How do I access ListIterator.previous() from another class

I have the following iteration in a "Statistics" Class

for (Number num : history) {
    // I do stuff with num here
    // ... and, occasionally want to read the previous value
} // for

Within my "history" class I have a ListIterator (rather than an Iterator as I want to go backwards as well).

   @Override
    public ListIterator<Number> iterator() {
       //System.out.println("In ITERATOR");
        ListIterator<Number> it = new ListIterator<Number>() {

            private int currentIndex = 0;

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

            @Override
            public boolean hasPrevious() {
                return currentIndex > 0;
            }

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

            @Override
            public Number previous() {
                if(hasPrevious()) {
                    return spins.get(currentIndex -1);
                }
                return null;
            }

            @Override
            public int previousIndex() {
                return currentIndex -1;

            }

            @Override
            public int nextIndex() {
                return currentIndex +1;
            }


            // The following functions are not implemented as we don't need them for this application
            @Override
            public void remove() {
                throw new UnsupportedOperationException();
            }

            @Override
            public void set(Number n) {
                throw new UnsupportedOperationException();
            }

            @Override
            public void add(Number n) {
                throw new UnsupportedOperationException();
            }

        };
        return it;
    }

... but I can't figure out, how I can get the previous element, when I am iterating through Numbers from the Statistics class.

Essentially, I want to iterate through Numbers but occasionally get the previous Number (after having moved past it).

I could store it, of course, but it seems that I should be able to call ListIterator.previous(), but I am unsure how to do that?

Any ideas?

G

Actually Kayaman already solved your problem using Iterator explicitly in two different ways.

I just wrote a simple demo following your request:

public class HelloWorld {
public static void main(String... args) {
    int size = 5;
    History<Integer> history = new HelloWorld().new History<>(size);
    for (int i = 0; i < size; ++i) {
        history.add(Integer.valueOf(i));
    }

    for (Integer a : history) {
        System.out.println(a);
        if (a % 3 == 1) {
            System.out.println(history.iterator().previous());
        }
    }
}

class History<T> implements Iterable<T> {
    T[] arr;
    int index;

    public History(int theSize) {
        index = -1;
        arr = (T[]) new Object[theSize];
    }

    public void add(T t) {
        arr[++index] = t;
    }

    @Override
    public ListIterator iterator() {
        return new ListIterator() {
            int i = 0;
            @Override
            public boolean hasNext() {
                return i <= index;
            }

            @Override
            public Object next() {
                return arr[i++];
            }

            @Override
            public boolean hasPrevious() {
                return i > 0;
            }

            @Override
            public Object previous() {
                return arr[i - 1];
            }

            @Override
            public int nextIndex() {
                return 0;
            }

            @Override
            public int previousIndex() {
                return 0;
            }

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

            @Override
            public void set(Object o) {
                throw new UnsupportedOperationException();
            }

            @Override
            public void add(Object o) {
                throw new UnsupportedOperationException();
            }
         };
      }
   }
}

And disasseambling its code, we have the following snippet:

  43: invokevirtual #9                  // Method com/company/HelloWorld$History.iterator:()Ljava/util/ListIterator;
  46: astore_2
  47: aload_2
  48: invokeinterface #10,  1           // InterfaceMethod java/util/Iterator.hasNext:()Z
  53: ifeq          101
  56: aload_2
  57: invokeinterface #11,  1           // InterfaceMethod java/util/Iterator.next:()Ljava/lang/Object;
  62: checkcast     #12                 // class java/lang/Integer
  65: astore_3
  66: getstatic     #13                 // Field java/lang/System.out:Ljava/io/PrintStream;
  69: aload_3
  70: invokevirtual #14                 // Method java/io/PrintStream.println:(Ljava/lang/Object;)V
  73: aload_3
  74: invokevirtual #15                 // Method java/lang/Integer.intValue:()I
  77: iconst_3
  78: irem
  79: iconst_1
  80: if_icmpne     98
  83: getstatic     #13                 // Field java/lang/System.out:Ljava/io/PrintStream;
  86: aload_1
  87: invokevirtual #9                  // Method com/company/HelloWorld$History.iterator:()Ljava/util/ListIterator;
  90: invokeinterface #16,  1           // InterfaceMethod java/util/ListIterator.previous:()Ljava/lang/Object;
  95: invokevirtual #14                 // Method java/io/PrintStream.println:(Ljava/lang/Object;)V

Clearly the for loop is actually using

while(history.hasNext()) {
     history.next();
}

to iterate the history. So if you use history.iterator().previous() directly in the for loop, you are actually creating a new iterator which will start from the very beginning 0 in your case.

And you don't want it, right?

Explicitly using iterator will suit your case as follows:

    ListIterator<Integer> iterator = history.iterator();
    while(iterator.hasNext()) {
        int a = iterator.next();
        System.out.println(a);
        if (a % 3 == 1) {
            System.out.println(iterator.previous());
        }

    }