如何实现迭代器？

Question

I'm having a problem with my class, which I cannot resolve. I want to implement an iterator to my Interval1 class. Here is my code:

public class Interval1 {
    private double first;
    private double last;
    private double step;
    private IntervalIterator e;

    public Interval1(double first, double last, double step,  IntervalIterator e) {
        //chequear los datos
        this.first = first;
        this.last = last;
        this.step = step;
        this.e = new IntervalIterator();
    }

    public  double at(int index) {
        checkIndex(index);
        return first + index*step;
    }

    private void checkIndex(int index) {
        if(index < 0 || index >= size())
            throw new IndexOutOfBoundsException("Invalid Index: " + index);
    }

    public int size() {
        return (int) ((last - first) / step);
    }

    public IntervalIterator e() {
        for (Double i : this)
            System.out.println(i);
        return e;
    }
}

This is the class IntervalIterator that I'm using and this give an error such in size() and at :

public class IntervalIterator implements Iterator<Double> {
    private int index = 0;
    private Double at;

    public boolean hasNext() {
        return index < size();
    }

    public Double next() {
        if (!hasNext()) {
            throw new NoSuchElementException();
        }
        return at(index++);
    }

    public void remove() {
        if (Interval1 <= 0) {
            return throw new UnsupportedOperationException("remove");
        } else {
            if (Interval1 >= 0) {
                //chekear el array e eliminarlo
            }
        }
    }
}

Answer 1

I'd use java8 streams to return an Iterator<Double> :

public class Interval
    implements Iterable<Double> {

    private final double first;
    private final double last;
    private final double step;
    private final long size;

    public Interval(double first, double last, double step) {
        this.first = first;
        this.last = last;
        this.step = step;
        this.size = (long) ((last - first) / step) + 1;
    }

    @Override
    public Iterator<Double> iterator() {
        return DoubleStream.iterate(this.first, n -> n + this.step)
            .limit(this.size)
            .iterator();
    }

    // TODO getters
}

I've added the final modifier to the fields and made the Interval class implement Iterable<Double> , so that it can be iterated. I've also fixed the calculation for size (it needed to be incremented by one).

I've used the DoubleStream.iterate() method , which receives a seed and a function that receives the current element of the stream as an input and returns the following element (for this I've added step to the current element). I also had to use the DoubleStream.limit() method , because the stream generated by DoubleStream.iterate() is infinite.

Usage:

Interval interval = new Interval(3.0, 9.0, 1.5);

for (double n : interval) {
    System.out.println(n);
}

The above code generates the following output:

3.0
4.5
6.0
7.5
9.0