小妹刚学java 照书上抄的代码怎么会有NullPointerException呢

Question

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class swingdemo01 extends JFrame
{  
private JPanel pane=null;
private JPanel p=null;
private CardLayout card=null;
private JButton button_1=null;
private JButton button_2=null;
private JButton b_1=null,b_2=null,b_3=null;
private JPanel p_1=null,p_2=null,p_3=null;
public swingdemo01()
{
super("测试用例");
try
{
UIManager.setLookAndFeel("com.sun.java.swing.plaf.windows.WindowsLookAndFeel");
}
catch(Exception e)
{
e.printStackTrace();
}
card=new CardLayout(5,5);
pane=new JPanel(card);
button_1=new JButton("<上一步");
button_2=new JButton("下一步 >");
b_1=new JButton("1");
b_2=new JButton("2");
b_3=new JButton("3");
b_1.setMargin(new Insets(2,2,2,2));
b_2.setMargin(new Insets(2,2,2,2));
b_3.setMargin(new Insets(2,2,2,2));
        p.add(button_1);
p.add(b_1);
p.add(b_2);
p.add(b_3);
p.add(button_2);
p_1=new JPanel();
p_2=new JPanel();
p_3=new JPanel();
p_1.setBackground(Color.RED);
p_2.setBackground(Color.BLUE);
p_3.setBackground(Color.GREEN);
p_1.add(new JLabel("jpanel_1"));
p_2.add(new JLabel("jpanel_2"));
p_3.add(new JLabel("jpanel_3"));
pane.add(p_1,"p1");
pane.add(p_2,"p2");
pane.add(p_3,"p3");
button_1.addActionListener(new ActionListener()
{
public void actionPerformed(ActionEvent e)
{
card.previous(pane);
}
});
button_2.addActionListener(new ActionListener()
{
public void actionPerformed(ActionEvent e)
{
card.next(pane);
}
});
b_1.addActionListener(new ActionListener()
{
public void actionPerformed(ActionEvent e)
{
card.show(pane,"p1");
}
});
b_2.addActionListener(new ActionListener()
{
public void actionPerformed(ActionEvent e)
{
card.show(pane,"p2");
}
});
b_3.addActionListener(new ActionListener()
{
public void actionPerformed(ActionEvent e)
{
card.show(pane,"p3");
}
});
this.getContentPane().add(pane);
this.getContentPane().add(p,BorderLayout.SOUTH);
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
this.setSize(300,200);
this.setVisible(true);

}
public static void main(String[] args)
{
new swingdemo01();
}

}

Answer 1

上边一行不是Pane=new JPanel(card)了吗?

Answer 2

小妹 你定义了两个JPanel：pane和p其中pane你new了 ，但是你找找你的p  new
了吗？报错的那一行是p.add报的错，因为p木有new

Answer 3

上面几楼说的很对，在使用声明的引用对象时，要给他指向地址的，new JPanel（)就是创建了一个新的对象，把它的地址赋给p，p才能使用，不然就会出现空指针异常