编译无问题 但是运行后只有标题框在桌面左上角？

Question

import java.awt.*;import javax.swing.*;import java.awt.event.*;class Login extends JFrame{JLabel l1;JLabel l2;JTextField t1;JTextField t2;JButton b1;JButton b2;public void Log(){l1=new JLabel("账号");l2=new JLabel("密码");t1=new JTextField(10);t2=new JTextField(10);b1=new JButton("确定");b2=new JButton("取消");Container c=getContentPane();setSize(300,200);setLocation(Toolkit.getDefaultToolkit().getScreenSize().width/3,Toolkit.getDefaultToolkit().getScreenSize().width/3);setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);c.add(l1);c.add(t1);c.add(l2);c.add(t2);c.add(b1);c.add(b2);}}class afkja{public static void main(String[] args){Login log=new Login();log.setVisible(true);}}

Answer 1

public void Log()
建议把这一句改成类的构造函数Login（）{。。。。。。}，如果对布局有要求，那么加个布局管理器c.setLayout(new FlowLayout());
setLocation(Toolkit.getDefaultToolkit().getScreenSize().width/3,Toolkit.getDefaultToolkit().getScreenSize().width/3);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
开头加个this.

Answer 2

要这样写：
log.setBounds(0, 0, 300, 300);
log.setVisible(true);

Answer 3

import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.*;

class Login extends JFrame implements ActionListener{
JLabel accountLabel=new JLabel("账号：");
JLabel pwdLabel=new JLabel("密码：");
JTextField name=new JTextField(10);
JPasswordField password=new JPasswordField();
JButton butnSure=new JButton("确定");
JButton butnCancel= new JButton("取消");;

public Login(String s) {
super(s);
setLayout(null);
add(accountLabel);
accountLabel.setBounds(45, 20, 100, 25);
add(pwdLabel);
pwdLabel.setBounds(45, 60, 100, 25);
add(name);
name.setBounds(105, 20, 110, 25);
add(password);
password.setBounds(105, 60, 110, 25);
add(butnSure);
butnSure.setBounds(45, 100, 80, 25);
add(butnCancel);
butnCancel.setBounds(135, 100, 80, 25);
butnSure.addActionListener(this);
butnCancel.addActionListener(this);
setVisible(true);
setSize(300, 200);
setLocation(Toolkit.getDefaultToolkit().getScreenSize().width / 3,
Toolkit.getDefaultToolkit().getScreenSize().width / 6);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
validate();//刷新
}

public static void main(String[] args) {
Login log = new Login("用户登陆");
}

public void actionPerformed(ActionEvent e) {
if (e.getSource() ==butnSure){
System.out.println("用户名："+name.getText());
System.out.println("密码："+name.getText());
if("wyjwsj".equals(name.getText().trim())&&"12345".equals(password.getText().trim())){
this.dispose();
new MainFrm("用户界面",name.getText().trim(),password.getText().trim());
}else {
JOptionPane.showMessageDialog(this, "用户名或密码不对，用户名为 wyjwsj,密码为 12345");
}
}else if(e.getSource()==butnCancel){
System.exit(1);
}
}
}

class MainFrm extends JFrame{
private JLabel info;

public MainFrm(String s,String name,String password) {
super(s);
setBounds(400, 200, 500, 400);
setLayout(new FlowLayout());
info=new JLabel("登陆成功，用户名："+name+"，密码："+password);
add(info);
setVisible(true);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
validate();
}
}