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”，其中包含她的属性、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该组件:

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());

    }
}

任何帮助将不胜感激..

Answer 1

我已经尝试过这段代码并且工作正常。我使用的是 Oracle，而不是 SQL 数据库，您可以更改

相应地。

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 ;
    }
}