How to convert ResultSet to string in Java

You could use an ArrayList and store the columns in there, such as:

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);
    }

Your ArrayList now contains String arrays, where each array represents one row. Now you can simply transform the string array entries into strings, e.g. using Arrays.toString(allRows.get(i));

I have not tried your code, but it looks workable at a glance.

Design-wise, we usually want to separate database access from other logic.

And likely you will want to define a class Course to hold this data and your business logic that operates on that data. For example, you may choose to implement a toHtmlTableRow method on Course that generates the HTML source. (In a more complex or sophisticated environment, you might also want to move that HTML-generation functionality to another class.) Something like this:

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 + '\'' +
        " }";
    }
}

Here is a complete working example app. For the sake of this demo, I crammed it all into a single .java file. In real work, I would not.

This example uses the H2 Database Engine. This example makes an in-memory database that never gets written to storage, again because this is a mere example.

Note the use of the try-with-resource & AutoCloseable syntax found in newer versions of Java to simplify working with JDBC.

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 + '\'' +
            " }";
        }
    }
}

When run.

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>