I am designing a simple Data Access Object for my Java application. I have a few classes (records) that represents a single row in tables like User
and Fruit
.
I would like to have a single method for getting all records of a specific type.
For the moment I have it like this:
public List<User> getAllUsers() {
...
}
public List<Fruit> getAllFruits() {
...
}
....
But I would like to have a single polymorphic method like this (wrong):
public List<T> getAllRecords(Class<T> type) {
if(type instanceof User) {
// Use JDBC and SQL SELECT * FROM user
} else if(type instanceof Fruit) {
// Use JDBC and SQL SELECT * FROM fruit
}
return collection;
}
Example for uses:
List<Fruit> fruits = myDataAccessObject.getAllRecrods(Fruit.class);
List<User> users = myDataAccessObject.getAllRecords(User.class);
How can I do this in Java?
Since you say that you don't want you data access methods in different classes(in the comment to anish's answer),I thought why not try something like this.
public class Records {
public interface RecordFetcher<T>{
public List<T> getRecords();
}
static RecordFetcher<Fruit> Fruit=new RecordFetcher<Fruit>(){
public List<Fruit> getRecords() {
...
}
};
static RecordFetcher<User> User=new RecordFetcher<User>(){
public List<User> getRecords() {
...
}
};
public static void main(String[] args) {
List<Fruit> fruitRecords=Records.Fruit.getRecords();
List<User> userRecords=Records.User.getRecords();
}
}
EDIT:
I would like to add one more of my implementation.
public class Test
{
public static void main(String[] args)
{
Test dataAccess=new Test();
List<Fruit> FruitList=dataAccess.getAllRecords(Fruit.myType);
List<User> UserList=dataAccess.getAllRecords(User.myType);
}
<T> List<T> getAllRecords(T cl)
{
List<T> list=new ArrayList<T>();
if(cl instanceof Fruit)
{
// Use JDBC and SQL SELECT * FROM fruit
}
else if(cl instanceof User)
{
// Use JDBC and SQL SELECT * FROM user
}
return list;
}
}
class Fruit
{
static final Fruit myType;
static {myType=new Fruit();}
}
class User
{
static final User myType;
static {myType=new User();}
}
EDIT:
I think this implementation is just as you have asked
public class Test
{
public static void main(String[] args) throws InstantiationException, IllegalAccessException
{
Test dataAccess=new Test();
List<Fruit> FruitList=dataAccess.getAllRecords(Fruit.class);
List<User> UserList=dataAccess.getAllRecords(User.class);
}
<T> List<T> getAllRecords(Class<T> cl) throws InstantiationException, IllegalAccessException
{
T inst=cl.newInstance();
List<T> list=new ArrayList<T>();
if(inst instanceof Fruit)
{
// Use JDBC and SQL SELECT * FROM fruit
}
else if(inst instanceof User)
{
// Use JDBC and SQL SELECT * FROM user
}
return list;
}
}