Writing a generic iterator in Java

Question

I was getting out of memory exceptions when I stored a collection of my class AbstractState in memory, so I am trying to write an iterable/iterator. I have a bit of C# knowledge but little Java experience. In C# I would write a method returning an IEnumerable<AbstractState>, but it doesn't seem so easy here.

AbstractState stores a pair of coordinates, although depending on the implementation, the operations on them differ. (It also extends a generic MyPair<Coordinate>.)

Within AbstractState, I define a constructor AbstractState(Coordinate A, Coordinate B){super(A,B);}. I override this in some but not all sub-classes. Coordinate is concrete. Here is my iterable:

import java.util.Iterator;

public class StateSpace<T extends AbstractState> implements Iterable<T> {
    @Override
    public Iterator<T> iterator() {
        return new StateIterator();
    }
}

and my Iterator:

public class StateIterator<T extends AbstractState> implements Iterator<T> {
    private Iterator<Coordinate> iX, iY;

    StateIterator(){
        iX = Main.GRID.iterator();
        iY = Main.GRID.iterator();
    }

    @Override
    public boolean hasNext() {
        return iX.hasNext() || iY.hasNext();
    }

    @Override
    public T next() {
        return null;
    }
}

(GRID here is a static range of Coordinates.)

How can I implement the next() method correctly? (Or, what is a better design for my code to solve this problem?)

Instantiating a T does not work, and I cannot instantiate an abstract class. I think I was close by trying

getDeclaredConstructor(Coordinate.class, Coordinate.class).newInstance(iX.next(), iY.next());

but I got a compiler warning for no such method.

I had unchecked casts when I casted to a T, so suspect that this is a bad idea. Having many iterators/iterables is unappealing, as I would be checking (via if statements or a switch) which iterator I need, which undermines my OO code design.

Any suggestions appreciated, thanks

what do you mean by "when I stored a collection of my class AbstractState in memory"? Please clarify: Do you already have a collection of AbstractState in memory and you get a memory exception when you add to it? in that case iterating it won't help you. — inor
– inor, Commented Apr 9, 2018 at 5:58
@inor sorry - just saw this. Previously I created the collection of AbstractState to be used later (in for loops). I could not even create it because it was too large. So I would like to iterate through an iterable so that storing every AbstractState in memory is not needed — Szmagpie
– Szmagpie, Commented Apr 13, 2018 at 14:39

K.Nicholas · Accepted Answer · 2018-04-08 01:29:54Z

1

Pass this to the iterator constructor. The StateSpace has access to the instantiated T type, and so it can implement methods to determine how to getNext() and hasNext(). It's not clear what iX and iY are about.

import java.util.Iterator;

public class StateSpace<T extends AbstractState> implements Iterable<T> {
    List<T> types;
    int pos;

    public StateSpace() {
        types = new ArrayList<>();
    }
    public void add(T type) {
        types.add(type);
    }
    T getNext() {
        return types.get(pos++);
    }
    boolean hasNext() {
         return pos < types.size()-1;
    }
    @Override
    public Iterator<T> iterator() {
        return new StateIterator(this);
    }
}

and

public class StateIterator<T extends AbstractState> implements Iterator<T> {
    private Iterator<Coordinate> iX, iY;
    private StateSpace<T> stateSpace;

    StateIterator(StateSpace<T> stateSpace){
        this.stateSpace = stateSpace;
        iX = Main.GRID.iterator();
        iY = Main.GRID.iterator();
    }

    @Override
    public boolean hasNext() {
        return iX.hasNext() || iY.hasNext();
    }

    @Override
    public T next() {
        return stateSpace.getNext(); // or whatever.
    }
}

maybe helps.

edited Apr 8, 2018 at 1:29

answered Apr 7, 2018 at 23:57

K.Nicholas

11.6k4 gold badges50 silver badges74 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Szmagpie Over a year ago

Thanks for reply. next() must return a T, but stateSpace is of type StateSpace<T>. I tried putting a method in StateSpace that returns a T, but have the same problems

K.Nicholas Over a year ago

Right, sorry, assuming StateSpace saves or has access to a type T then stateSpace should be able to getNext(); Updated.

Szmagpie Over a year ago

Ahaa, that makes sense: if you can't getNext then it isn't "Iterable" really. I'll need to keep track of the current AbstractSpace so I can iterate to the next

Collectives™ on Stack Overflow

Writing a generic iterator in Java

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related