Question

我想将结果集转换为字符串。之后，我将使用该字符串写一个html文件。课程是一个包含courseid（String），name（String），先决条件（String）连接数据库的表。这是我的代码和想法。你能评估我的想法或给我一些更好的解决方案吗？

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Answer 1

您可以使用ArrayList并将列存储在那里，例如：

List allRows = new ArrayList();
    while(rs.next()){
        String[] currentRow = new String[numberColumns];
        for(int i = 1;i<=numberColumns;i++){
            row[i-1]=rs.getString(i);
        }
        rows.add(row);
    }

您的ArrayList现在包含String数组，其中每个数组代表一行。现在您可以简单地将字符串数组条目转换为字符串，例如使用Arrays.toString(allRows.get(i));

Answer 2

我还没有尝试过您的代码，但是乍一看似乎可行。

在设计方面，我们通常希望将数据库访问与其他逻辑分开。

可能您将需要定义一个类Course来保存此数据以及对这些数据进行操作的业务逻辑。例如，您可以选择在toHtmlTableRow上实现一种生成HTML源的Course方法。（在更复杂或更复杂的环境中，您可能还希望将HTML生成的功能移至另一个类。）类似这样的东西：

class Course {
    String id, name, prereq;

    public Course ( String id , String name , String prereq ) {
        this.id = id;
        this.name = name;
        this.prereq = prereq;
    }

    public CharSequence toHtmlTableRow () {
        StringBuilder html = new StringBuilder();
        html.append( "<tr>\n" );
        html.append( "<td>" + this.id + "</td><td>" + this.name + "</td><td>" + this.prereq + "</td>\n" );
        html.append( "</tr>\n" );
        return html;
    }

    // Override `Object`.
    @Override
    public String toString () {
        return "Course{ " +
        "id='" + id + '\'' +
        " | name='" + name + '\'' +
        " | prereq='" + prereq + '\'' +
        " }";
    }
}

这是一个完整的工作示例应用程序。为了这个演示，我将它们全部塞入了一个.java文件中。在实际工作中，我不会。

此示例使用H2 Database Engine。此示例使内存数据库永远不会写入存储，这再次是一个示例。

请注意，使用Java新版本中的try-with-resource和AutoCloseable语法可以简化JDBC的使用。

package com.basilbourque.example;

import java.sql.*;
import java.time.LocalDate;
import java.time.ZoneId;
import java.util.ArrayList;
import java.util.List;
import java.util.UUID;

public class DbToText {

    public static void main ( String[] args ) {
        DbToText app = new DbToText();
        app.doIt();
    }

    private void doIt () {

        try {
            Class.forName( "org.h2.Driver" );
        } catch ( ClassNotFoundException e ) {
            e.printStackTrace();
        }

        List< Course > courses = new ArrayList();

        try (
        Connection conn = DriverManager.getConnection( "jdbc:h2:mem:db_to_text" ) ;
        Statement stmt = conn.createStatement() ;
        ) {
            String sql = "CREATE TABLE course_ ( \n" +
            "  id_ VARCHAR NOT NULL PRIMARY KEY , \n" +
            "  name_ VARCHAR NOT NULL , \n" +
            "  prereq_ VARCHAR NOT NULL \n" +
            ");";
            stmt.execute( sql );

            // Insert row.
            sql = "INSERT INTO course_ ( id_ , name_ , prereq_ ) VALUES ( ? , ? , ? ) ;";
            try (
            PreparedStatement preparedStatement = conn.prepareStatement( sql ) ;
            ) {
                preparedStatement.setString( 1 , "C01" );
                preparedStatement.setString( 2 , "Course 1" );
                preparedStatement.setString( 3 , "None" );
                preparedStatement.executeUpdate();

                preparedStatement.setString( 1 , "C02" );
                preparedStatement.setString( 2 , "Course 2" );
                preparedStatement.setString( 3 , "C01" );
                preparedStatement.executeUpdate();

                preparedStatement.setString( 1 , "C03" );
                preparedStatement.setString( 2 , "Course 3" );
                preparedStatement.setString( 3 , "C02" );
                preparedStatement.executeUpdate();
            }

            // Query all.
            sql = "SELECT * FROM course_";
            try ( ResultSet rs = stmt.executeQuery( sql ) ; ) {
                while ( rs.next() ) {
                    //Retrieve by column name
                    String id = rs.getString( "id_" );
                    String name = rs.getString( "name_" );
                    String prereq = rs.getString( "prereq_" );

                    // Instantiate a `Course` object for this data.
                    Course c = new Course( id , name , prereq );
                    courses.add( c );
                }
            }
        } catch ( SQLException e ) {
            e.printStackTrace();
        }

        System.out.println( "List of courses: \n" + courses );

        System.out.println( "Courses as HTML table rows: " );
        for ( Course course : courses ) {
            System.out.println( course.toHtmlTableRow() );
        }
    }

    class Course {
        String id, name, prereq;

        public Course ( String id , String name , String prereq ) {
            this.id = id;
            this.name = name;
            this.prereq = prereq;
        }

        public CharSequence toHtmlTableRow () {
            StringBuilder html = new StringBuilder();
            html.append( "<tr>\n" );
            html.append( "<td>" + this.id + "</td><td>" + this.name + "</td><td>" + this.prereq + "</td>\n" );
            html.append( "</tr>\n" );
            return html;
        }

        // Override `Object`.
        @Override
        public String toString () {
            return "Course{ " +
            "id='" + id + '\'' +
            " | name='" + name + '\'' +
            " | prereq='" + prereq + '\'' +
            " }";
        }
    }
}

运行时。

List of courses: 
[Course{ id='C01' | name='Course 1' | prereq='None' }, Course{ id='C02' | name='Course 2' | prereq='C01' }, Course{ id='C03' | name='Course 3' | prereq='C02' }]
Courses as HTML table rows: 
<tr>
<td>C01</td><td>Course 1</td><td>None</td>
</tr>

<tr>
<td>C02</td><td>Course 2</td><td>C01</td>
</tr>

<tr>
<td>C03</td><td>Course 3</td><td>C02</td>
</tr>

如何在Java中将ResultSet转换为字符串

2 个答案: