//下面是一个注册小示例
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.EventListener;
import java.awt.event.ItemListener;
import *;
public class huxiang extends Frame implements ActionListener{
    Panel panel1=new Panel();
    Panel panel2=new Panel();
    Panel panel3=new Panel();
    Panel panel4=new Panel();
    Panel panel5=new Panel();
    Panel panel6=new Panel();
    Panel panel7=new Panel();
    Panel panel8=new Panel();
    Panel panel9=new Panel();
    Label label=new Label("QQ注册窗体");
    Label label1=new Label("账 号");
    Label label2=new Label("密 码");
    Label label3=new Label("昵 称");
    Label label4=new Label("性 别");
    Label label5=new Label("EMAIL：");
    Label label6=new Label("地 址");
    Label label7=new Label("           ");
    Choice choice1=new Choice();
    TextField text1=new TextField(12);
    TextField text2=new TextField(12);
    TextField text3=new TextField(12);
    TextField text4=new TextField(12);
    TextField text5=new TextField(12);
    Button button1=new Button("确定");
    Button button2=new Button("取消");
    public huxiang(){
        this.setTitle("QQ注册");
        this.setResizable(false);
        this.setLayout(new BorderLayout());
        button1.addActionListener(this);
        button2.addActionListener(this);
        choice1.addItem("男");
        choice1.addItem("女");
        choice1.select(1);
        panel8.add(label);
        panel1.add(label1);
        panel1.add(text1);
        panel2.add(label2);
        panel3.add(label3);
        panel4.add(label4);
        panel5.add(label5);
        panel6.add(label6);
        panel2.add(text2);
        panel3.add(text3);
        panel4.add(choice1);
        panel5.add(text4);
        panel6.add(text5);
        panel7.add(button1);
        panel7.add(button2);
        panel9.setLayout(new GridLayout(7,1));
        panel9.add(panel1);
        panel9.add(panel2);
        panel9.add(panel3);
        panel9.add(panel4);
        panel9.add(panel5);
        panel9.add(panel6);
        panel9.add(panel7);
        this.add(panel8,BorderLayout.NORTH);
        this.add(panel9,BorderLayout.CENTER);
        this.setVisible(true);
        this.setSize(200,320);
        this.show();
        }
    public static void main(String args[]){
        new huxiang();
        }
    public void actionPerformed(ActionEvent e){
        if(e.getSource()==button1){
            String s1=text1.getText();
            String s2=text2.getText();
            String s3=text3.getText();
            String s5=text4.getText();
            String s6=text5.getText();
            String s4=choice.getSelectItem();
            if(s1.equals("")||s2.equals("")||s3.equals("")||s5.equals("")||s6.equals("")){
                System.out.println("注册信息失败：请填写完整的信息...");
                }
            else{
                String regMsg="regMsg:"+s1+"_"+s2+"_"+s3+"_"+s4+"_"+s5+"_"+s6+"_";
                try{
                    FileWriter output=new FileWriter("reginfor.txt",true);
                    PrintWriter pw=new PrintWriter(output,true);
                    pw.println(reMsg);
                    pw.flush();
                    System.out.println("注册成功...\n注册信息为："+regMsg);
                    }catch(Exception e1){
                        e1.printStackTrace();
                        }
                    }
                }
        else(e.getSource()==button2){                  //编译器提示这个地方错误 not a statement
                System.out.println("退出程序...");
                System.exit(0);
            }
        }
    }
    //大侠们帮我看看吧，我实在是找不出问题了，谢谢了

else --> else if

public void actionPerformed(ActionEvent e){
        if(e.getSource()==button1){
            String s1=text1.getText();
            String s2=text2.getText();
            String s3=text3.getText();
            String s5=text4.getText();
            String s6=text5.getText();
            String s4=choice.getSelectItem();
            if(s1.equals("")||s2.equals("")||s3.equals("")||s5.equals("")||s6.equals("")){
                System.out.println("注册信息失败：请填写完整的信息...");
                }
            else{
                String regMsg="regMsg:"+s1+"_"+s2+"_"+s3+"_"+s4+"_"+s5+"_"+s6+"_";
                try{
                    FileWriter output=new FileWriter("reginfor.txt",true);
                    PrintWriter pw=new PrintWriter(output,true);
                    pw.println(reMsg);
                    pw.flush();
                    System.out.println("注册成功...\n注册信息为："+regMsg);
                    }catch(Exception e1){
                        e1.printStackTrace();
                        }
                    }
                }
        else if(e.getSource()==button2){                 
                System.out.println("退出程序...");
                System.exit(0);
            }
        }

注意if的格式：
1、if(  ..  ){
    .........
    }else{
    .........
    }

2、if(...){
    .....
    }else if(...){
    .....
    }else{
    .....
    }

3、if(...){
    .....
    }else if(...){
    .....
    }

else后面没有()来加条件，直接加{}，如果的()，就要用else if()

我刚好再做这个，写完了 给你借鉴一下

import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.EventListener;//监听器
import java.awt.event.ItemListener;//接受器
import *;
public class RegFrame extends Frame implements ActionListener{
   
    Panel p1=new Panel();
    Panel p2=new Panel();
    Panel p3=new Panel();
    Panel p4=new Panel();
    Panel p5=new Panel();
    Panel p6=new Panel();
    Panel p7=new Panel();
    Panel p8=new Panel();
    Panel p9=new Panel();
   
    Label label =new Label("QQ注册窗体");
    Label label1=new Label("账号");
    Label label2=new Label("密码");
    Label label3=new Label("昵称");
    Label label4=new Label("性别");
    Label label5=new Label("Email");
    Label label6=new Label("地址");
    Label label7=new Label("               ");
     
    Choice choice1=new Choice();
           
   
    TextField text1=new TextField(12);
    TextField text2=new TextField(12);
    TextField text3=new TextField(12);
    TextField text4=new TextField(12);
    TextField text5=new TextField(12);
   
     Button button1=new Button("确定");
    Button button2=new Button("取消");
   
     public RegFrame(){
        
        
        this.setTitle("QQ注册窗体");
        this.setResizable(false);
        this.setLayout(new BorderLayout());
        
        button1.addActionListener(this);//设置一个监听器（如果点击这个按钮的时候，联系public void actionPerformed(ActionEvent e){事件}能产生点击事件。）
        button2.addActionListener(this);//设置一个监听器
        
        choice1.addItem("男");
         choice1.addItem("女");
        choice1.select(1);//设置初始的选择项0是男，1是女
        
        
        p8.add(label);
        
        p1.add(label1);
        p1.add(text1);
   
         p2.add(label2);
        p2.add(text2);
        
        p3.add(label3);
        p3.add(text3);
        
        p4.add(label4);
        p4.add(label7);
        p4.add(choice1);
        p4.add(label7);
      
        p5.add(label5);
        p5.add(text4);
        
        p6.add(label6);
        p6.add(text5);
        
        p7.add(button1);
        p7.add(button2);
        
        
        p9.setLayout(new GridLayout(7,1));
        p9.add(p1);
        p9.add(p2);
        p9.add(p3);
        p9.add(p4);
        p9.add(p5);
        p9.add(p6);
        p9.add(p7);
        
        
        this.add(p8,BorderLayout.NORTH);  //设置面板8在北边
        this.add(p9,BorderLayout.CENTER);//设置面板9在中间
        this.setVisible(true);   //设置窗体为可见
        this.setSize(300,300);
        this.show( );
        }
public static void main(String args[]){
   
    new RegFrame();
    }        
public void actionPerformed(ActionEvent e){
    String s1=text1.getText();//账号
    String s2=text2.getText();//密码
    String s3=text3.getText();//昵称
    String s4=choice1.getSelectedItem();//性别
    String s5=text4.getText();//email
    String s6=text5.getText();//地址
   

    if(e.getSource()==button1||s1.equals("")||s2.equals("")||s3.equals("")||s5.equals("")||s6.equals("")){
        System.out.println("注册信息失败：请填写完整的信息...");
        }   
    else{
        String regMsg="regMsg:"+s1+"_"+s2+"_"+s3+"_"+s4+"_"+s5+"_"+s6;
        
        try{
            FileWriter output=new FileWriter("regInfor.txt",true);
            PrintWriter pw=new PrintWriter(output,true);
            pw.println(regMsg);
            pw.flush();
            System.out.println("注册成功...\n 注册信息为:"+regMsg);
            }
        catch(Exception d1){d1.printStackTrace();}
        }
    if(e.getSource()==button2){
    System.out.println("退出程序...");
    System.exit(0);
    }
        
        
        }
   
    }