Question

我试图用左边的按钮看一下基本的，然后它们链接到的面板应该会弹出但是我有一些问题让它正常工作

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

public class MtnDew extends JApplet implements ActionListener
{
    JButton home, first, faq, contact;
    Image img;
JLabel title, logo, copyright, ntbkImg, firstMsg, ntbkDesc, infoSheets, welcomeMsg;
JLabel basket;
Color blackColor = new Color(10, 10, 10);
Color greenColor = new Color( 10,200,50);
JPanel leftside, top, center, welcome, firstPanel,contactPanel, separator;                                                                                         
Jpanel faqPanel;
public void init ()
{
    setLayout( new BorderLayout());
    doTitle();
    doLeftSide();
    doBottom();
    doCenter();
    setupFAQPanel();
}
public void setupButton( JButton b)
{
    b.setContentAreaFilled( false );
    b.setBorderPainted (false );
    b.setFocusable( false );
    b.setForeground ( blackColor );
    b.addActionListener ( this );
    leftside.add( b );
}
public void doTitle()
{
    title = new JLabel("<HTML>The DewMocracy",JLabel.CENTER);
    img = getImage( getCodeBase(), "MtnDew1.jpeg");
    logo = new JLabel (new ImageIcon( img));
    img = getImage( getCodeBase(), "MtnDew2.jpeg");
    title = new JLabel (new ImageIcon( img));
    top = new JPanel (new FlowLayout());
    top.add( logo );
    top.add( title );
    top.setBackground ( blackColor );
    add( top, BorderLayout.NORTH);
}
public void doLeftSide()
{
    //Links on left bar
    leftside = new JPanel (new GridLayout(5, 1));
    leftside.setBackground(greenColor);
    home = new JButton ("Home");
    first = new JButton ("First DewMocracy");
    faq = new JButton ("FAQ");
    contact = new JButton ("Contact Us");
    setupButton ( home );
    setupButton ( first );
    setupButton ( faq );
    setupButton ( contact );
    basket = new JLabel( new ImageIcon( getImage(   getCodeBase(),"MtnDew3.jpeg")));
    leftside.add(basket);
    add( leftside, BorderLayout.WEST);
}
public void doBottom()
{
    copyright = new JLabel("<HTML>Text from dobottom",JLabel.CENTER);
    copyright.setForeground( blackColor);
    copyright.setOpaque( true);
    copyright.setBackground(greenColor);
    add( copyright, BorderLayout.SOUTH);
}

public void doCenter()
{
    center = new JPanel (new BorderLayout());
    center.setBackground( blackColor );
    separator = new JPanel();
    separator.setBackground (greenColor);
    separator.setPreferredSize(new Dimension (10, 20));
    center.add(separator, BorderLayout.NORTH);

    welcome = new JPanel( new FlowLayout());
    welcome.setOpaque(false);
    welcomeMsg = new JLabel("<html><center>"
            + "<H2>this is a welcome message</H2>");
    welcomeMsg.setForeground(greenColor);
    welcome.add(welcomeMsg);
    center.add(welcome,BorderLayout.CENTER);
    add( center, BorderLayout.CENTER);
}
public void setupcontactPanel ()
{
    JLabel contactText = new JLabel ( "<HTML><H1>FAQ</H1>"
            + "contact panel test");
    contactPanel = new JPanel (new FlowLayout());
    contactPanel.setOpaque(false);
    contactPanel.add(contactText);
}
public void setupFAQPanel ()
{
    JLabel faqText = new JLabel ( "<HTML><H1>FAQ</H1>"
            + "lots bla text can go here");
    faqPanel = new JPanel (new FlowLayout());
    faqPanel.setOpaque(false);
    faqPanel.add(faqText);
}
public void actionPerformed (ActionEvent ae)
{
    JButton src = (JButton)ae.getSource();
    center.removeAll();
    separator = new JPanel ();
    separator.setBackground (greenColor);
    separator.setPreferredSize( new Dimension (10, 20));
    center.add(separator, BorderLayout.NORTH);
    if (src.equals(home))
    {
        center.removeAll();
        center.add(welcome,BorderLayout.CENTER);
    }
    else if (src.equals(contact))
    {
        center.removeAll();
        center.add( contactPanel, BorderLayout.CENTER);
    }
    else if (src.equals(faq))
    {
        center.removeAll();
        center.add( faqPanel, BorderLayout.CENTER);
    }
    else if (src.equals(first))
    {
        center.removeAll();
        center.add( firstPanel, BorderLayout.CENTER);
    }
    center.repaint();
    validate();
    repaint();
    }

}

我一直试图让它工作，我甚至开始做一个简单的版本，但它也没有工作。似乎这与我的if语句有关，或者我不理解动作事件的关键元素

编辑：我将If语句更改为代码中现在的内容，但仍然无效。我可以点击主页和常见问题，但我的其他两个jbuttons似乎没有工作。我试图用联系按钮测试它

Answer 1

由于Contact Us，

First DewMocracy和NullPointerException按钮无效。 contactPanel和firstPanel从未初始化，当您尝试将其添加到null中的center面板时actionPerformed()。

另外，请查看A Visual Guide to Layout Managers。 CardLayout可能会有所帮助，而不是手动替换面板。

Answer 2

变化

Object src = ae.getSource();

到

Jbutton src = (JButton)ae.getSource();

然后

if(src.equals(home))

或

if(src.equals(contact))

依旧......

Answer 3

你应该将它们添加到代码顶部的容器中，初始化它们，然后在你的else if语句下使用;

 contactPanel.setVisible(true);

等。对于他们所有人

JPanels没有通过else if语句显示

3 个答案: