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How can I correctly implement the methods in my class?

I have a class SimpHisto that takes in an array of generic type SL, and returns the size and the count of a specific element of that array. My code is provided down below.

public class SimpHisto<SL> implements Histo<SL>, Iterable<SL> {
    DT[] items;

    public SimpHisto() {
    }

    public SimpHisto(SL[] items) {

        this.items = items;
    }

    public int getCount(SL item) {
        var n = 0;
        Iterator<SL> L = this.iterator();
        while (L.hasNext()) {
            for (DT dt : items) {
                if (dt == item) {
                    n++;
                }
            }
        }
        return n;
    }
    @Override
    public Iterator<SL> iterator() {

        return new Iterate();
    }

    @Override
    public int getTotalCount() {
        return items.length;
    }

    private class Iterate implements Iterator<SL> {
        int index = 0;
        boolean lastRemoved = false;

        @Override
        public boolean hasNext() {
            return (index < items.length);
        }

        @Override
        public SL next() {
            if (index <= items.length)
                throw new NoSuchElementException("No element at index");
            SL object = items[index];
            index++;
            lastRemoved = false;
            return object;
        }
    }

After I tried to run my code using the snippet down below…

public class SimpleHistogramTest {

    @Test
    public void testHistogram() {
        Character[] s = {'a','b','c','a'};
        Histo<Character> h = new SimpHisto<>(s);
        Iterator<Character> iter = h.iterator();
        int elemCount = 0;
        while(iter.hasNext()) {
            iter.next();
            elemCount++;
        }

        assertEquals(3, elemCount);
        assertEquals(2, h.getCount('a'));
        assertEquals(1, h.getCount('b'));
        assertEquals(1, h.getCount('c'));
        assertEquals(4, h.getTotalCount());
    }
}

I get an error saying:

> Task :compileJava UP-TO-DATE
> Task :processResources UP-TO-DATE
> Task :classes UP-TO-DATE
> Task :compileTestJava
> Task :processTestResources NO-SOURCE
> Task :testClasses
> Task :test FAILED

No element at index

Can someone please explain what’s wrong with my code and provide any explanations on solutions?

Thank you

P.S. this is interface Histo which is implemented by SimpHisto

public interface Histo<DT> extends Iterable<DT> {

    public int getTotalCount();
    public int getCount(DT item);
    public void setCount(DT item, int count);
}

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Answer

The error is in your Iterate. You throw that exception if (index <= items.length). index starts with 0 so this is always true. You only have to change to >=

As mentioned by 英語は苦手 your while in getCount does not call next. Since you iterate over the items directly I would prefer to remove the Iterator at all. And a Stream may increase readability:

return (int) Stream.of(this.items)
    .filter(dt -> dt == item)
    .count();

EDIT: without Stream-API you can iterate over your items directly:

for (SL dt : this.items) {
    if (dt == item) {
        n++;
    }
}

If you want to use the Iterator you have to call next

while (L.hasNext()) {
    SL dt = L.next();
    if (dt == item) {
        n++;
    }
}

BTW: why assertEquals(3, elemCount);? There are 4 elements

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