I need to iterate over the set of values for each iteration of for loop, but only for first iteration it works fine. Thereafter the itr.hasNext()
returns false
.
Iterator<String> itr = getQuestionIterator(File file); for(Person p : persons) { while(itr.hasNext()) { String question = itr.next(); ........ ........ } }
This behaviour is clear to me.
One solution could be calling getQuestionIterator(File file)
method in for loop so for each for loop iteration it gets reinitialized. But this is very inefficient approach as itr
is independent.
I tried this Iterator<String> temp = itr
, but it didn't work also as it holds the reference only.
Is there any way to copy the iterator into another one or any other better approach?
iterators are not reusable; you need to get a fresh Iterator from the Iterable collection each time you want to iterate over the elements.
Iterator interface provides the following methods: boolean hasNext() - Returns true if the iteration has more elements. E next() - Returns the next element in the iteration. void remove() - Removes from the underlying collection the last element returned by the iterator (optional operation).
Iterator and for-each loop are faster than simple for loop for collections with no random access, while in collections which allows random access there is no performance change with for-each loop/for loop/iterator.
An Iterator
is the smallest possible API to work off data sequentially, hence it abstracts from the underlying data source. As it can only move forwards (next()
) without any option to reset or rewind, it is a one-way object that must be thrown away after usage. And due to the limited API it offers, it is not possible to simply "copy" it without knowing the implementation and/or the underlying data source.
So there are four ways to handle your problem:
(1) Re-aquire a new iterator from the underlying data source
Just call getQuestionIterator(File file)
every time you need to iterate over the data (again).
(2) Combine all processing code into one single iterating loop
Instead of...
iterator = /* get new iterator */ while (iterator.hasNext()) { String question = iterator.next(); /* first processing step */ } iterator = /* get new iterator */ while (iterator.hasNext()) { String question = iterator.next(); /* second processing step */ } iterator = /* get new iterator */ while (iterator.hasNext()) { String question = iterator.next(); /* third processing step */ } ...
...combine all steps:
iterator = /* get new iterator */ while (iterator.hasNext()) { String question = iterator.next(); /* first processing step */ /* second processing step */ /* third processing step */ ... }
(3) Copy all elements into a local cache (Collection
)
Iterate over all items once and put them into a local collection that you can use to aquire an arbitrary number of iterators:
// read everything into a local cache Collection<String> cache = new ArrayList<>(); while (iterator.hasNext()) cache.add(iterator.next()); // now you can get as many iterators from cache as required: Iterator<String> iter = cache.iterator(); // use iter iter = cache.iterator(); // once more // use iter ...
(4) Modify your data source API to let its implementation handle the problem
Meaning: Change getQuestionIterator(File file)
to return an Iterable<String>
instead of an Iterator<String>
. You can gain an arbitrary number of iterators from an Iterable
:
Iterable<String> iterable = getQuestionIterator(File file); Iterator<String> iter = iterable.iterator(); // use iter iter = iterable.iterator(); // once more // use iter
It depends on the exact contents of your code block, but why not flip the loops? Have the outer loop go over the file, and for each iteration, go over all the Person
s:
Iterator<String> itr = getQuestionIterator(File file); while(itr.hasNext()) { String question = itr.next(); for(Person p : persons) { ........ ........ } }
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