Class AbstractIterator<T>

  • All Implemented Interfaces:

    public abstract class AbstractIterator<T>
    extends UnmodifiableIterator<T>
    This class provides a skeletal implementation of the Iterator interface, to make this interface easier to implement for certain types of data sources.

    Iterator requires its implementations to support querying the end-of-data status without changing the iterator's state, using the hasNext() method. But many data sources, such as, do not expose this information; the only way to discover whether there is any data left is by trying to retrieve it. These types of data sources are ordinarily difficult to write iterators for. But using this class, one must implement only the computeNext() method, and invoke the endOfData() method when appropriate.

    Another example is an iterator that skips over null elements in a backing iterator. This could be implemented as:

     public static Iterator<String> skipNulls(final Iterator<String> in) {
       return new AbstractIterator<String>() {
         protected String computeNext() {
           while (in.hasNext()) {
             String s =;
             if (s != null) {
               return s;
           return endOfData();

    This class supports iterators that include null elements.

    Kevin Bourrillion
    • Constructor Detail

      • AbstractIterator

        protected AbstractIterator()
        Constructor for use by subclasses.
    • Method Detail

      • computeNext

        protected abstract T computeNext()
        Returns the next element. Note: the implementation must call endOfData() when there are no elements left in the iteration. Failure to do so could result in an infinite loop.

        The initial invocation of hasNext() or next() calls this method, as does the first invocation of hasNext or next following each successful call to next. Once the implementation either invokes endOfData or throws an exception, computeNext is guaranteed to never be called again.

        If this method throws an exception, it will propagate outward to the hasNext or next invocation that invoked this method. Any further attempts to use the iterator will result in an IllegalStateException.

        The implementation of this method may not invoke the hasNext, next, or peek() methods on this instance; if it does, an IllegalStateException will result.

        the next element if there was one. If endOfData was called during execution, the return value will be ignored.
        RuntimeException - if any unrecoverable error happens. This exception will propagate outward to the hasNext(), next(), or peek() invocation that invoked this method. Any further attempts to use the iterator will result in an IllegalStateException.
      • endOfData

        protected final T endOfData()
        Implementations of computeNext() must invoke this method when there are no elements left in the iteration.
        null; a convenience so your computeNext implementation can use the simple statement return endOfData();
      • hasNext

        public final boolean hasNext()
        Description copied from interface: java.util.Iterator
        Returns true if the iteration has more elements. (In other words, returns true if would return an element rather than throwing an exception.)
        true if the iteration has more elements
      • peek

        public final T peek()
        Returns the next element in the iteration without advancing the iteration, according to the contract of PeekingIterator.peek().

        Implementations of AbstractIterator that wish to expose this functionality should implement PeekingIterator.