object is not static, I can't use methods like name.setText(String s) or name.setBackground(Color c);.

I need the objects to be nonstatic because I need to run multiple instances of them simultaneously in multiple windows.

回答1:

You cannot display multiple instances of a Swing component, only one at time. You can have multiple JTextAreas with the same Document, and thus they would show the same text, but this would not change the background color.

I suggest that you create an ArrayList<JTextArea> field along with a public method that allows other objects to change the background color of all fields. Pass the color into the method's parameter, and inside of the method loop through the list of JTextAreas, changing the background color within the loop.

Note: you state that you need the "objects to be nonstatic..." -- note that objects are neither static nor non-static. Variables on the other hand can be one or the other, and I suggest that you avoid use of static fields and use of public fields as this will help reduce the complexity of your code and thus help reduce the potential for bugs.

For example:

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Component;
import java.awt.Dialog.ModalityType;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.ArrayList;
import java.util.List;

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

@SuppressWarnings("serial")
public class MultTextAreasMain extends JPanel {
    private static final int DIALOG_COUNT = 4; //  how many dialogs?

    // Single Document shared by all JTextAreas -- thus they share text
    private PlainDocument document = new PlainDocument();
    private List<JTextArea> textAreas = new ArrayList<>(); // list of all text areas
    private MultTextAreaPanel panel1 = new MultTextAreaPanel(this);

    public MultTextAreasMain() {
        textAreas.add(panel1.getTextArea());
        setLayout(new BorderLayout());
        add(panel1);
    }

    public PlainDocument getDocument() {
        return document;
    }

    // public method to allow addition of text areas to the array list
    public void addTextArea(JTextArea textArea) {
        textAreas.add(textArea);
    }

    // sets the background color all text areas held by the array list
    public void setTextAreaBackground(Color value) {
        for (JTextArea jTextArea : textAreas) {
            jTextArea.setBackground(value);
        }
    }

    private static void createAndShowGui() {
        MultTextAreasMain main = new MultTextAreasMain();

        JFrame frame = new JFrame("MultTextAreas");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(main);
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);

        for (int i = 0; i < DIALOG_COUNT; i++) {
            MultTextAreaPanel mtaPanel = new MultTextAreaPanel(main);
            main.addTextArea(mtaPanel.getTextArea());
            String title = "Dialog " + (i + 1);
            JDialog dialog = new JDialog(frame, title, ModalityType.MODELESS);
            dialog.add(mtaPanel);
            dialog.pack();
            dialog.setLocationByPlatform(true);
            dialog.setVisible(true);
        }
    }

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

@SuppressWarnings("serial")
class MultTextAreaPanel extends JPanel {
    public static final int GRAY = 150;
    public static final Color[] COLORS = { 
            new Color(255, GRAY, GRAY), 
            new Color(GRAY, 255, GRAY),
            new Color(GRAY, GRAY, 255), 
            new Color(255, 255, GRAY), 
            new Color(255, GRAY, 255),
            new Color(GRAY, 255, 255) };
    private JTextArea textArea = new JTextArea(10, 30);
    private MultTextAreasMain main;
    JComboBox<Color> colorsCombo = new JComboBox<>(COLORS);

    public MultTextAreaPanel(MultTextAreasMain main) {
        this.main = main;
        PlainDocument doc = main.getDocument();
        textArea.setDocument(doc);
        textArea.setWrapStyleWord(true);
        textArea.setLineWrap(true);
        colorsCombo.setRenderer(new ColorListCellRenderer());
        colorsCombo.addActionListener(new ColorsListener());

        JPanel comboPanel = new JPanel();
        comboPanel.add(colorsCombo);

        setLayout(new BorderLayout());
        add(new JScrollPane(textArea));
        add(comboPanel, BorderLayout.PAGE_END);
    }

    public JTextArea getTextArea() {
        return textArea;
    }

    // combo box listener that gets the selected color from the combo box
    // and then calls the main class's method to change the background
    // color of all the JTextAreas held by the ArrayList
    private class ColorsListener implements ActionListener {
        @Override
        public void actionPerformed(ActionEvent e) {
            Color value = (Color) colorsCombo.getSelectedItem();
            if (value != null) {
                main.setTextAreaBackground(value);
            }
        }
    }

    // a renderer for the JComboBox. This displays the RGB constants for colors along with 
    // showing each item's background color. A selected item is darker. 
    private class ColorListCellRenderer extends DefaultListCellRenderer {
        private final int darker = 20;

        @Override
        public Component getListCellRendererComponent(JList<?> list, Object value, int index, boolean isSelected,
                boolean cellHasFocus) {
            DefaultListCellRenderer renderer = (DefaultListCellRenderer) super.getListCellRendererComponent(list, value,
                    index, isSelected, cellHasFocus);
            renderer.setOpaque(true);
            Color color = (Color) value;
            String text = String.format("Color [%d, %d, %d]", color.getRed(), color.getGreen(), color.getBlue());
            renderer.setText(text);
            if (isSelected) {
                int r = color.getRed() == 255 ? 255 : darker;
                int g = color.getGreen() == 255 ? 255 : darker;
                int b = color.getBlue() == 255 ? 255 : darker;
                color = new Color(r, g, b);
            }
            renderer.setBackground(color);
            return renderer;
        }
    }
}

来源：https://stackoverflow.com/questions/37398838/java-changing-background-color-of-a-nonstatic-jtextarea-that-needs-to-appear-i