如何在JTextArea上添加JScrollPane

Question

I am going to make a Simple Notepad. I have used JTextArea for writing some text in it and I want scrollbar on JTextArea . I have written few lines of code.

package project.notepad;

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

public class Notepad extends JFrame {
    private JTextArea area;
    private JMenu filemenu;
    private JMenu editmenu;
    private JMenu formatmenu;
    private JMenu helpmenu;
    private JScrollPane scroll;

    private JMenuBar menubar;
    private JMenuItem newmenuitem;
    private JMenuItem openmenuitem;
    private JMenuItem savemenuitem;
    private JMenuItem exitmenuitem;


    public Notepad() {
        initComponents();
        setComponents();

        setTitle("Simple Notepad");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setVisible(true);
        setResizable(true);
        setSize(600, 600);
        setJMenuBar(menubar);


        menubar.add(filemenu);
        menubar.add(editmenu);
        menubar.add(formatmenu);
        menubar.add(helpmenu);
        filemenu.add(newmenuitem);
        filemenu.add(openmenuitem);
        filemenu.add(savemenuitem);
        filemenu.add(exitmenuitem);

        add(area);
        add(scroll);
    }


    public final void initComponents() {
        scroll = new JScrollPane(area, JScrollPane.VERTICAL_SCROLLBAR_ALWAYS, JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS);
        area = new JTextArea();
        menubar = new JMenuBar();
        filemenu = new JMenu("    File");
        editmenu = new JMenu("    Edit");
        formatmenu = new JMenu("    Format");
        helpmenu = new JMenu("    Help");
        newmenuitem = new JMenuItem("    New");
        openmenuitem = new JMenuItem("    Open");
        savemenuitem = new JMenuItem("    Save");
        exitmenuitem = new JMenuItem("    Exit");
    }

    public final void setComponents() {
        area.setSize(600, 600);
        area.setBackground(Color.WHITE);
    }


    public static void main(String[] args) {
        new Notepad();
    }
}

I'm not sure where the issue lies.

Answer 1

There are three problems here :

1) You add the area to the JScrollPane , before area is initialized.

So you end up with a JScrollPane containing a null component.

To fix this, instantiate area before adding it to the JScrollPane .

2) You add area to the JFrame , then you add the JScrollPane containing area .

This is wrong, a Component can't be added several times. The last addition will win, so you end up with your JFrame containing a mix between a JTextArea , and a JScrollPane now containing null .

To fix this, juste remove add(area); .

3) You call setVisible too early

You should call setVisible(true) , only when all components have been added.

The following code shows the according modifications to the two relevant methods (comments have been added for changes) :

public Notepad() {
    initComponents();
    setComponents();

    setTitle("Simple Notepad");
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    setResizable(true);
    setSize(600, 600);
    setJMenuBar(menubar);

    menubar.add(filemenu);
    menubar.add(editmenu);
    menubar.add(formatmenu);
    menubar.add(helpmenu);
    filemenu.add(newmenuitem);
    filemenu.add(openmenuitem);
    filemenu.add(savemenuitem);
    filemenu.add(exitmenuitem);

    //add(area); // remove this, the textarea is already added to the scrollpane
    add(scroll);

    // set the frame visible, only once all components have been added
    setVisible(true);
}

public final void initComponents() {

    area = new JTextArea(); // instantiate the textarea, before adding to the scrollpane
    scroll = new JScrollPane(area, JScrollPane.VERTICAL_SCROLLBAR_ALWAYS, JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS);

    menubar = new JMenuBar();
    filemenu = new JMenu("    File");
    editmenu = new JMenu("    Edit");
    formatmenu = new JMenu("    Format");
    helpmenu = new JMenu("    Help");
    newmenuitem = new JMenuItem("    New");
    openmenuitem = new JMenuItem("    Open");
    savemenuitem = new JMenuItem("    Save");
    exitmenuitem = new JMenuItem("    Exit");
}

Answer 2

You added the area to the scroll pane before instantiating the area. Just switch scroll and area instantiation lines.

area = new JTextArea();    
scroll = new JScrollPane (area,JScrollPane.VERTICAL_SCROLLBAR_ALWAYS,JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS);   

Should be enough