木構造を再帰呼び出しで処理したくない人向けの Iterable なインタフェース。Iterator インタフェースで tree retrieval (pre-order) できます。

Retrievable.java

package hoge;

import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;
import java.util.Stack;

public interface Retrievable<T extends Retrievable> extends Iterable<T> {
    List<T> children();
    
    public static class PreOrderRetriever<T extends Retrievable> implements Iterator<T> {
        private Stack<Iterator<T>> stack = new Stack<Iterator<T>>();

        private T nextObject;

        public PreOrderRetriever(T source, boolean returnsSourceAtFirst) {
            stack.push(source.children().iterator());

            if (returnsSourceAtFirst) {
                nextObject = source;
            }
        }

        @Override
        public boolean hasNext() {
            if (nextObject != null) {
                return true;
            }

            Iterator<T> usingItr = null;
            while (!stack.isEmpty()) {
                Iterator<T> i = stack.pop();
                if (!i.hasNext()) {
                    continue;
                }

                usingItr = i;
                break;
            }

            if (usingItr == null) {
                return false;
            }

            nextObject = usingItr.next();
            stack.push(usingItr);
            stack.push(nextObject.children().iterator());

            return true;
        }

        @Override
        public T next() {
            if (!hasNext()) {
                throw new NoSuchElementException();
            }

            T result = nextObject;
            nextObject = null;

            return result;
        }

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