java trouble making a correct interface

Question

I want to make a class that can interact with a database, which has the following desired functionality:

It has a method to return all fields from the database, later can be changed such that it can also limit with it returns.
It has a method to insert a specific instance of this class.
It has a method to update a specific instance of this class. I will show the code in a moment after further explanation.

Now I want to extract an interface, or rather an abstract class I think might be more appriopiate, to be sure that all classes/datafields follow the same 'interface', and to be able to use them as a supertype in Lists etc.

The data class, in this case Account.java, should represent a table in a database which stores {Username, Password}, I am omitting an explicite unique identifier for now, still not sure if I will make an additional ID field or use the uniqueness of the Username field.

It would be best if the abstract class itself would handle all the MySQL interaction 'mess'.

Account.java so far:

package testthing; import java.util.Map; /** * * @author Frank */ public class Account { private final static String ALL_QUERY = "SELECT * FROM accounts"; private final static String INSERT_QUERY = "INSERT INTO accounts (username, password) VALUES(?, ?)"; private final static String UPDATE_QUERY = "UPDATE accounts SET password=? WHERE username=?"; private String username; private String password; public Account(final String username, final String password) { this.username = username; this.password= password; } public String getUsername() { return username; } public void setUsername(final String username) { this.username = username; } public String getPassword() { return password; } public void setPassword(final String password) { this.password = password; } public static Map<String, Account> getAll() { //return a map using the ALL_QUERY string } public void insert() { //insert this using INSERT_QUERY } public void update() { //update this using UPDATE_QUERY } }

I know that I haven't been that clear I'm afraid, but I hope this is enough to help me get going.

Basically I want to always be able to use the followings methods when working with a TableObject, which Account would be a subset of:

Account.getAll();
new Account("test", "test").insert();
currentAccount.setPassword("newPassword"); currentAccount.update();

All the nasty SQL stuff should be hidden inside the proposed abstract class. The only thing you cannot escape in a TableObject class is the definition of the SQL queries.

Regards.

EDIT: In current example Account.getAll() returns a Map<String, Account>, but in reality the first generic argument should be the type of the key in the database. So if you would use an unique ID then it would need to return a Map<Integer, Account>. I hope this change makes it in time for people to read it.

JDBC statements should not be in the parent class. What if your child class has its own fields? — Sotirios Delimanolis
– Sotirios Delimanolis, Commented May 13, 2013 at 19:23

christopher · Accepted Answer · 2013-05-13 19:23:43Z

Is it not more logical, to have the connection code and all the "nasty" stuff in the superclass, but simply have a more general method in the superclass, that is used by it's sub classes. For example:

public void executeUpdate(String query) { // Code to execute update. } public Map<String, Data> getData(String query) { // Code to get data. return data; }

This way, these methods are more general. It means that you can implement several classes that simply pass query data, rather than constantly having to update a superclass every single time you want to add new functionality.

Obviously I've just assumed a type Data here, but that might be something to look into. The aim here is to decouple your classes as much as possible. This means you can add as many new classes as you want, and they can use their supertype without hinderence.

This also means things like

Account.getAll();

is a little less complicated, because if you have getAll in your superclass, where is the reference that you want to get all accounts? If the code is actually in the Account class, you can customize the query, and send it off to the getData method to be executed.

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