从数据库中读取数据并存储在数组列表对象中答案

【问题标题】：Reading Data From Database and storing in Array List object从数据库中读取数据并存储在数组列表对象中
【发布时间】：2020-07-26 18:25:14
【问题描述】：

您好，我想在 html 页面上显示我的数据库表的全部内容。我正在尝试首先从数据库中获取记录并存储在 ArrayList 但是当我在 html 页面上返回数组列表时，它只会重复显示最后一条记录我的数据库表的计数。下面是代码：

public ArrayList<CustomerDTO> getAllCustomers() 
{
    ArrayList<CustomerDTO> customers = new ArrayList<CustomerDTO>();
    CustomerDTO customer = null;
    Connection c;
    try {
        c = openConnection();
        Statement statement = c.createStatement();
        String s = "SELECT * FROM customer";

        ResultSet rs = statement.executeQuery(s);
        int g =0;

        while (rs.next()) {

            customer.setId(rs.getInt("id"));
            customer.setName(rs.getString("name"));

            customer.setAddress(rs.getString("address"));
            customer.setPhone(rs.getString("phone"));
            customer.setEmail(rs.getString("email"));
            customer.setBountPoints(rs.getInt("bonuspoint"));
            customer.setTotalsale(rs.getInt("totalsale"));

            customers.add(customer);
        }

        rs.close();
    } catch (Exception e) {
        System.out.println(e);
    }

    return customers;
}

【问题讨论】：

标签： java jdbc

【解决方案1】：

试试下面的代码

public static ArrayList<Customer> getAllCustomer() throws ClassNotFoundException, SQLException {
    Connection conn=DBConnection.getDBConnection().getConnection();
    Statement stm;
    stm = conn.createStatement();
    String sql = "Select * From Customer";
    ResultSet rst;
    rst = stm.executeQuery(sql);
    ArrayList<Customer> customerList = new ArrayList<>();
    while (rst.next()) {
        Customer customer = new Customer(rst.getString("id"), rst.getString("name"), rst.getString("address"), rst.getDouble("salary"));
        customerList.add(customer);
    }
    return customerList;
}

这是我的模型类

public class Customer {
private String id;
private String name;
private String salary;
private String address;
public String getId() {
    return id;
}
public void setId(String id) {
    this.id = id;
}
public String getName() {
    return name;
}
public void setName(String name) {
    this.name = name;
}
public String getSalary() {
    return salary;
}
public void setSalary(String salary) {
    this.salary = salary;
}
public String getAddress() {
    return address;
}
public void setAddress(String address) {
    this.address = address;
}
}

这是我的视图方法

  private void reloadButtonActionPerformed(java.awt.event.ActionEvent evt) {                                             
    try {
        ArrayList<Customer> customerList = null;
        try {
            try {
                customerList = CustomerController.getAllCustomer();
            } catch (SQLException ex) {
                Logger.getLogger(veiwCustomerFrame.class.getName()).log(Level.SEVERE, null, ex);
            }
        } catch (Exception ex) {
            Logger.getLogger(ViewCustomerForm.class.getName()).log(Level.SEVERE, null, ex);
        }
        DefaultTableModel tableModel = (DefaultTableModel) customerTable.getModel();
        tableModel.setRowCount(0);
        for (Customer customer : customerList) {
            Object rowData[] = {customer.getId(), customer.getName(), customer.getAddress(), customer.getSalary()};
            tableModel.addRow(rowData);
        }


    } catch (Exception ex) {
        Logger.getLogger(ViewCustomerForm.class.getName()).log(Level.SEVERE, null, ex);
    }

}

【讨论】：

【解决方案2】：

您必须在每次迭代中创建一个新的客户对象，然后在迭代的最后将该新创建的对象添加到 ArrayList 中。

【讨论】：

【解决方案3】：

您正在重用customer 引用。 Java 通过引用为对象工作。不适用于基元。

您正在做的是将相同的customer 添加到列表中，然后对其进行修改。从而为所有对象设置相同的值。这就是为什么你看到最后一个。因为都是一样的。

 while (rs.next()) {
        Customer customer = new Customer();
        customer.setId(rs.getInt("id"));

        ...

【讨论】：

【解决方案4】：

尝试每次都创建新的客户实例，例如

         while (rs.next()) {

        Customer customer = new Customer();
        customer.setId(rs.getInt("id"));
        customer.setName(rs.getString("name"));

        customer.setAddress(rs.getString("address"));
        customer.setPhone(rs.getString("phone"));
        customer.setEmail(rs.getString("email"));
        customer.setBountPoints(rs.getInt("bonuspoint"));
        customer.setTotalsale(rs.getInt("totalsale"));

        customers.add(customer);


    }

【讨论】：

【解决方案5】：

试试这个

import java.sql.ResultSet;
import java.util.ArrayList;

import com.rcb.dbconnection.DbConnection;
import com.rcb.model.Docter;


public class DocterService {



public ArrayList<Docter> getAllDocters() {
    ArrayList<Docter> docters = new ArrayList<Docter>();

    try {
        String sql = "SELECT tbl_docters";

        ResultSet rs = db.getData(sql);
        while (rs.next()) {
            Docter docter = new Docter();

            docter.setD_id(rs.getInt("d_id"));
            docter.setD_FName(rs.getString("d_fname"));
            docter.setD_LName(rs.getString("d_lname"));

            docters.add(docter);

        }

    } catch (Exception e) {
        System.out.println("getAllDocters()");
        e.printStackTrace();
    }

    return (docters);
}

public static void main(String args[]) {
    DocterService ds = new DocterService();
    ArrayList<Docter> doctersList = ds.getAllDocters();
    String s[] = null;
    for (int i = 0; i < doctersList.size(); i++) {

        System.out.println(doctersList.get(i).getD_id());
        System.out.println(doctersList.get(i).getD_FName());
    }

  }
}

【讨论】：

【解决方案6】：

我正在尝试先从数据库中获取记录并存储在 ArrayList 中但是当我在 html 页面上返回数组列表时，它只显示最后一条记录重复作为我的数据库表的计数

这部分大部分已经被之前的所有答案覆盖了。因此，您需要在while 循环中创建CustomerDTO 的新实例并将其添加到ArrayList。

还有一件事我想评论：

确保在使用完所有资源后释放它们。从您发布的代码中，您尚未关闭您的 Statement 或 Connection 对象（不确定您是否正在池化您的连接，在这种情况下，您需要释放此连接到游泳池)

因此，当您考虑这些要点时，您的代码结构可能如下所示：

public ArrayList<CustomerDTO> getAllCustomers() 
{
    ArrayList<CustomerDTO> customers = new ArrayList<CustomerDTO>();
    Connection c = null;
    Statement statement = null;
    ResultSet rs        = null;

    try {
        c           = openConnection();
        statement   = c.createStatement();
        String s    = "SELECT * FROM customer";

        rs          = statement.executeQuery(s);
        int g =0;

        while (rs.next()) {
            CustomerDTO customer = new CustomerDTO();
            //Code to fill up your DTO
            customers.add(customer);
        }
    } catch (Exception e) {
        System.out.println(e);
    }finally{
        //Code to release your resources
    }

    return customers;
}

【讨论】：

【解决方案7】：

如果您的客户类有静态变量，请将它们删除，这样您的类应该看起来像这样。

public class customer {

     private int id;
     private String name;
     private String DOB;

    public int getId() {
        return id;
    }
    public String getName() {
        return name;
    }
    public String getDOB() {
        return DOB;
    }
     public void setId(int id) {
        this.id = id;
    }
    public void setName(String name) {
        this.name = name;
    }
    public void setDOB(String dOB) {
        this.DOB = dOB;
    }

而不是像

public class customer {

     private static int id;
     private static String name;
     private static String DOB;

    public static int getId() {
        return id;
    }
    public static String getName() {
        return name;
    }
    public static String getDOB() {
        return DOB;
    }
     public static void setId(int id) {
        custumer.id = id;
    }
    public  static void setName(String name) {
        customer.name = name;
    }
    public static void setDOB(String dOB) {
        customer.DOB = dOB;
    }

【讨论】：

【解决方案8】：

Instead ofnull, use CustomerDTO customers =new CustomerDTO()`;

CustomerDTO customer = null;


  private static List<Author> getAllAuthors() {
    initConnection();
    List<Author> authors = new ArrayList<Author>();
    Author author = new Author();
    try {
        stmt = (Statement) conn.createStatement();
        String str = "SELECT * FROM author";
        rs = (ResultSet) stmt.executeQuery(str);

        while (rs.next()) {
            int id = rs.getInt("nAuthorId");
            String name = rs.getString("cAuthorName");
            author.setnAuthorId(id);
            author.setcAuthorName(name);
            authors.add(author);
            System.out.println(author.getnAuthorId() + " - " + author.getcAuthorName());
        }
        rs.close();
        closeConnection();
    } catch (Exception e) {
        System.out.println(e);
    }
    return authors;
}

【讨论】：

【解决方案9】：

 while (rs.next()) {

            customer.setId(rs.getInt("id"));
            customer.setName(rs.getString("name"));

            customer.setAddress(rs.getString("address"));
            customer.setPhone(rs.getString("phone"));
            customer.setEmail(rs.getString("email"));
            customer.setBountPoints(rs.getInt("bonuspoint"));
            customer.setTotalsale(rs.getInt("totalsale"));

            customers.add(customer);
             customer = null;
        }

尝试用上述代码替换您的 while 循环代码。这里我们所做的是在做customers.add(customer)之后我们在做customer = null;`

【讨论】：

【解决方案10】：

每次在while循环中创建CustomerDTO对象

检查以下代码

    while (rs.next()) {

    Customer customer = new Customer();

    customer.setId(rs.getInt("id"));
    customer.setName(rs.getString("name"));
    customer.setAddress(rs.getString("address"));
    customer.setPhone(rs.getString("phone"));
    customer.setEmail(rs.getString("email"));
    customer.setBountPoints(rs.getInt("bonuspoint"));
    customer.setTotalsale(rs.getInt("totalsale"));

    customers.add(customer);
}

【讨论】：

【解决方案11】：

此外，如果您希望结果集数据在列表中。请在 LOC 下方使用：

public List<String> dbselect(String query)
  {
      List<String> dbdata=new ArrayList<String>();
      try {
        dbResult=statement.executeQuery(query);
        ResultSetMetaData metadata=dbResult.getMetaData();
        for(int i=0;i>=metadata.getColumnCount();i++)
        {
            dbdata.add(dbResult.getString(i));
        }
        return dbdata;
    } catch (SQLException e) {
        return null;
    }
      
      
  }

【讨论】：