Java Iterator for a parameterised type

Question

I am trying to write an itertor for a type of ArrayList, where Number is a class.

I can almost do it, but I get errors:

This is the class definition

public class History implements Iterable<ArrayList<Number>> {

This is the definition of the type I wish to iterate over:

private ArrayList<Number> spins = new ArrayList<Number>();

... and here are the iterator functions:

   @Override
    public Iterator<ArrayList<Number>> iterator() {
        Iterator<ArrayList<Number>> it = new Iterator<ArrayList<Number>>() {

            private int currentIndex = 0;

            @Override
            public boolean hasNext() {
                return currentIndex < gethistorySize() && spins.get(currentIndex) != null;
            }

            @Override
            public ArrayList<Number> next() {
                return spins.get(currentIndex++); // ** ERROR **
            }

            @Override
            public void remove() {
                throw new UnsupportedOperationException();
            }
        };
        return it;
    }

I get an error at the lines marked ** ERROR **

The error I get is:

Multiple markers at this line - The type of expression must be an array type but it resolved to ArrayLisy - Type mismatch: Cannot convert from Number to ArrayList

I am not quite sure what to return here.

Can anybody help?

Thx

Multiple markers at this line - The type of expression must be an array type but it resolved to ArrayLisy<Number> Type mismatch: Cannot conver from Number to ArrayList<Number> — Graham
– Graham, Commented Nov 27, 2016 at 10:32
Could you add error message so people with same problem would be able to find your question and possible answers? — Pshemo
– Pshemo, Commented Nov 27, 2016 at 10:32
Done above, sorry - and, just to note, it is not a run time error, but a "screen error" so cannot complie — Graham
– Graham, Commented Nov 27, 2016 at 10:35

Eran · Accepted Answer · 2016-11-27 10:32:31Z

1

In order for spins.get(currentIndex++) to return an ArrayList<Number>, the type of spins would have to be List<ArrayList<Number>>, not ArrayList<Number>.

If the type of spins is correct, perhaps what you need is to implement an Iterator<Number>, not an Iterator<ArrayList<Number>> (and History should implement Iterable<Number>).

answered Nov 27, 2016 at 10:32

Eran

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1 Comment

Graham Over a year ago

Thnak you - I will try these suggestions

Luminous_Dev · Accepted Answer · 2016-11-27 10:37:44Z

0

Change

        @Override
        public ArrayList<Number> next() {
            return spins.get(currentIndex++); // ** ERROR **
        }

To

        @Override
        public Number next() {
            return spins.get(currentIndex++); // ** ERROR **
        }

Return type is wrong hence the compile error. When you call next() method, you want to iterate each Number object within An ArrayList<Number>.

answered Nov 27, 2016 at 10:37

Luminous_Dev

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3 Comments

Graham Over a year ago

Thansk - just for info - that removs the original error, but gives a (similar) error on the changed line

Luminous_Dev Over a year ago

@Graham link try some of this example

Graham Over a year ago

Many thanks all - changing the type to <Number> respolved this.

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