急求解答!!

[aaa1810]

import javawt.*;
import javawt.event.*;
class AWT4 extends WindowAdapter implements ActionListener,WindowListener
{
Frame f=new Frame("模拟面板3号实验品");
MenuBar mb;
Menu mf,me,mh,ms;
CheckboxMenuItem cm1;
Panel p1,p2;
String a[]={"1","2","3","4","5","6","7","8","9","*","0","#"};
Button b[];
TextField tf=new TextField(50);

public void setMenuBar()
{
mf=new Menu("文件(F)");
me=new Menu("编辑(E)");
mh=new Menu("帮助(H)");
ms=new Menu("菜单栏示范");
mb.add(mf);
mb.add(me);
mb.add(mh);
mb.add(ms);
mf.add(new MenuItem("打开"));
mf.add(new MenuItem("保存"));
mf.add(new MenuItem("关闭"));
mf.add(new CheckboxMenuItem("我是NB",true));
mf.add(new CheckboxMenuItem("我不是NB"));
}
public void listenter()
{
f.addWindowListener(this);
mf.addActionListener(this);
me.addActionListener(this);
mh.addActionListener(this);
ms.addActionListener(this);
}

public void windowClosing(WindowEvent e)
{
System.exit(0);
}
public void actionPerformed(ActionEvent e)
{
if(e.getActionCommand()=="打开")
tf.setText("已经打开");
if(e.getActionCommand()=="保存")
tf.setText("已经保存");
if(e.getActionCommand()=="关闭")
System.exit(0);
}
public void display()
{
f.setSize(480,360);
f.setMenuBar(mb);
f.setLayout(new FlowLayout());
p1=new Panel();
p1.add(tf);
p2=new Panel();
p2.setLayout(new GridLayout(4,3,5,5));
for(int i=0; i <a.length;i++)
{
Button b[]=new Button[a.length];
p2.add(b[i]);
}

f.setVisible(true);
}
public static void main(String arg[])
{
(new AWT4()).display();
}

}

编译通过
运行失败，说是没找到指针，这是怎么回事？求指正！

[临远]

import java.awt.*;
import java.awt.event.*;

class AWT4 extends WindowAdapter implements ActionListener,WindowListener {
    Frame f=new Frame("模拟面板3号实验品");
    MenuBar mb;
    Menu mf,me,mh,ms;
    CheckboxMenuItem cm1;
    Panel p1,p2;
    String a[]={"1","2","3","4","5","6","7","8","9","*","0","#"};
    Button b[];
    TextField tf=new TextField(50);

    public void setMenuBar() {
        mf=new Menu("文件(F)");
        me=new Menu("编辑(E)");
        mh=new Menu("帮助(H)");
        ms=new Menu("菜单栏示范");
        mb.add(mf);
        mb.add(me);
        mb.add(mh);
        mb.add(ms);
        mf.add(new MenuItem("打开"));
        mf.add(new MenuItem("保存"));
        mf.add(new MenuItem("关闭"));
        mf.add(new CheckboxMenuItem("我是NB",true));
        mf.add(new CheckboxMenuItem("我不是NB"));
    }

    public void listenter() {
        f.addWindowListener(this);
        mf.addActionListener(this);
        me.addActionListener(this);
        mh.addActionListener(this);
        ms.addActionListener(this);
    }

    public void windowClosing(WindowEvent e) {
        System.exit(0);
    }

    public void actionPerformed(ActionEvent e) {
        if(e.getActionCommand()=="打开")
            tf.setText("已经打开");
        if(e.getActionCommand()=="保存")
            tf.setText("已经保存");
        if(e.getActionCommand()=="关闭")
            System.exit(0);
    }

    public void display() {
        f.setSize(480,360);
        f.setMenuBar(mb);
        f.setLayout(new FlowLayout());
        p1=new Panel();
        p1.add(tf);
        p2=new Panel();
        p2.setLayout(new GridLayout(4,3,5,5));

        for(int i=0; i <a.length;i++) {
            //Button b[]=new Button[a.length];
            //p2.add(b[i]);
            p2.add(new Button("" + i));
        }
        f.add(p2);

        f.setVisible(true);
    }
    public static void main(String arg[]) {
        (new AWT4()).display();
    }

}

[老紫竹]

楼上正解
Button b[] = new Button[a.length];
p2.add(b[i]);

这里声明了一个对象数组，可是没有对数组里面的对象进行初始化。导致b[i] 为null

[w358581619]

Button b[] = new Button[a.length];
p2.add(b[i]);