IDEA GUI-管理系统登录界面报错

Question

package view;

import dao.UserDao;
import model.User;
import Until.DBUtils;
import Until.StringUtil;

import javax.swing.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.sql.Connection;
import java.sql.SQLException;

public class login {
    private JPanel login;
    private JTextField username;
    private JPasswordField Password;
    private JButton Buttond;
    private JButton Buttonc;


    private DBUtils dbUtil=new DBUtils();
    private UserDao userDao=new UserDao();

    public login() {
        Buttonc.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                resetvalueaction(e);
            }
        });
        Buttond.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                loginaction(e);
            }
        });
    }

//重置事件处理
    private void resetvalueaction(ActionEvent evt) {
        this.Password.setText("");
        this.username.setText("");
    }
//登录事件处理
    private void loginaction(ActionEvent evt) {
       String username=this.username.getText();
       String password=new String(this.Password.getPassword());
        if (StringUtil.isEmpty(username)){
            JOptionPane.showMessageDialog(null,"用户名不能为空！");
            return;
        }
        if(StringUtil.isEmpty(password)){
            JOptionPane.showMessageDialog(null,"密码不能为空！");
            return;
        }
        User user=new User(username,password);
        Connection con=null;
        try {
            con=dbUtil.getConn();
            User cc=userDao.login(con,user);
            if(cc!=null) JOptionPane.showMessageDialog(null,"登录成功");
            else JOptionPane.showMessageDialog(null,"用户名密码错误！");
        } catch (SQLException throwables) {
            throwables.printStackTrace();
        }
    }

    public static void main(String[] args) {
        JFrame frame = new JFrame("login");
        frame.setContentPane(new login().login);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.pack();
        frame.setVisible(true);
    }

    private void createUIComponents() {
        // TODO: place custom component creation code here
    }


    }

下面为报错：

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "java.awt.Container.setLayout(java.awt.LayoutManager)" because "<local1>" is null
	at view.login.$$$setupUI$$$(login.java)
	at view.login.<init>(login.java:22)
	at view.login.main(login.java:71)

Answer 1

frame.setContentPane(new login().login); 错误

setContentPane方法里面只能设置布局方式，如

setContentPane(new BorderLayout());

整个程序有很多问题，控件都没有创建，也没有放到界面。参考

package java3.T6;

import java.awt.Button;
import java.awt.Frame;
import java.awt.Label;
import java.awt.TextField;

public class LoginUI extends Frame {
	
	public LoginUI() {
		init();
	}
	public LoginUI(String title) {
		//设置窗口标题
//		super("用户登录");
		init();
	}
	private void init() {
		//设置窗口标题
		setTitle("用户登录");
		//取消布局
		setLayout(null);
		
		Label lblUser = new Label("用户名称：",Label.RIGHT);
		Label lblPass = new Label("登录密码：",Label.RIGHT);
		//文本框
		TextField txtUser = new TextField(10);
		TextField txtPass = new TextField(10);
		//按钮
		Button btnLogin = new Button("登录");
		Button btnClose = new Button("关闭");
		//设置控件在窗口上的位置(x,y,width,height)
		lblUser.setBounds(100, 110, 100, 20);
		lblPass.setBounds(100, 160, 100, 20);
		txtUser.setBounds(200, 110, 200, 20);
		txtPass.setBounds(200, 160, 200, 20);
		
		btnLogin.setBounds(280, 230, 90, 30);
		btnClose.setBounds(390, 230, 90, 30);
		add(lblUser);
		add(lblPass);
		add(txtUser);
		add(txtPass);
		add(btnLogin);
		add(btnClose);
		//设置窗口大小
		setSize(530,300);
		//显示窗口
		setVisible(true);
		
	}
	public static void main(String[] args) {
		new LoginUI();
	}
}