GenericApp.java

public class GenericApp {
    private DataSource<FacebookItem> mFbSource = new FacebookDataSource();
    private DataSource<InstagramItem> mIgSource = new InstagramDataSource();

    public DataSource<? extends Item> getDataSource(int section) {
        switch (section) {
            case 0:
                return mIgSource;
            case 1:
            default:
                return mFbSource;
        }
    }

    public Item getItem(int section) {
        DataSource<? extends Item> source = getDataSource(section);
        Item item = source.getItem(0);
        item.getWidth();
        return item;
    }

    static class FacebookItem extends Item {

    }
    static class InstagramItem extends Item {

    }

    static abstract class Item {
        public int getWidth() {
            return 100;
        }

        public int geHeight() {
            return 100;
        }
    }

    interface DataSource<T extends Item> {
        T[] getItems();
        T getItem(int idx);
        int getCount();
    }

    static class FacebookDataSource implements DataSource<FacebookItem> {

        @Override
        public FacebookItem[] getItems() {
            return new FacebookItem[0];
        }

        @Override
        public FacebookItem getItem(int idx) {
            return null;
        }

        @Override
        public int getCount() {
            return 0;
        }
    }
    static class InstagramDataSource implements DataSource<InstagramItem> {

        @Override
        public InstagramItem[] getItems() {
            return new InstagramItem[0];
        }

        @Override
        public InstagramItem getItem(int idx) {
            return null;
        }

        @Override
        public int getCount() {
            return 0;
        }
    }
}

GenericApp.swift

class GenericApp {
    let fbSource = FacebookDataSource()
    let igSource = InstagramDataSource()
    
    // Error : Cannot specialize non-generic type 'DataSource'
    private func getSource<T>(section : Int) -> DataSource<Any> {
        switch section {
        case 0:
            return fbSource
        default:
            return igSource
        }
    }

    func getItem(section : Int, index : Int) -> PickerItem? {
        return getSource(section:section).getItem(index)
    }
    
    func getCount(section : Int) -> Int {
        return getSource(section:section).count
    }
}

class PickerItem {
    func getWidth() -> Int {
        return 100
    }

}

class FacebookPickerItem : PickerItem {
}

class InstagramPickerItem : PickerItem {
}
protocol DataSource {
    associatedtype Item : PickerItem

    var count: Int { get }
    func getItem(idx: Int) -> Item
}

class FacebookDataSource: DataSource {
    
    var items = [FacebookPickerItem]()

    typealias Item = FacebookPickerItem
    
    var count: Int {
        return items.count
    }
    func getItem(idx: Int) -> FacebookPickerItem {
        return items[idx]
    }
    
}

class InstagramDataSource: DataSource {
    
    var items = [InstagramPickerItem]()
    
    typealias Item = InstagramPickerItem
    
    var count: Int {
        return items.count
    }
    func getItem(idx: Int) -> InstagramPickerItem {
        return items[idx]
    }
    
}