Cyclic Iteration : Iterator « Collections Data Structure « Java

Cyclic Iteration


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import java.util.Iterator;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.SortedMap;

/** Provide an cyclic {@link Iterator} for a {@link SortedMap}.
 * The {@link Iterator} navigates the entries of the map
 * according to the map's ordering.
 * If the {@link Iterator} hits the last entry of the map,
 * it will then continue from the first entry.
public class CyclicIteration<K, V> implements Iterable<Map.Entry<K, V>> {
  private final SortedMap<K, V> navigablemap;
  private final SortedMap<K, V> tailmap;
  private final K startingkey;

  /** Construct an {@link Iterable} object,
   * so that an {@link Iterator} can be created  
   * for iterating the given {@link SortedMap}.
   * The iteration begins from the starting key exclusively.
  public CyclicIteration(SortedMap<K, V> navigablemap, K startingkey) {
    this.startingkey = startingkey;
    if (navigablemap == null || navigablemap.isEmpty()) {
      this.navigablemap = null;
      this.tailmap = null;
    else {
      this.navigablemap = navigablemap;
      this.tailmap = navigablemap.tailMap(startingkey);

  /** {@inheritDoc} */
  public Iterator<Map.Entry<K, V>> iterator() {
    return new CyclicIterator();

  /** An {@link Iterator} for {@link CyclicIteration}. */
  private class CyclicIterator implements Iterator<Map.Entry<K, V>> {
    private boolean hasnext;
    private Iterator<Map.Entry<K, V>> i;
    /** The first entry to begin. */
    private final Map.Entry<K, V> first;
    /** The next entry. */
    private Map.Entry<K, V> next;
    private CyclicIterator() {
      hasnext = navigablemap != null;
      if (hasnext) {
        i = tailmap.entrySet().iterator();
        Map.Entry<K, V> e = nextEntry();
        if (e.getKey().equals(startingkey)) {
          e = nextEntry();

        first = e; 
        next = first;
      else {
        i = null;
        first = null;
        next = null;

    private Map.Entry<K, V> nextEntry() {
      if (!i.hasNext()) {
        i = navigablemap.entrySet().iterator();

    /** {@inheritDoc} */
    public boolean hasNext() {
      return hasnext;

    /** {@inheritDoc} */
    public Map.Entry<K, V> next() {
      if (!hasnext) {
        throw new NoSuchElementException();

      final Map.Entry<K, V> curr = next;
      next = nextEntry();
      hasnext = !next.equals(first);
      return curr;

    /** Not supported */
    public void remove() {
      throw new UnsupportedOperationException("Not supported");


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