Column anno

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface Column {
    String name();
    int primary() default 0;
    int updatable() default 0;
}

Dao

import java.lang.reflect.Field;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class Dao {

    private static String getTableName(Class clazz){
        if (!clazz.isAnnotationPresent(Table.class)) {
            throw new IllegalArgumentException("Annotation @Table is not present");
        }
        Table annotation = (Table) clazz.getAnnotation(Table.class);

        return annotation.name().isEmpty() ? clazz.getName() : annotation.name();
    }

    static String save(Object obj) throws Exception {
        StringBuilder query = new StringBuilder("INSERT INTO ");

        Class clazz = obj.getClass();

        // add table name to query
        String tableName = getTableName(clazz);

        query.append(tableName);
        query.append(" (");

        // collect column names and values of corresponding fields
        List<String> columnNames = new ArrayList<>();
        List<Object> valuesToInsert = new ArrayList<>();

        for (Field field : clazz.getDeclaredFields()) {
            Column columnAnnotation = field.getAnnotation(Column.class);

            if (columnAnnotation != null) {
                // get column name
                String columnName = columnAnnotation.name().isEmpty() ?
                        field.getName() : columnAnnotation.name();
                columnNames.add(columnName);

                // get value for this column
                field.setAccessible(true);
                Object columnValue = field.get(obj);
                valuesToInsert.add(columnValue);
                field.setAccessible(false);
            }
        }

        // just for info (DON'T USE SUCH TECHNIQUE FOR DEBUG PURPOSES!!!)
        printColumnNamesAndValues(columnNames, valuesToInsert);

        // generate rest of query
        appendColumnNames(query, columnNames);
        query.append(")");
        appendColumnValues(query, valuesToInsert);

        return query.toString();
    }


    private static void printColumnNamesAndValues(List<String> columnNames, List<Object> valuesToInsert) {
        System.out.println("Column names and values list:");

        Iterator<String> namesIterator = columnNames.iterator();
        Iterator<Object> valuesIterator = valuesToInsert.iterator();

        while (namesIterator.hasNext()) {
            System.out.println(namesIterator.next() + " -> " + valuesIterator.next());
        }

    }

    private static void appendColumnNames(StringBuilder query, List<String> columnNames) {


        for (String columnName : columnNames) {
            query.append(columnName);
            query.append(", ");
        }

        // remove last ", "
        query.setLength(query.length() - 2);

    }

    private static void appendColumnValues(StringBuilder query, List<?> columnValues) {
        query.append(" VALUES (");

        for (Object columnValue : columnValues) {
            query.append(columnValue);
            query.append(", ");
        }

        // remove last ", "
        query.setLength(query.length() - 2);

        query.append(");");
    }

    //  TODO: Generate SQL to get all instances of clazz from DB
    public static String getAll(Class clazz) {
        StringBuilder query = new StringBuilder("SELECT ");

        String tableName = getTableName(clazz);

        // collect column names and values of corresponding fields
        List<String> columnNames = new ArrayList<>();
        List<Object> valuesToInsert = new ArrayList<>();

        for (Field field : clazz.getDeclaredFields()) {
            Column columnAnnotation = field.getAnnotation(Column.class);

            if (columnAnnotation != null) {
                // get column name
                String columnName = columnAnnotation.name().isEmpty() ?
                        field.getName() : columnAnnotation.name();
                columnNames.add(columnName);

            }
        }

        // generate rest of query
        appendColumnNames(query, columnNames);

        query.append(" FROM ");

        query.append(tableName);
        query.append(";");

        // just for info (DON'T USE SUCH TECHNIQUE FOR DEBUG PURPOSES!!!)
        //printColumnNamesAndValues(columnNames, valuesToInsert);


        return query.toString();
    }

    //  TODO: Generate SQL which can remove Object of clazz by it's id
    //  Hint: you will probably need to indicate somehow that current @Column is primary key in table
    public static String deleteById(Object id, Class clazz) {
        StringBuilder query = new StringBuilder("DELETE FROM ");

        String tableName = getTableName(clazz);

        query.append(tableName);
        query.append(" WHERE ");

        for (Field field : clazz.getDeclaredFields()) {
            Column columnAnnotation = field.getAnnotation(Column.class);

            if ((columnAnnotation != null) && (columnAnnotation.primary() == 1)) {

                query.append(columnAnnotation.name());
                query.append(" = ").append(id).append(";");
                break;

            }
        }

        return query.toString();
    }

    // TODO: Generate SQL which can Update Object representation in DB
    public static String updateSalaryForIdByRate(Class clazz, int id, int rate) {
        StringBuilder query = new StringBuilder("UPDATE ");

        String tableName = getTableName(clazz);

        query.append(tableName);
        query.append(" SET ");
        String strId = "";
        String strSalary = "";

        for (Field field : clazz.getDeclaredFields()) {
            Column columnAnnotation = field.getAnnotation(Column.class);

            if (columnAnnotation != null) {
                if (columnAnnotation.primary() == 1) {
                    strId = columnAnnotation.name();
                }
                if (columnAnnotation.updatable() == 1) {
                    strSalary = columnAnnotation.name();
                }
            }

        }
        query.append(strSalary).append(" = ").append(strSalary).append(" * ").append(rate).append(" WHERE ").append(strId).append(" = ").append(id).append(";");

        return query.toString();

    }


}

DaoTest

import org.junit.Test;

import static org.junit.Assert.assertEquals;

public class DaoTest {

    @Test
    public void testSave() throws Exception {
        User user = new User(1, "Tolik", 100.0d);
        String expectedQuery = "INSERT INTO dan_user (id, name, sal) VALUES (1, Tolik, 100.0);";

        String actualQuery = Dao.save(user);

        assertEquals(expectedQuery, actualQuery);
    }

    @Test
    public void testGetAll() throws Exception {
        String expectedQuery = "SELECT id, name, sal FROM dan_user;";
        String actualQuery = Dao.getAll(User.class);

        assertEquals(expectedQuery, actualQuery);
    }

    @Test
    public void testDeleteById() throws Exception {
        String expectedQuery = "DELETE FROM dan_user WHERE id = 1;";
        String actualQuery = Dao.deleteById(1, User.class);

        assertEquals(expectedQuery, actualQuery);
    }

    @Test
    public void testUpdate() throws Exception {
        String expectedQuery = "UPDATE dan_user SET sal = sal * 2 WHERE id = 1;";
        String actualQuery = Dao.updateSalaryForIdByRate(User.class, 1, 2);

        assertEquals(expectedQuery, actualQuery);
    }
}

User

@Table(name="dan_user")
public class User {
    @Column(name="id", primary = 1)
    private int id;
    @Column(name="name")
    private String name;
    @Column(name="sal", updatable = 1)
    private double salary;

    public User(int id, String name, double salary) {
        this.id = id;
        this.name = name;
        this.salary = salary;
    }
}