使用類的Java登錄程序

Question

我必須使用公共類和主驅動程序控制臺創建一個簡單的登錄程序。

這裏是我的代碼：

主要

import java.util.Scanner; 
public class main 
{ 

    public static void main(String[] args) 
    { 

     Scanner input = new Scanner (System.in); 

     String username; 
     String password; 


     System.out.println("Welcome to your Social network site!"); 
     System.out.println("\nEnter your username and password to login to your account.");  

     System.out.println("Username: "); 
     username = input.nextLine(); 

     System.out.println("Password: "); 
     password = input.nextLine(); 

     UserAccount login = new UserAccount(username, password); 

     if(login.checkPassword()) 
      System.out.println("You are logged in!"); 
     else 
      System.out.println("The username and password you entered are incorrect."); 
    } 


} 

用戶類別

public class UserAccount 
{ 

    private String username; 
    private String password; 
    private String[][] accounts = {{"anthony", "supernova"},{"steve", "java1"}}; 

    public UserAccount(String username, String password) 
    { 
     String user = username; 
     String pass = password; 
     boolean active; 
    } 

    public boolean checkPassword() 
    { 

     if((username.equals(accounts[0][0])) && (password.equals(accounts[0][1]))) 
      return true; 
     else 
      return false; 
    } 

    public void deactivateAccount() 
    { 
     boolean active = false; 
    } 

} 

的問題是我不斷收到此錯誤後，我輸入正確的信息： 異常線程「main 「java.lang.NullPointerException在UserAccount.checkPassword（UserAccount.java:19）at main.main（main.java:25）

Answer 1

您沒有在您的UserAccount構造函數中分配對象屬性。更改如下：

public UserAccount(String username, String password) { 
    this.username = username; 
    this.password = password; 
} 

此外，如果你打算做與active布爾的東西，你必須將其添加爲一個屬性類爲好。只需在您的構造函數/方法中聲明它就好像您現在正在做的一樣，不會有任何效果。

Answer 2

您的UserAccount構造函數應該初始化類變量。這樣的 -

public UserAccount(String username, String password) 
    { 
     this.username = username; 
     this.password = password; 
     boolean active; 
    } 

HTH

Answer 3

public UserAccount(String username, String password) 
{ 
    this.username = username; 
    this.password = password; 
    // boolean active; //not a good coding standard 
    //in constructor only initialization of class variables must be done 

} 

this will show how to use constructor - 良好的編碼標準 （重要：構造函數必須只能用於初始化不是delcaration）