Java different type objects as same parameter

Question

I am currently writing an function for manipulate several ArrayList.

All the elements have same constructor, how can I do it like below?

Class A{
public A (Cursor data){...}
}

Class B{
public B (Cursor data){...}
}

Class C{
public C (Cursor data){...}
}

public void dataManipulation(ArrayList<**?**> list, Cursor cursor){
cursor.moveToFirst();
for (int i = 0; i < cursor.getCount(); i++) {
            list(new **?**(cursor));
            if (!cursor.isAfterLast())
                cursor.moveToNext();

}

public void main(){
ArrayList<A> listA = new ArrayList<A>();
ArrayList<B> listB = new ArrayList<B>();
ArrayList<C> listC = new ArrayList<C>();

Cursor cursor = IMPORT_CURSOR_FROM_FILE;

dataManipulation(listA, cursor);
dataManipulation(listB, cursor);
dataManipulation(listC, cursor);
}

Create a common interface for A, B and C to implement which guarantees compatibility for the functionality that you require — MadProgrammer
– MadProgrammer, Commented Nov 28, 2014 at 5:04
Thanks for reply, could you show some sample code? As I am a newbie of programming — Booker Chan
– Booker Chan, Commented Nov 28, 2014 at 5:05
You don't call your example right. I'm not sure where you're getting your cursor from that main method (which isn't the same as public static void main(String[] args), mind you). Also, what is list(new **?**(cursor)) meant to represent? — Makoto
– Makoto, Commented Nov 28, 2014 at 5:15

MadProgrammer · Accepted Answer · 2014-11-28 05:19:24Z

4

Start by taking a look at Interfaces and Inheritance, basically an interface is a contract that gurentees that any object that implements that interface will provide the contractual functionality...

For example...

public interface CursorContainer {
    public Cursor getCursor();
}

You "common" class would implement this interface and provide implementations for the getCursor (and any other required) method...

Class A implements CursorContainer {
    public A (Cursor data){...}
}

Class B implements CursorContainer  {
    public B (Cursor data){...}
}

Class C implements CursorContainer  {
    public C (Cursor data){...}
}

Then you could use generics...

public void dataManipulation(ArrayList<CursorContainer> list, Cursor cursor){

Your next problem is know how to create a particular implementation, for this, you will need a factory...

public interface CursorContainerFactory {
    public CursorContainer create(Cursor cursor);
}

You would need a factory for each type of container you want to create...

public Class CursorContainerAFactory implements CursorContainerFactory {
    public CursorContainer create(Cursor cursor) {
        return new A(cursor);
    }
}

You would then need to supply the factory to your dataManipulation method...

public void dataManipulation(ArrayList<CursorContainer> list, CursorContainerFactory factory, Cursor cursor){
    cursor.moveToFirst();
    for (int i = 0; i < cursor.getCount(); i++) {
        list(factory.create(cursor));
        if (!cursor.isAfterLast())
            cursor.moveToNext();

}

And finally, call it...

dataManipulation(listA, new CursorContainerAFactory(), cursor);

Remember, classes can implement many interfaces, but only extend from one...

edited Nov 28, 2014 at 5:19

answered Nov 28, 2014 at 5:13

MadProgrammer

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6 Comments

Makoto Over a year ago

Is there any reason that it couldn't be List<? extends CursorContainer>?

Makoto Over a year ago

If that's the case, then it could be shifted to List<? super CursorContainer>. I didn't see anywhere that we were writing to the list at first (I guess that's what list(new **?**(cursor)); means), but I also don't see anywhere that we read from the list, either.

ajb Over a year ago

Just to spoil your fun even further, implements has an s on the end.

MadProgrammer Over a year ago

@ajb I also implemented CursorContainer on the factory implementation, but at least I caught that one :P

MadProgrammer Over a year ago

@Makoto Sure, but what's the benefit (of <? super CursorContainer> over <CursorContainer>)? I'm assuming the OP wants to read it later...

|

frgnvola · Accepted Answer · 2014-11-28 05:12:13Z

2

You would need to create an interface for each of your classes (A, B, C) to implement.

See this: https://docs.oracle.com/javase/tutorial/java/concepts/interface.html

answered Nov 28, 2014 at 5:12

frgnvola

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4 Comments

MadProgrammer Over a year ago

Hmm, and what about list(new **?**(cursor));?

frgnvola Over a year ago

You could just substitute cursor for A,B, or C

MadProgrammer Over a year ago

But how do you know which one is to be used?

frgnvola Over a year ago

I don't, since I have no clue what the program is being used for. I would imagine you could use a switch statement for this, however.

Elliott Frisch · Accepted Answer · 2014-11-28 05:13:00Z

You might use reflection, but you'll need to pass in the Class because of type-erasure. Something like,

public <T> void dataManipulation(Class<T> cls, ArrayList<T> list,
        Cursor cursor) {
    cursor.moveToFirst();
    for (int i = 0; i < cursor.getCount(); i++) {
        Constructor<T>[] ctors = (Constructor<T>[]) cls.getConstructors();
        for (Constructor<T> c : ctors) {
            Type[] types = c.getGenericParameterTypes();
            if (types.length == 1) {
                if (types[0].getClass().equals(Cursor.class)) {
                    list.add(c.newInstance(cursor));
                    break;
                }
            }
        }
        if (!cursor.isAfterLast()) {
            cursor.moveToNext();
        }
    }
}

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