如何「簡化」MySQL數據庫程序的Java代碼？

這裏是我的代碼：

package com.company; 

import com.mysql.jdbc.exceptions.MySQLDataException; 

import java.sql.Connection; 
import java.sql.DriverManager; 
import java.sql.PreparedStatement; 
import java.sql.ResultSet; 
import java.util.Scanner; 

public class Main { 

public static void main(String[] args) { 

    Connection mysql = null; 
    PreparedStatement pst = null; 
    ResultSet data = null; 

    try 
    { 
     Class.forName("com.mysql.jdbc.Driver"); 
     String connectionStringURL = "jdbc:mysql://us-cdbr-azure-west-b.cleardb.com:3306/database"; 
     mysql = DriverManager.getConnection(connectionStringURL, "username", "password"); 

     if(mysql == null) 
      System.out.println("Connection Failed"); 
     else 
      System.out.println("Success"); 

     System.out.println("What would you like to do?"); 
     System.out.println("Type 1 to display all students."); 
     System.out.println("Type 2 to Add/Update students."); 
     System.out.println("Type 3 to Delete students."); 
     System.out.println("Type 4 to Search for a student."); 

     Scanner scan = new Scanner(System.in); 
     int input = scan.nextInt(); 

     if(input == 1) 
     { 
      pst = mysql.prepareStatement("SELECT * FROM Student;"); 
      data = pst.executeQuery(); 
      while (data.next()) { 
       System.out.println(data.getString("studentId")); 
       System.out.println(data.getString("firstname")); 
       System.out.println(data.getString("lastname")); 
       System.out.println(data.getInt("gpa")); 
       System.out.println(data.getString("major")); 
       System.out.println(data.getString("facultyAdvisor")); 
       System.out.println(); 
      } 
      data.close(); 
     } 

     if(input == 2) 
     { 
      System.out.println("What would you like to do?"); 
      System.out.println("Type Add to add a student."); 
      System.out.println("Type Update to update a student's information."); 
      String answer = scan.next(); 

      if(answer.equals("Add")) 
      { 
       System.out.println("Enter the student First Name"); 
       String firstname = scan.next(); 
       System.out.println("Enter the student's Last Name"); 
       String lastname = scan.next(); 
       System.out.println("Enter the student's GPA"); 
       int GPA = scan.nextInt(); 
       System.out.println("Enter the student's Major"); 
       String Major = scan.next(); 
       System.out.println("Enter the student's Faculty Advisor"); 
       String FacultyAdvisor = scan.next(); 

       pst = mysql.prepareStatement("INSERT INTO Student (FirstName, LastName, GPA, Major, FacultyAdvisor) VALUES (?, ?, ?, ?, ?)"); 
       pst.setString(1, firstname); 
       pst.setString(2, lastname); 
       pst.setInt(3, GPA); 
       pst.setString(4, Major); 
       pst.setString(5, FacultyAdvisor); 
       pst.executeUpdate(); 
      } 

      if(answer.equals("Update")) 
      { 
       System.out.println("Please enter the student's Id"); 
       int id = scan.nextInt(); 
       System.out.println("You may only update the Major or the Advisor. Which would you like to update?"); 
       System.out.println("Enter Major to update the major."); 
       System.out.println("Enter Advisor to update the advisor."); 
       String result = scan.next(); 

       if(result.equals("Major")) 
       { 
        System.out.println("Please enter the new Major"); 
        String Major = scan.next(); 
        pst = mysql.prepareStatement("UPDATE Student SET Major = ? WHERE StudentId = ?"); 
        pst.setString(1, Major); 
        pst.setInt(2, id); 
        pst.executeUpdate(); 
       } 

       if(result.equals("Advisor")) 
       { 
        System.out.println("Please enter the new Advisor"); 
        String Advisor = scan.next(); 
        pst = mysql.prepareStatement("UPDATE Student SEt FacultyAdvisor = ? WHERE StudentId = ?"); 
        pst.setString(1, Advisor); 
        pst.setInt(2, id); 
        pst.executeUpdate(); 
       } 
      } 
     } 

     if(input == 3) 
     { 
      System.out.println("Enter the student's ID"); 
      int Studentid = scan.nextInt(); 
      pst = mysql.prepareStatement("DELETE FROM Student WHERE StudentId = ?"); 
      pst.setInt(1, Studentid); 
      pst.executeUpdate(); 
     } 

     if(input == 4) { 
      System.out.println("How would you like to search for students?"); 
      System.out.println("Type Major to search by the student's Major."); 
      System.out.println("Type GPA to search by the student's GPA."); 
      System.out.println("Type Advisor to search by the student's Faculty Advisor."); 
      String search = scan.next(); 

      if (search.equals("Major")) { 
       System.out.println("Enter the Student's Major"); 
       String major = scan.next(); 
       pst = mysql.prepareStatement("SELECT * FROM Student WHERE Major = ?"); 
       pst.setString(1, major); 
       data = pst.executeQuery(); 
       while (data.next()) 
       { 
        System.out.println(data.getString("studentId")); 
        System.out.println(data.getString("firstname")); 
        System.out.println(data.getString("lastname")); 
        System.out.println(data.getInt("gpa")); 
        System.out.println(data.getString("major")); 
        System.out.println(data.getString("facultyAdvisor")); 
        System.out.println(); 
       } 
       data.close(); 
      } 

      if (search.equals("GPA")) { 
       System.out.println("Enter the Student's GPA"); 
       int GPA = scan.nextInt(); 
       pst = mysql.prepareStatement("SELECT * FROM Student WHERE GPA = ?"); 
       pst.setInt(1, GPA); 
       data = pst.executeQuery(); 
       while (data.next()) 
       { 
        System.out.println(data.getString("studentId")); 
        System.out.println(data.getString("firstname")); 
        System.out.println(data.getString("lastname")); 
        System.out.println(data.getInt("gpa")); 
        System.out.println(data.getString("major")); 
        System.out.println(data.getString("facultyAdvisor")); 
        System.out.println(); 
       } 
       data.close(); 
      } 

      if (search.equals("Advisor")) { 
       System.out.println("Enter the Student's Advisor"); 
       String advisor = scan.next(); 
       pst = mysql.prepareStatement("SELECT * FROM Student WHERE FacultyAdvisor = ?"); 
       pst.setString(1, advisor); 
       data = pst.executeQuery(); 
       while (data.next()) 
       { 
        System.out.println(data.getString("studentId")); 
        System.out.println(data.getString("firstname")); 
        System.out.println(data.getString("lastname")); 
        System.out.println(data.getInt("gpa")); 
        System.out.println(data.getString("major")); 
        System.out.println(data.getString("facultyAdvisor")); 
        System.out.println(); 
       } 
       data.close(); 
      } 
     } 
     mysql.close(); 
    } 
    catch(Exception ex) 
    { 
     ex.printStackTrace(); 
    } 
} 
}

我需要什麼正在以連接到我的MySQL數據庫到它自己的方法（或東西的那種（如果你不頂部的部分幫助相信這是真的有必要請讓我知道））。我已經在這裏（和其他網站）上看過其他人的代碼，以瞭解如何做到這一點，但是每當我嘗試做類似於他們所做的事情時，我都會遇到大量的錯誤，所以這就是爲什麼我'已經離開它了。

此外，我想使每個部分（如果（輸入= 1）等..）到他們自己的方法，以便我可以從主要方法調用它們。我多年前就學習了Java，迄今爲止我所做的研究並沒有真正幫助我。最後一件事，如果任何人有任何提示如何使這更「用戶友好」（< - 只有我能想到的東西），那麼請讓我知道。

非常感謝你提前。

P.S.我使用的軟件是JetBrains IntelliJ（用於實際編碼）和JetBrains DataGrip（用於製作表格/數據庫並檢查是否發生變化）

P.P.S.此外，一切似乎都在正常工作，並按照預期進行。然而，在IntelliJ中，只要在整個代碼中有一個「.prepareStatement（」某些SQL代碼「）」，它就會突出顯示它，並告訴我「此方法可能調用java.lang.NullPointerException」，這是我應該擔心的？

編輯：

所以這裏是我現在有：

public class Main { 

private static Connection mysql; 
private static PreparedStatement pst; 
private static ResultSet data; 

private static void method1() 
{ 
    PreparedStatement pst = null; 
    pst = mysql.prepareStatement("SELECT * FROM Student;"); 
    data = pst.executeQuery(); 
    while (data.next()) { 
     System.out.println(data.getString("studentId")); 
     System.out.println(data.getString("firstname")); 
     System.out.println(data.getString("lastname")); 
     System.out.println(data.getInt("gpa")); 
     System.out.println(data.getString("major")); 
     System.out.println(data.getString("facultyAdvisor")); 
     System.out.println(); 
    } 
} 

private static void method2() 
{ 
    System.out.println("What would you like to do?"); 
    System.out.println("Type Add to add a student."); 
    System.out.println("Type Update to update a student's information."); 
    String answer = scan.next(); 

    if(answer.equals("Add")) 
    { 
     System.out.println("Enter the student First Name"); 
     String firstname = scan.next(); 
     System.out.println("Enter the student's Last Name"); 
     String lastname = scan.next(); 
     System.out.println("Enter the student's GPA"); 
     int GPA = scan.nextInt(); 
     System.out.println("Enter the student's Major"); 
     String Major = scan.next(); 
     System.out.println("Enter the student's Faculty Advisor"); 
     String FacultyAdvisor = scan.next(); 

     pst = mysql.prepareStatement("INSERT INTO Student (FirstName, LastName, GPA, Major, FacultyAdvisor) VALUES (?, ?, ?, ?, ?)"); 
     pst.setString(1, firstname); 
     pst.setString(2, lastname); 
     pst.setInt(3, GPA); 
     pst.setString(4, Major); 
     pst.setString(5, FacultyAdvisor); 
     pst.executeUpdate(); 
    } 

    if(answer.equals("Update")) 
    { 
     System.out.println("Please enter the student's Id"); 
     int id = scan.nextInt(); 
     System.out.println("You may only update the Major or the Advisor. Which would you like to update?"); 
     System.out.println("Enter Major to update the major."); 
     System.out.println("Enter Advisor to update the advisor."); 
     Scanner scanner= new Scanner(System.in); 
     String result = scanner.next(); 

     if(result.equals("Major")) 
     { 
      System.out.println("Please enter the new Major"); 
      String Major = scanner.next(); 
      pst = mysql.prepareStatement("UPDATE Student SET Major = ? WHERE StudentId = ?"); 
      pst.setString(1, Major); 
      pst.setInt(2, id); 
      pst.executeUpdate(); 
     } 

     if(result.equals("Advisor")) 
     { 
      System.out.println("Please enter the new Advisor"); 
      String Advisor = scanner.next(); 
      pst = mysql.prepareStatement("UPDATE Student SEt FacultyAdvisor = ? WHERE StudentId = ?"); 
      pst.setString(1, Advisor); 
      pst.setInt(2, id); 
      pst.executeUpdate(); 
     } 
    } 
} 

private static void method3() 
{ 
    System.out.println("Enter the student's ID"); 
    int Studentid = scan.nextInt(); 
    pst = mysql.prepareStatement("DELETE FROM Student WHERE StudentId = ?"); 
    pst.setInt(1, Studentid); 
    pst.executeUpdate(); 
} 


private static void method4() 
{ 
    System.out.println("How would you like to search for students?"); 
    System.out.println("Type Major to search by the student's Major."); 
    System.out.println("Type GPA to search by the student's GPA."); 
    System.out.println("Type Advisor to search by the student's Faculty Advisor."); 
    Scanner find = new Scanner(System.in); 
    String search = find.next(); 

    if (search.equals("Major")) { 
     System.out.println("Enter the Student's Major"); 
     String major = find.next(); 
     pst = mysql.prepareStatement("SELECT * FROM Student WHERE Major = ?"); 
     pst.setString(1, major); 
     data = pst.executeQuery(); 
     while (data.next()) 
     { 
      System.out.println(data.getString("studentId")); 
      System.out.println(data.getString("firstname")); 
      System.out.println(data.getString("lastname")); 
      System.out.println(data.getInt("gpa")); 
      System.out.println(data.getString("major")); 
      System.out.println(data.getString("facultyAdvisor")); 
      System.out.println(); 
     } 
     data.close(); 
    } 

    if (search.equals("GPA")) { 
     System.out.println("Enter the Student's GPA"); 
     int GPA = find.nextInt(); 
     pst = mysql.prepareStatement("SELECT * FROM Student WHERE GPA = ?"); 
     pst.setInt(1, GPA); 
     data = pst.executeQuery(); 
     while (data.next()) 
     { 
      System.out.println(data.getString("studentId")); 
      System.out.println(data.getString("firstname")); 
      System.out.println(data.getString("lastname")); 
      System.out.println(data.getInt("gpa")); 
      System.out.println(data.getString("major")); 
      System.out.println(data.getString("facultyAdvisor")); 
      System.out.println(); 
     } 
     data.close(); 
    } 

    if (search.equals("Advisor")) { 
     System.out.println("Enter the Student's Advisor"); 
     String advisor = find.next(); 
     pst = mysql.prepareStatement("SELECT * FROM Student WHERE FacultyAdvisor = ?"); 
     pst.setString(1, advisor); 
     data = pst.executeQuery(); 
     while (data.next()) 
     { 
      System.out.println(data.getString("studentId")); 
      System.out.println(data.getString("firstname")); 
      System.out.println(data.getString("lastname")); 
      System.out.println(data.getInt("gpa")); 
      System.out.println(data.getString("major")); 
      System.out.println(data.getString("facultyAdvisor")); 
      System.out.println(); 
     } 
     data.close(); 
    } 
} 

public static void main(String[] args) { 
    connect(); 

    try 
    { 
     System.out.println("What would you like to do?"); 
     System.out.println("Type 1 to Display all students."); 
     System.out.println("Type 2 to Add/Update students."); 
     System.out.println("Type 3 to Delete students."); 
     System.out.println("Type 4 to Search for a student."); 

     Scanner scan = new Scanner(System.in); 
     int input = scan.nextInt(); 

     switch (input) 
     { 
      case 1: 
       method1(); 
       break; 
      case 2: 
       method2(); 
       break; 
      case 3: 
       method3(); 
       break; 
      case 4: 
       method4(); 
       break; 
      default: 
       System.out.println("You chose incorrectly and this program will now terminate."); 
     } 
    } 
    catch(Exception ex) 
    { 
     ex.printStackTrace(); 
    } 
} 

private static void connect() 
{ 
    Class.forName("com.mysql.jdbc.Driver"); 
    String connectionStringURL = "jdbc:mysql://us-cdbr-azure-west-b.cleardb.com:3306/acsm_0a00c1270f36f77"; 
    mysql = DriverManager.getConnection(connectionStringURL, "b448ce74b4a1f1", "595da2aa"); 

    if(mysql == null) 
     System.out.println("Connection Failed"); 
    else 
     System.out.println("Success"); 
}

}

到處它說：「mysql.preparestatement（」 // ... 「）」或「數據。 // ...「或」DriverManager.getConnection（// ...）「他們都給我了同樣的錯誤：未處理的異常：java.sql.SQLException。

它說「Class.forname（// ...）」它說：未處理的異常：java.lang.ClassNotFoundException。我如何解決這些問題？

來源

2016-09-25 Marc Karam

在第一步中，您可以使用switch語句來處理菜單輸入。爲連接和每個輸入案例創建方法。加場舉辦的MySQL聯接...

像

switch(input){ 
    case 1: 
     method1(); 
     break; 
}

private static void method1(){ 
    //move code from input 1 here 
}

，那麼你需要一個字段來保存了連接

private static Connection mysql;

編輯方法：

public class Main { 

    private static Connection mysql; 

    public static void main(String[] args) { 
     connect(); 
     //... 
    } 

    private static void connect(){ 
      Class.forName("com.mysql.jdbc.Driver"); 
      String connectionStringURL = "jdbc:mysql://us-cdbr-azure-west-b.cleardb.com:3306/database"; 
      mysql = DriverManager.getConnection(connectionStringURL, "username", "password"); 
     if(mysql == null){ 
      System.out.println("Connection Failed"); 
     }else{ 
      System.out.println("Success"); 
     } 
    } 

//... 

}

來源

2016-09-25 23:52:06 Tokazio

是該方法內的連接字段？如果是這樣，我需要把它放在所有的方法？ –

抱歉，我的意思是'班'字段。然後，您可以在類中的任何地方使用它，例如在使用連接方法初始化它之後。請參閱編輯。 – Tokazio

如何「簡化」MySQL數據庫程序的Java代碼？

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