如何正確選擇mysql數據庫中的數據？ ResultSet錯誤

Question

我正在處理一個項目，並從打印數據庫中選擇的數據時遇到一些問題。 這是MySQL連接：

import java.sql.*; 

public class MysqlConnect { 

    public Connection conn = null; 
    public String url = "jdbc:mysql://localhost:3306/"; 
    public String dbName = "jdbctutorial"; 
    public String driver = "com.mysql.jdbc.Driver"; 
    public String userName = "birthday"; 
    public String password = "123456"; 
    public Statement stmt = null; 
    public String query = ""; 
    public int rs = 0; 
    public PreparedStatement pstmt = null; 
    public ResultSet rrs = null; 

    public void crearedatabase() { 
     try { 
      Class.forName(driver).newInstance(); 
      conn = DriverManager.getConnection(url + dbName, userName, password); 
      // System.out.println("Connected to the database"); 
      stmt = conn.createStatement(); 
     } catch (Exception e) { 
      System.out.println("Baza de date nu a fost creata"); 
     } 

    } 

    public void executareSql() { 
     try { 
      rs = stmt.executeUpdate(query); 
     } catch (Exception e) { 
      System.out.println("Eroare sql"); 
     } 
    } 

    public void executaresSqlPrepareSt(String nume, String prenume, int ziua, int luna, int an) { 
     try { 
      pstmt = conn.prepareStatement("INSERT INTO persoane(Nume,Prenume,Ziua,Luna,An)VALUES(?,?,?,?,?);"); 

      pstmt.setString(1, nume); 
      pstmt.setString(2, prenume); 
      pstmt.setInt(3, ziua); 
      pstmt.setInt(4, luna); 
      pstmt.setInt(5, an); 
      pstmt.execute(); 

     } catch (SQLException e) { 
      System.out.println("Eroare prepareSql."); 
     } 
    } 

    public void selectsql() { 
     try { 
      rrs = stmt.executeQuery(query); 
     } catch (SQLException e) { 
      System.out.println("Error"); 
     } 
    } 

    public void returnareRezultat() throws SQLException { 
     while (rrs.next()) { 
      String nume = rrs.getString("FirstName"); 
      String prenume = rrs.getString("LastName"); 
      int ziua = rrs.getInt("Day"); 
      int luna = rrs.getInt("Month"); 
      int an = rrs.getInt("Year"); 

      // print the results 
      System.out.format("%s, %s, %s, %s, %s\n", nume, prenume, ziua, luna, an); 
     } 
    } 
} 

而這正是我調用該方法從我的GUI部分：

........................  

INT zi_actuala = data_azi.get（Calendar.DAY_OF_MONTH）; int luna_actuala = data_azi.get（Calendar.MONTH）;

mysql.query = "SELECT Nume ,Prenume,Ziua,Luna,An FROM persoane WHERE (Ziua="+zi_actuala+" AND Luna="+luna_actuala+"); "; 
    mysql.selectsql(); 

.................

我的錯誤是在rrs = stmt.executeQuery(query);（顯示java.lang.NullPointerException）

這是完全錯誤at MysqlConnect.selectsql(MysqlConnect.java:63) at GuiApp.<init>(GuiApp.java:120) at GuiApp$1.run(GuiApp.java:55) at java.awt.event.InvocationEvent.dispatch(Unknown Source) at java.awt.EventQueue.dispatchEventImpl(Unknown Source) at java.awt.EventQueue.access$200(Unknown Source) at java.awt.EventQueue$3.run(Unknown Source) at java.awt.EventQueue$3.run(Unknown Source) at java.security.AccessController.doPrivileged(Native Method) at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source) at java.awt.EventQueue.dispatchEvent(Unknown Source) at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source) at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source) at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source) at java.awt.EventDispatchThread.pumpEvents(Unknown Source) at java.awt.EventDispatchThread.pumpEvents(Unknown Source) at java.awt.EventDispatchThread.run(Unknown Source) 你能幫我嗎？提前致謝。

Answer 1

NullPointerException，可能是因爲你沒有調用crearedatabase（），所以stmt爲null。

Answer 2

是這樣做的。

mysql.query = "SELECT Nume ,Prenume,Ziua,Luna,An FROM persoane WHERE (Ziua="+zi_actuala+" AND Luna="+luna_actuala+"); "; 
    mysql.crearedatabase(); 
    mysql.selectsql();