Using the iterator over the same set more than once in java

Question

Say I'm iterating over a set in java, e.g.

Iterator<_someObject_> it = _someObject_.iterator();

well, first I want to go through every object in the set and make a change to each object I visit. So I use:

while(it.hasNext()) {
        _someObject_ = it.next();
        _someObject_.add3();       //made up method
    }

Now that I've done this, I decide I want to iterate through the entire set from start to finish once more to carry out another method on them, given that I've just made changes to each element through my first iteration.

Can I just use

while(it.hasNext())

again?

I thought maybe the iterator has a pointer so once it's reached the last element on the first run through, if I call while(it.hasNext()) again, the pointer would still be at the end of the set, meaning my second while loop would be pointless.

Why don't you write a simple test and you can feel the satisfaction of figuring out if this works by yourself. — Buhb
– Buhb, Commented May 1, 2011 at 14:18

ditkin · Accepted Answer · 2011-05-01 14:13:03Z

15

No, you can not do this. Just ask the Iterable object for another iterator.

answered May 1, 2011 at 14:13

ditkin

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1 Comment

Gressie Over a year ago

Of course you can't assume that the order is still the same if the underlying Set implementation doesn't enforce strict order. So you would be taking a risk assuming two calls for an Iterator will give you the same order for the content. ;)

Gressie · Accepted Answer · 2011-05-01 14:20:38Z

7

Sorry, but the contract of an Iterator states that you can only go through it once.

Given that you can't know for sure the order of the next Iterator, or even the content for that matter (it might change!), you're better off doing all the modifications or method calls in the same block.

answered May 1, 2011 at 14:20

Gressie

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Comments

shyam · Accepted Answer · 2011-05-01 14:13:43Z

2

Iterators are single use only.

answered May 1, 2011 at 14:13

shyam

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Comments

corlettk · Accepted Answer · 2011-05-01 14:14:33Z

0

As I guess you've discovered Iterator has no "reset" method... so you just grab ANOTHER Iterator. Simple.

answered May 1, 2011 at 14:14

corlettk

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Wim · Accepted Answer · 2011-05-01 14:14:36Z

0

it.hasNext() is false after the loop, this is exactly why the while loop ends. If you do another while(it.hasNext()) afterwards it.hasNext() will still be false so this loop will exit immediately. You need a new iterator for your new loop.

answered May 1, 2011 at 14:14

Wim

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Comments

cabreracanal · Accepted Answer · 2011-05-01 14:42:27Z

0

It's true that iterators are single use only, but there is a class called ListIterator that allows you to do this by looking back on the list. An example would be:

ArrayList<String> list = new ArrayList<String>();
list.add("hey!");
list.add("how");
list.add("are");
list.add("you?");
ListIterator it = list.listIterator();
while(it.hasNext()){
   System.out.println(it.next() +" ");//you print all the elements
}
//Now you go back. You can encapsulate this while sentence into a void reset method.
while(it.hasPrevious()){
   it.previous();
}
//Now you check that you've reached the beginning of the list
while(it.hasNext()){
   System.out.println("back " + it.next() +" ");//you print all the elements
}

I think it could be useful for you that class.

answered May 1, 2011 at 14:42

cabreracanal

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1 Comment

cabreracanal Over a year ago

You can always create your own Iterator and customize it as you want.

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