三国杀主公有哪些:谁能帮我解释一下~~谢谢了

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.border.*;
import java.sql.*;
import java.util.*;
import java.text.*;
public class CardManagement implements ActionListener
{ private JTextField jtfd1,jtfd2,jtfd3,jtfd4;
private JButton btn1,btn2,btn3,btn4;
private Statement stmt;
private PreparedStatement pstmt;
private Connection con;
private JFrame frame;

public CardManagement()
{JFrame.setDefaultLookAndFeelDecorated(true);
frame=new JFrame("借阅证管理");

Container content=frame.getContentPane();
JLabel lb1=new JLabel("借阅证号:");
JLabel lb2=new JLabel("按借阅证号:");
JLabel lb3=new JLabel("按有效期:");
JLabel lb4=new JLabel("借阅证号:");
jtfd1=new JTextField();
jtfd2=new JTextField();
jtfd3=new JTextField();
jtfd4=new JTextField();
jtfd3.addFocusListener(new FocusHandler());
btn1=new JButton("挂失");
btn2=new JButton("注销");
btn3=new JButton("注销");
btn4=new JButton("取消挂失");
btn3.addActionListener(this);
btn1.addActionListener(this);
btn2.addActionListener(this);
btn4.addActionListener(this);
JPanel pl1=new JPanel();
JPanel pl2=new JPanel();
JPanel pl3=new JPanel();
pl1.setBorder(BorderFactory.createTitledBorder("挂失"));
pl2.setBorder(BorderFactory.createTitledBorder("注销"));
pl3.setBorder(BorderFactory.createTitledBorder("取消挂失"));
GridBagLayout gbl=new GridBagLayout();
GridBagConstraints gbc=new GridBagConstraints();
gbc.fill=GridBagConstraints.BOTH;
gbc.weightx=gbc.weighty=10.0;
pl1.setLayout(gbl);

gbc.gridwidth=1;
gbc.weightx=1.0;
gbl.setConstraints(lb1,gbc);

pl1.add(lb1);

gbc.weightx=8.0;
gbc.gridheight=2;
gbc.gridwidth=2;
gbc.insets=new Insets(15,15,15,5);
gbl.setConstraints(jtfd1,gbc);
pl1.add(jtfd1);

gbc.insets=new Insets(19,0,19,5);
gbc.weightx=1.0;
gbc.gridheight=2;
gbc.gridwidth=3;
gbc.gridwidth=gbc.REMAINDER;
gbl.setConstraints(btn1,gbc);
pl1.add(btn1);

GridBagLayout gbl2=new GridBagLayout();
GridBagConstraints gbc2=new GridBagConstraints();
gbc2.fill=GridBagConstraints.BOTH;
gbc2.weightx=gbc2.weighty=10;
pl2.setLayout(gbl2);
gbc2.gridwidth=1;
gbc2.weightx=1;
gbl2.setConstraints(lb2,gbc2);

pl2.add(lb2);

gbc2.gridwidth=2;
gbc2.insets=new Insets(0,0,0,5);
gbc2.weightx=8;
gbc2.gridheight=2;
gbl2.setConstraints(jtfd2,gbc2);
pl2.add(jtfd2);

gbc2.gridwidth=gbc2.REMAINDER;
gbc2.insets=new Insets(5,0,5,5);
gbc2.weightx=1;
gbc2.gridwidth=2;
gbc2.gridheight=2;
gbl2.setConstraints(btn2,gbc2);
pl2.add(btn2);

gbc2.gridx=0;
gbc2.insets=new Insets(5,0,0,0);
gbc2.gridwidth=1;
gbc2.weightx=1;
gbl2.setConstraints(lb3,gbc2);
pl2.add(lb3);

gbc2.gridx=1;
gbc2.gridwidth=2;
gbc2.insets=new Insets(5,0,0,5);
gbc2.weightx=8;
gbl2.setConstraints(jtfd3,gbc2);
pl2.add(jtfd3);
gbc2.gridx=3;
gbc2.gridwidth=gbc2.REMAINDER;
gbc2.insets=new Insets(10,0,5,5);
gbc2.weightx=1;
gbl2.setConstraints(btn3,gbc2);
pl2.add(btn3);

}