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package playground.library.functional.iterator; | |
import java.lang.reflect.Array; | |
import java.util.Arrays; | |
import java.util.Iterator; | |
import java.util.NoSuchElementException; | |
import java.util.Stack; | |
/** | |
* An iterator that 'flattens out' collections, iterators, arrays, etc. | |
* | |
* That is it will iterate out their contents in order, descending into any | |
* iterators, iterables or arrays provided to it. | |
* | |
* An example (not valid Java for brevity - some type declarations are ommitted): | |
* | |
* new FlattingIterator({1, 2, 3}, {{1, 2}, {3}}, new ArrayList({1, 2, 3})) | |
* | |
* Will iterate through the sequence 1, 2, 3, 1, 2, 3, 1, 2, 3. | |
* | |
* Note that this implements a non-generic version of the Iterator interface so | |
* may be cast appropriately - it's very hard to give this class an appropriate | |
* generic type. | |
* | |
* @author david | |
*/ | |
public class FlatteningIterator implements Iterator | |
{ | |
// Marker object. This is never exposed outside this class, so can be guaranteed | |
// to be != anything else. We use it to indicate an absense of any other object. | |
private final Object blank = new Object(); | |
/* This stack stores all the iterators found so far. The head of the stack is | |
* the iterator which we are currently progressing through */ | |
private final Stack<Iterator<?>> iterators = new Stack<Iterator<?>>(); | |
// Storage field for the next element to be returned. blank when the next element | |
// is currently unknown. | |
private Object next = blank; | |
public FlatteningIterator(Object... objects){ | |
this.iterators.push(Arrays.asList(objects).iterator());} | |
public void remove(){ | |
/* Not implemented */} | |
private void moveToNext(){ | |
if ((next == blank) && !this.iterators.empty() ) { | |
if (!iterators.peek().hasNext()){ | |
iterators.pop(); | |
moveToNext();} | |
else{ | |
final Object next = iterators.peek().next(); | |
if (next instanceof Iterator){ | |
iterators.push((Iterator<?>)next); | |
moveToNext();} | |
else if (next instanceof Iterable){ | |
iterators.push(((Iterable)next).iterator()); | |
moveToNext();} | |
else if (next instanceof Array){ | |
iterators.push(Arrays.asList((Array)next).iterator()); | |
moveToNext();} | |
else this.next = next;}}} | |
/** | |
* Returns the next element in our iteration, throwing a NoSuchElementException | |
* if none is found. | |
*/ | |
public Object next() { | |
moveToNext(); | |
if (this.next == blank) throw new NoSuchElementException(); | |
else{ | |
Object next = this.next; | |
this.next = blank; | |
return next; | |
}} | |
/** | |
* Returns if there are any objects left to iterate over. This method | |
* can change the internal state of the object when it is called, but repeated | |
* calls to it will not have any additional side effects. | |
*/ | |
public boolean hasNext(){ | |
moveToNext(); | |
return (this.next != blank);} | |
} |
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This was originally from DZone snippets. You can find the original at http://snippets.dzone.com/posts/show/3523. | |
The original comment was: | |
A trivial utility class for iterating through a collection of objects in a 'flat' manner, descending into any collections (in this case defined as iterators, iterables or arrays, rather than elements of the Collection interface ) it finds and iterating through their elements. This preserves order, so {a, b, {c, d, {e}}} will be iterated through as a, b, c, d, e.It's not very complicated, but the implementation amused me so I thought I'd share it. |
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