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POReturning the same object from Iterator<?>
 

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    <p>Is it valid - i.e., within the contact, for <code>Iterator</code> to return the same (mutable) object as a prior iteration, or a new object with shared mutable state as prior object? That is, may one call to <code>next()</code> to invalidate the object returned by a prior call, as long as it is documented?</p> <p>I understand perfectly well that this isn't a good idea <em>in general</em>, and I don't make a habit of it.</p> <p>However, there are some cases where reusing the same underlying object, or new object with shared internals (thus possibly invalidating internals of an object returned earlier) has a significant and measurable in some cases. A good example would be when the object returned from the iterator points with some offset and length into a buffer array held by the containing <code>Iterable</code>, and this array may be overwritten when the subsequent object is retrieved.</p> <p>Granted this violates the principle of least astonishment, and may be client unfriendly - but I'm curious if it violates the contact of <code>Iterator</code> - as long as I document it, am I playing within the rules?</p> <p>The <a href="http://docs.oracle.com/javase/6/docs/api/java/util/Iterator.html" rel="nofollow">javadoc</a> seems to have been written with a focus on collections, where sharing state between <code>next()</code> calls is probably meaningless, but <code>Iterator</code> is a general purpose interface (you can implement without a backing collection or even without a backing <em>anything</em>).</p> <p>Basically I'm trying to implement something comparable, performance-wise, to the flyweight pattern, but to retain the niceties of Java iterators.</p> <p>Iterators following this pattern would be safe to use in the following way:</p> <pre><code>for (Foo foo : FooIterable) { if (foo.method()) { System.println("ha!"); } } </code></pre> <p>since each instance of <code>foo</code> never escapes from one iteration to the other. However, something like the following would not work in general:</p> <pre><code>List&lt;Foo&gt; allTheFoos = ...; for (Foo foo : FooIterable) { allTheFoos.add(foo); } for (Foo foo : allTheFoos) { ... } </code></pre> <p>Since the <code>foo</code>s in the first loop are stored away and thus escape their iteration. When you access them later, their state may be invalid (e.g., they may all look like the last <code>foo</code> returned from the initial iteration.</p>
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