java登录引发NullPointerException

Question

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

public class Remote implements ActionListener {
    Action  action;
    Gui     gui;
    String  output;
    Boolean result;

public Remote(Gui g, Action a) {
    action = a;
    gui = g;
    actionListenerMeth(this);
}

@Override
public void actionPerformed(ActionEvent arg0) {
    try {
        String a = gui.username_tf.getText();
        char[] c = gui.password_tf.getPassword();
        String b = new String(c);
        result = action.login(a, b);
    } catch (Exception ee) {
        ee.printStackTrace();
    }
    if (result == true) { //<--this is where Eclipse shows me the error ...
        output = "You are Successfuly Loged In!";
    } else {
        output = "Username or Password is Wrong!";
    }
    gui.result_lb.setText(output);
}

public void actionListenerMeth(ActionListener ae) {
    gui.login_bt.addActionListener(ae);
}

}

下面是Action类中的.login方法：

import java.sql.*;

public class Action {
String url = "jdbc:mysql://localhost/";
String dbName = "test";
String driver = "com.mysql.jdbc.Driver";
String username = "root";
String password = "password";
Boolean result;
///
public Boolean login(String x, String y)
{
    String user_var = x;
    String pass_var = y;
    ///
    try {
        Class.forName(driver).newInstance();
        Connection conn = DriverManager.getConnection(url + dbName, username, password);
        Statement st = conn.createStatement();
        ResultSet res = st.executeQuery("SELECT * FROM java where username='"+user_var+"' and password='"+pass_var+"' ");

        while(res.next())
        {
            String user = res.getString("username");
            String pass = res.getString("password");

            if((user_var.equals(user)) && (pass_var.equals(pass))){
                result = true;
            }else{
                result = false;
            }
        }
        conn.close();
    } catch (Exception e) {
        e.printStackTrace();
    }
    return (result);
}

}

Answer 1

在使用框类型boolean之前，需要学习自动装箱和取消装箱。box类型boolean与unbox类型的布尔值等于true或false。如果使用box类型，则代码应为

Boolean result;    
if (result != null) 

或者如果使用unbox类型,则代码应为

boolean result;
if (result == true) {//TODO}

实际上ubox boolean不需要==。你可以这样申报

if (result){//DO SOMETHING}