问题描述:

I have a class Polygon on which I wish to implement two iterators: one to run through all elements (vertices and edges in alternating order) just ONCE, and another to run through them ad infinitum (cyclically).

From a for-each usage standpoint, my guess is that I am only going to be able to have one of the above be the default iterator that can be used with for-each, via implementation of Iterable.iterator(). Is this correct? Or is there a way I could use for-each with both?

网友答案:

Just add two methods returning two different Iterators, one for each case:

public Iterable<String> eachOnce() {
    List<String> allResults = new ArrayList<String>();
    // fill list
    return allResults;
}

public Iterable<String> eachCyclic() {
    return new Iterable<String>() {

        public Iterator<String> iterator() {
            return new Iterator<String>() {

                public boolean hasNext() {
                    return true;
                }

                public String next() {
                    // TODO implement
                    return null;
                }

                public void remove() {
                    // do nothing
                }
            };

        }
    };
}

This is just an example with a List of Strings, just adapt.

Instead of

for (Polygon p : polygons) { }

just use

for (Polygon p : polygons.eachOnce()) { }

or the cyclic edition

网友答案:

An answer I think is better than those already presented is a method that turns any Iterable into a cyclic one.

public class IterableUtils {
  public static class CyclicIterator<T> implements Iterator<T> {
    private final Iterable<T> inner;
    private Iterator<T> currentIter;
    public CyclicIterator(Iterable<T> inner) {
      this.inner = inner;
    }
    public boolean hasNext() {
      if (currentIter == null || !currentIter.hasNext()) {
        currentIter = inner.iterator();
      }
      return currentIter.hasNext();
    }
    public T next() {
      if (currentIter == null || !currentIter.hasNext()) {
        currentIter = inner.iterator();
      }
      return currentIter.next();
    }
    public void remove() {
      currentIter.remove();
    }
  }
  public static <T> Iterable<T> cycle(final Iterable<T> i) {
    return new Iterable<T>() {
      public Iterator<T> iterator() { return new CyclicIterator<T>(i); }
    };
  }
}

Then you can just implement the single iterator method in the Polygon class and use

for (Element e: polygon) {
  ...
}

to iterate once and

for (Element e: cycle(polygon)) { ... }

to iterate endlessly. As a bonus, the cycle modifier can be applied to any iterable.

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