今天阅读Guava源码发现一个通用的迭代器设计。
package com.google.common.collect;
import static com.google.common.base.Preconditions.checkState;
import com.google.common.annotations.GwtCompatible;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.util.NoSuchElementException;
public abstract class AbstractIterator<T> extends UnmodifiableIterator<T> {
private State state = State.NOT_READY;
/** Constructor for use by subclasses. */
protected AbstractIterator() {}
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. <b>Note:</b> 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.
*
* <p>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.
*
* <p>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}.
*
* <p>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} <b>must</b> 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();}
*/
@CanIgnoreReturnValue
protected final T endOfData() {
state = State.DONE;
return null;
}
@CanIgnoreReturnValue // TODO(kak): Should we remove this? Some people are using it to prefetch?
@Override
public final boolean hasNext() {
checkState(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;
}
@CanIgnoreReturnValue // TODO(kak): Should we remove this?
@Override
public final T next() {
if (!hasNext()) {
throw new NoSuchElementException();
}
state = State.NOT_READY;
T result = next;
next = null;
return result;
}
/**
* Returns the next element in the iteration without advancing the iteration,
* according to the contract of {@link PeekingIterator#peek()}.
*
* <p>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;
}
}
有两个实例字段：
private State state = State.NOT_READY;
private T next;state有四种状态
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,
}所有的迭代器都需要实现hasNext和next这两个方法。
这个类的设计思路是：
每次执行hasNext的时候如果 next的值准备好了就返回true。没有准备好就会尝试着去寻找这个next的值。在寻找的过程中先假设会寻找失败将state设置为FAILED。寻找到了这个next的值就把state设置为ready。这个寻找的过程是一个抽象的过程，所以这一个类有一个抽象的方法：
protected abstract T computeNext();在返回值赋值给next.如果在寻找的过程中到达了序列的结尾在computeNext()这个方法里面需要把state的值设置为Done。这就是这个抽象方法的语义。
在状态为ready只会执行next方法就是把next返回出来同时，设置next值为空，然设置state为NOT_READY。此时又可以进行下一轮的hasNext和next过程了。
这里给出一个基于这个类的最简单的实现。实现了computeNext的方法完全符合语义。寻找得到的时候返回这个值，在序列终止的时候设置状态为Done
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();
}
};Guava中的Splitter就是基于这样的一个模式设计的。Splitter认为被切割的字符串是一个待返回的迭代器。
