CODEWITHSUNDEEP
javagenericsarraylist
Rayden
Rayden

Reputation: 3

How to return a generic ArrayList<T>

Sorry didnt know who to formulate the question best, but i will try to explain my Problem here, i have following class:

public class ReadOnlyTable<T extends Model> implements ReadOnly<T> {
...

protected ReadOnlyTable() {
    initTable();
}

protected void reloadDataSource() {
    initTable();
}

...

@Override
public ArrayList<T> findAll() {
    ArrayList<T> result = null;

    try {
        long startTime = System.currentTimeMillis();
        result = datasource.getAllEntries();
        //  // Logger.getLogger().write("FindAllQuery time was " + (System.currentTimeMillis() - startTime) + "ms");
    } catch (Exception e) {
        e.printStackTrace();
        return null;
    }

    return result;
}

Lets say T is class "Galaxy", then i tried to loop over all elements in the returned Array, with following Code

    for (Galaxy gal : gDAO.findAll()) {

    }

Why does it give me an error that Galaxy is required, but Object is found?? What am i doing wrong, that i have return type ArrayList and not ArrayList<T>

EDIT 1:

gDAO is defined this way

private static GalaxyDAO gDAO = (GalaxyDAO) DAOFactory.get(GalaxyDAO.class);

And DAOFactory looks like this

    public static <T> T get(Class clazz) {
    if (!instanceList.containsKey(clazz)) {
        try {

            GenericDAO genericDAO = null;
            Constructor constructor;
            constructor = clazz.getDeclaredConstructor();
            constructor.setAccessible(true);
            genericDAO = (GenericDAO) constructor.newInstance();
            instanceList.put(clazz, genericDAO);

            return (T)genericDAO;
        } catch (NoSuchMethodException ex) {
            DebugBuffer.writeStackTrace(DAOFactory.class.getName(), ex);
        } catch (SecurityException ex) {
            DebugBuffer.writeStackTrace(DAOFactory.class.getName(), ex);
        } catch (InstantiationException ex) {
            DebugBuffer.writeStackTrace(DAOFactory.class.getName(), ex);
        } catch (IllegalAccessException ex) {
            DebugBuffer.writeStackTrace(DAOFactory.class.getName(), ex);
        } catch (InvocationTargetException ex) {
            DebugBuffer.writeStackTrace(DAOFactory.class.getName(), ex);
        }
    }else{
        return (T)instanceList.get(clazz);
    }

    return null;
}

and finally

public class GalaxyDAO extends ReadWriteTable<Galaxy> implements GenericDAO {

and yeah .. ReadWriteTable extends ReadOnlyTable

public abstract class ReadWriteTable<T extends Model> extends ReadOnlyTable implements ReadWrite<T> {
public ReadWriteTable() {              
    super();
}

Appendix to acknowledged solution

I had a mistake in my inferfaces which prevented to give the Type to ReadOnlyTable see

public interface ReadWrite<T> extends ReadOnly {

instead of 

public interface ReadWrite<T> extends ReadOnly<T> {

after fixing that i could also change following line

public abstract class ReadWriteTable<T extends Model> extends ReadOnlyTable<T> implements ReadWrite<T> {

Upvotes: 0

Views: 1269

Answers (3)

ixx
ixx

Reputation: 32273

gDAO is an instance of your class ReadOnlyTable<T extends Model> ? What is it's type parameter (how did you declare it)?

It has to be Galaxy, otherwise it will not work.

Update: Now that you posted more code, the problem is that you have to pass the type parameter Galaxy to the class ReadOnlyTable<T extends Model>, where the findAll method is, otherwise it doesn't know what T is, and will return ArrayList<Object>.

This line is the problem:

public abstract class ReadWriteTable<T extends Model> extends ReadOnlyTable implements ReadWrite<T>

The class ReadOnlyTable also has to receive the type parameter:

ReadOnlyTable<T extends Model>

So the line has to be:

public abstract class ReadWriteTable<T extends Model> extends ReadOnlyTable<T extends Model> implements ReadWrite<T>

Upvotes: 1

shem
shem

Reputation: 4712

the problem is the you declare GalaxyDAO as a row type, it should be declared like that:

public class GalaxyDAO<Galaxy> extends ReadWriteTable<Galaxy> implements GenericDAO

I'm suer that you got a compiler warning on it, don't ignore warning, you should threat them almost like errors!

Upvotes: 0

Ian Roberts
Ian Roberts

Reputation: 122364

The problem is the ReadWriteTable declaration, you've declared it as

public abstract class ReadWriteTable<T extends Model> extends ReadOnlyTable implements ReadWrite<T>

extending the raw type ReadOnlyTable rather than the parameterized ReadOnlyTable<T>. Change it to

public abstract class ReadWriteTable<T extends Model> extends ReadOnlyTable<T> implements ReadWrite<T>

Upvotes: 0

Related Questions