JScrollPane在JTextArea中代理

Question

Hi there fellow comrades!
Thanks to you all I've managed to get my small experiment project going further but as you go further there are always obsticles so I again ran into one. I got a JTextArea outputting a list of running processes but JScrollPane isn't showing up but when I resize the window itself the JScrollPane hovers over the whole JTextArea.
Looks like this:

After sever hours of scratching the back of my head I decided to ask you guys!
Code is:

    import java.awt.BorderLayout;
    import java.awt.Component;
    import java.awt.Container;
    import java.awt.Font;
    import java.awt.GridBagLayout;
    import java.io.*;
    import javax.swing.*;

    public class JTask {

        JButton button = new JButton();
        JFrame frame = new JFrame();
        JTextArea area = new JTextArea();
        JScrollPane scrollPane = new JScrollPane();

        JTask() throws IOException{
            frame.setBounds(100, 100, 1000, 700);
            frame.setLayout(null);
            area.setFont(new Font("monospaced", Font.PLAIN, 14));
            area.setBounds(5,25,972,500);
            scrollPane.add(area);
            scrollPane = new JScrollPane(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS,     JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS);
            scrollPane.setBounds(5,25,972,500);
            scrollPane.setViewportView(area);

            scrollPane.setEnabled(true);
            scrollPane.setVisible(true);
            scrollPane.repaint();
            scrollPane.revalidate();
            area.setVisible(true);  

            frame.add(scrollPane);
            frame.add(area);

            frame.setVisible(true);

            try {
                String line;
                Process p = Runtime.getRuntime().exec(System.getenv("windir") +"\\system32\\"+"tasklist.exe");
                BufferedReader input = new BufferedReader(new InputStreamReader(p.getInputStream()));
                while ((line = input.readLine()) != null) {
                    System.out.println(line);
                    area.append(line);
                    area.append("\n");
                }
                input.close();
            } catch (Exception e) {
                e.printStackTrace();
            }

            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        }

    public static void main(String args []) throws IOException{
        JTask task = new JTask();

    }
}

I apologize for a messy code and if anything could be done better but I hope you understand that I am just a beginner that is trying to understand the mechanics of Java programming language and sink in into the world of Java. Thank you all in advance!

Answer 1

Why are you using two JScrollPane ? Simply add JScrollPane in JFrame and JTextArea in JScrollPane .

sample code:

public JTask() throws IOException {
    JFrame  frame = new JFrame();
    JTextArea  area = new JTextArea();
    area.setFont(new Font("monospaced", Font.PLAIN, 14));
    JScrollPane  scrollPane = new JScrollPane(area);
    frame.add(scrollPane);
    ...
    // set text in JTextArea
    ...
    frame.pack();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setLocationRelativeTo(null);
    frame.setVisible(true);
}

It's worth reading Swing Tutorial on How to Use Text Areas to learn more and find more detail examples.

Some points

• Try Java7 - The try-with-resources Statement to close the resources after the program is finished with it.

• First don't use null layout. There are lots of layout manager that is designed for this purpose.

  See Swing Tutorial on How to Use Various Layout Managers

• Call frame.setVisible(true) in the end to make sure everything is added in JFrame

• Use SwingUtilities.invokeLater() or EventQueue.invokeLater() to make sure that EDT is initialized properly.

  Read more...

Answer 2

You may try this code

JScrollPane scrollPane = new JScrollPane(area);
scrollPane.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
scrollPane.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS);

not necessary to use view port in this case, the same is concerned to revalidation and repainting before frame is visible.

all you need is frame.getContentPane().add(scrollPane);