java如何遍历一个列表数据_求助，Java数据库编程，遍历表中数据，

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import java.awt.*;

import java.awt.event.ActionEvent;

import java.awt.event.ActionListener;

import java.awt.event.MouseAdapter;

import java.awt.event.MouseEvent;

import javax.swing.*;

import java.sql.*;

public class SQL extends JFrame {

JPanel jp1,jp2,jp3,jp4;

JButton jb1,jb2,jb3;

JLabel jlb1,jlb2,jlb3;

JTextField jtf1;

JPasswordField jpw1,jpw2;

public SQL() {

jp1=new JPanel();

jp2=new JPanel();

jp3=new JPanel();

jp4=new JPanel();

jlb1=new JLabel("用户名");

jlb2=new JLabel("密码");

jlb3=new JLabel("再次输入密码");

jtf1=new JTextField(16);

jpw1=new JPasswordField(22);

jpw2=new JPasswordField(22);

jb1=new JButton("检测");//后面的程序有问题

jb1.addActionListener(new ActionListener() {

public void actionPerformed(ActionEvent e) {

String st;

st=jtf1.getText();

try {

Class.forName("com.mysql.jdbc.Driver").newInstance();

} catch (Exception e1) {

// TODO Auto-generated catch block

e1.printStackTrace();

}

try {

String url ="jdbc:mysql://localhost:3306/userdb"; //数据库操作

Connection conn= DriverManager.getConnection(url,"root","");

String str="insert into user values(?,?)";

PreparedStatement ps=conn.prepareStatement(str);

ps.setString(1, st);

Statement stmt=conn.createStatement();

String sql="select * from user";

ResultSet rs=stmt.executeQuery(sql);

String id=rs.getString("id");

if(st==id)

JOptionPane.showMessageDialog(null, "此用户名已被注册！");

//select count(*) as cont from usertable where username= *regname *

//if (rs.getInt( "cont ")> 0){

//重复

//}

} catch (SQLException e1) {

// TODO Auto-generated catch block

}

}

});

jb2=new JButton("注册");

jb2.addActionListener(new ActionListener() {

@SuppressWarnings("deprecation")

public void actionPerformed(ActionEvent e) {

String st1,st2,st3,st;

st1=jtf1.getText();

st2=jpw1.getText();//getPassword().toString().trim();

st3=jpw2.getText();//getPassword().toString().trim();

st=String.valueOf(jpw1.getPassword());

if(st2.equals(st3))

{

try {

Class.forName("com.mysql.jdbc.Driver").newInstance();

} catch (Exception e1) {

// TODO Auto-generated catch block

e1.printStackTrace();

}

try {

String url ="jdbc:mysql://localhost:3306/userdb"; //数据库操作

Connection conn= DriverManager.getConnection(url,"root","");

String str="insert into user values(?,?)";

PreparedStatement ps=conn.prepareStatement(str);

ps.setString(1, jtf1.getText());

ps.setString(2,st);

ps.executeUpdate();

//ps.executeUpdate();

//ps.execute();

JOptionPane.showMessageDialog(null, "注册成功！");

conn.close();

} catch (SQLException e1) {

// TODO Auto-generated catch block

e1.printStackTrace();

}

}

else{

JOptionPane.showMessageDialog(null, "请重新输入密码！");

}

}

});

jb3=new JButton("重输");

jb3.addMouseListener(new MouseAdapter() {

@Override

public void mouseClicked(MouseEvent e) {//重置为空

jtf1.setText("");

jpw1.setText("");

jpw2.setText("");

}

});

this.setLayout(new GridLayout(4,1));

jp1.add(jlb1);

jp1.add(jtf1);

jp1.add(jb1);

jp1.setLayout(new FlowLayout(FlowLayout.RIGHT));

jp2.add(jlb2);

jp2.add(jpw1);

jp2.setLayout(new FlowLayout(FlowLayout.RIGHT));

jp3.add(jlb3);

jp3.add(jpw2);

jp3.setLayout(new FlowLayout(FlowLayout.RIGHT));

jp4.add(jb2);

jp4.add(jb3);

jp4.setLayout(new FlowLayout(FlowLayout.RIGHT));

this.add(jp1);

this.add(jp2);

this.add(jp3);

this.add(jp4);

this.setSize(350, 200);

this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

this.setLocation(200, 200);

this.setVisible(true);

pack();

}

public static void main(String[] args) {

SQL sq=new SQL();

}

}