链表-JavaBeans组件java.lang.NullPointerException

Question

我正在尝试做一个javaBeans组件，但它给了我这个错误：

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

组件：

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 ;
    }
}

我有另一个类"Persona"，包含她的属性、构造函数、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;
    }


}

最后，我创建了另一个项目来测试组件：

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());
    }
}

任何帮助都将不胜感激。。

Answer 1

我试过这段代码，它运行得很好。您可以更改，而不是我使用的SQL数据库Oracle

因此。

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 ;
    }
}

Answer 2

方法comp.Mostrar()返回null。Componente类中的lista属性从未设置。comp.Mostrar()方法将Persona实例添加到Personas类中的lista变量中。我不确定要求是什么，但您可能需要从comp.Mostrar()方法返回p.lista。