leap.lang.collection.UnmodifiableIteratorBase Maven / Gradle / Ivy
/*
* Copyright (C) 2007 The Guava Authors
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package leap.lang.collection;
import java.util.NoSuchElementException;
import leap.lang.Assert;
/**
* This class provides a skeletal implementation of the {@code Iterator} interface, to make this interface easier to
* implement for certain types of data sources.
*
*
* {@code Iterator} requires its implementations to support querying the end-of-data status without changing the
* iterator's state, using the {@link #hasNext} method. But many data sources, such as {@link java.io.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 {@link #computeNext} method, and invoke the {@link #endOfData} method when appropriate.
*
*
* Another example is an iterator that skips over null elements in a backing iterator. This could be implemented as:
*
*
* {@code
*
* public static Iterator skipNulls(final Iterator in) {
* return new AbstractIterator() {
* 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.
*
* @author Kevin Bourrillion
* @since 2.0 (imported from Google Collections Library)
*/
//copy from google guava library 15.0-SNAPSHOT,under Apache License 2.0
public abstract class UnmodifiableIteratorBase extends UnmodifiableIterator {
private State state = State.NOT_READY;
/** Constructor for use by subclasses. */
protected UnmodifiableIteratorBase() {
}
private enum State {
/** We have computed the next element and haven't returned it yet. */
READY,
/** We haven't yet computed or have already returned the element. */
NOT_READY,
/** We have reached the end of the data and are finished. */
DONE,
/** We've suffered an exception and are kaput. */
FAILED,
}
private T next;
/**
* Returns the next element. Note: the implementation must call {@link #endOfData()} when there are no
* elements left in the iteration. Failure to do so could result in an infinite loop.
*
*
* The initial invocation of {@link #hasNext()} or {@link #next()} calls this method, as does the first invocation
* of {@code hasNext} or {@code next} following each successful call to {@code next}. Once the implementation either
* invokes {@code endOfData} or throws an exception, {@code computeNext} is guaranteed to never be called again.
*
*
* If this method throws an exception, it will propagate outward to the {@code hasNext} or {@code next} invocation
* that invoked this method. Any further attempts to use the iterator will result in an
* {@link IllegalStateException}.
*
*
* The implementation of this method may not invoke the {@code hasNext}, {@code next}, or {@link #peek()} methods on
* this instance; if it does, an {@code IllegalStateException} will result.
*
* @return the next element if there was one. If {@code endOfData} was called during execution, the return value
* will be ignored.
* @throws RuntimeException if any unrecoverable error happens. This exception will propagate outward to the
* {@code hasNext()}, {@code next()}, or {@code peek()} invocation that invoked this method. Any further
* attempts to use the iterator will result in an {@link IllegalStateException}.
*/
protected abstract T computeNext();
/**
* Implementations of {@link #computeNext} must invoke this method when there are no elements left in the
* iteration.
*
* @return {@code null}; a convenience so your {@code computeNext} implementation can use the simple statement
* {@code return endOfData();}
*/
protected final T endOfData() {
state = State.DONE;
return null;
}
@Override
public final boolean hasNext() {
Assert.isTrue(state != State.FAILED);
switch (state) {
case DONE:
return false;
case READY:
return true;
default:
}
return tryToComputeNext();
}
private boolean tryToComputeNext() {
state = State.FAILED; // temporary pessimism
next = computeNext();
if (state != State.DONE) {
state = State.READY;
return true;
}
return false;
}
@Override
public final T next() {
if (!hasNext()) {
throw new NoSuchElementException();
}
state = State.NOT_READY;
return next;
}
/**
* Returns the next element in the iteration without advancing the iteration, according to the contract of
* {@link PeekingIterator#peek()}.
*
*
* Implementations of {@code AbstractIterator} that wish to expose this functionality should implement
* {@code PeekingIterator}.
*/
public final T peek() {
if (!hasNext()) {
throw new NoSuchElementException();
}
return next;
}
}