@GwtCompatible public abstract class AbstractIterator<T> extends UnmodifiableIterator<T>
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 Reader.read(), 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 = in.next();
if (s != null) {
return s;
}
}
return endOfData();
}
};
}
This class supports iterators that include null elements.| Modifier | Constructor and Description |
|---|---|
protected |
AbstractIterator()
Constructor for use by subclasses.
|
| Modifier and Type | Method and Description |
|---|---|
protected abstract T |
computeNext()
Returns the next element.
|
protected T |
endOfData()
Implementations of
computeNext() must invoke this method when
there are no elements left in the iteration. |
boolean |
hasNext() |
T |
next() |
T |
peek()
Returns the next element in the iteration without advancing the iteration,
according to the contract of
PeekingIterator.peek(). |
removeprotected abstract T computeNext()
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.
endOfData was called
during execution, the return value will be ignored.java.lang.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.protected final T endOfData()
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();public final boolean hasNext()
public final T next()
public final T peek()
PeekingIterator.peek().
Implementations of AbstractIterator that wish to expose this
functionality should implement PeekingIterator.