JavaBeans組件顯示java.lang.NullPointerException

Question

我試圖做一個JavaBeans組件，但它給我這個錯誤：

Exception in thread "main" java.lang.NullPointerException 
    at probarcomponente.ProbarComponente.main(ProbarComponente.java:37) 
Java Result: 1 

組件：我有另一個類

public class Componente implements Serializable { 
    private List<Persona> lista; 


    public Componente() { 
    } 


    public class Personas { 

     List<Persona> lista = new LinkedList<Persona>(); 

    } 

    public List <Persona> Mostrar() { 

     try { 
      Connection conexion = DriverManager.getConnection("jdbc:mysql://192.168.100.128/mibase", "asis", "titanic24"); 
      java.sql.Statement comando = conexion.createStatement(); 
      String sql = "select * from dades_pers"; 
      ResultSet result = comando.executeQuery(sql); 

      while (result.next()) { 

       String nif = result.getString(1); 
       String nom = result.getString(2); 
       String cognoms = result.getString(3); 
       String tel = result.getString(4); 

      Personas p = new Personas(); 

      // Inserción en la lista 
      p.lista.add(new Persona(nif,nom,cognoms,tel)); 


      } 

     } catch (SQLException ex) { 
      System.out.println("Error al mostrar datos"); 
      System.out.println(ex.getMessage()); 

     } 
     return lista ; 

    } 

} 

「假面」與她的屬性，構造函數，getter和setter。

public class Persona { 
    private String nif ; 
    private String nom; 
    private String cognoms; 
    private String telf; 

    public Persona(String nif, String nom, String cognoms, String telf) { 
     this.nif = nif; 
     this.nom = nom; 
     this.cognoms = cognoms; 
     this.telf = telf; 
    } 

    public String getNif() { 
     return nif; 
    } 

    public String getNom() { 
     return nom; 
    } 

    public String getCognoms() { 
     return cognoms; 
    } 

    public String getTelf() { 
     return telf; 
    } 

    public void setNif(String nif) { 
     this.nif = nif; 
    } 

    public void setNom(String nom) { 
     this.nom = nom; 
    } 

    public void setCognoms(String cognoms) { 
     this.cognoms = cognoms; 
    } 

    public void setTelf(String telf) { 
     this.telf = telf; 
    } 




} 

，最後我創造另一個項目來測試組件：

public class ProbarComponente { 

    /** 
    * @param args the command line arguments 
    */ 
    public static void main(String[] args) throws SQLException { 

     Componente comp = new Componente(); 

     List<Persona> lista = new LinkedList<Persona>(); 
     lista= comp.Mostrar(); 
     System.out.print("Tamaño de la lista es"+lista.size()); 

    } 
} 

任何幫助將不勝感激..

Answer 1

方法comp.Mostrar()將返回null。 Componente類中的lista屬性從不設置。 comp.Mostrar()方法正在將Persona實例添加到Personas類中的lista變量中。我不確定需求是什麼，但您可能需要從comp.Mostrar()方法中返回p.lista。

Answer 2

我試過這段代碼，它工作正常。而不是SQL數據庫我已經使用Oracle，您可以相應地更改

。

public class Componente implements Serializable { 
      private static final long serialVersionUID = 1L; 

      List<Persona> lista = new LinkedList<Persona>(); 

      public Componente() { 
      } 


    public List <Persona> Mostrar() { 

     try { 
      Class.forName("oracle.jdbc.driver.OracleDriver"); 
      Connection conexion = DriverManager.getConnection("jdbc:oracle:thin:@localhost:1521:xe", "system", "oracle"); 
      Statement comando = conexion.createStatement(); 
      String sql = "select * from dades_pers"; 
      ResultSet result = comando.executeQuery(sql); 

      while (result.next()) { 

       String nif = result.getString(1); 
       String nom = result.getString(2); 
       String cognoms = result.getString(3); 
       String tel = result.getString(4); 

       lista.add(new Persona(nif,nom,cognoms,tel)); 
      } 

     } catch (SQLException | ClassNotFoundException ex) { 
      System.out.println("Error al mostrar datos"); 
      System.out.println(ex.getMessage()); 

     } 
     return lista ; 
    } 
}