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POHow to implement iterator on nested collection in Java?
 

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  1. POHow to implement iterator on nested collection in Java?
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    <p>I have a nested collection with this representation <code>Collection&lt;Collection&lt;T&gt;&gt;</code>. I have implemented the Iterator on the class, but the next() method is not giving the right results. It is fetching only the first element of each list. Example <code>List&lt;List&lt;String&gt;&gt;</code> and values are <code>{"1","2"},{"3","4"},{"5","6"}</code>. The Complete layout of class.</p> <pre><code>import java.util.ArrayList; import java.util.Collection; import java.util.Iterator; import java.util.List; public class NestedCollectionIterator implements Iterator&lt;Object&gt; { private Collection&lt;? extends Collection&lt;? extends Object&gt;&gt; _collOfColl = null; private Iterator&lt;? extends Collection&lt;? extends Object&gt;&gt; itCollection = null; private Iterator&lt;? extends Object&gt; innerIterator = null; Object next = null; public NestedCollectionIterator( Collection&lt;? extends Collection&lt;? extends Object&gt;&gt; collofColl){ _collOfColl = collofColl; itCollection = _collOfColl.iterator(); } @Override public boolean hasNext() { if(itCollection.hasNext()){ innerIterator = itCollection.next().iterator(); if(innerIterator != null || innerIterator.hasNext()){ next = innerIterator.next(); return true; } } return false; } public Object next() { if(hasNext()){ Object obj = next; //Need some changes here. return obj; } return null; } @Override public void remove() {} } </code></pre> <p>Class to test the implementation</p> <pre><code>class Sample{ public static void main(String[] args){ List&lt;List&lt;String&gt;&gt; Nestedlist = new ArrayList&lt;List&lt;String&gt;&gt;(); List&lt;String&gt; l = new ArrayList&lt;String&gt;(); l.add("1"); l.add("2"); Nestedlist.add(l); l = new ArrayList&lt;String&gt;(); l.add("3"); l.add("4"); Nestedlist.add(l); l = new ArrayList&lt;String&gt;(); l.add("5"); l.add("6"); Nestedlist.add(l); NestedCollectionIterator cc = new NestedCollectionIterator(Nestedlist); while(cc.hasNext()){ System.out.println(cc.next.toString()); } } } </code></pre> <p>the results is 1,3,5. How make the list iterate over all the elements in list first and then move to next collection item inside it?</p> <p>Thanks.</p>
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      1. POHow to implement iterator on nested collection in Java?
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    1. COWell currently your "implementation" of the `Iterator` interface is just going to say that you haven't got any elements. What have you tried, and what happened? ("is not giving the right results" is not enough information.)
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      1. POHow to implement iterator on nested collection in Java?
      1. USJon Skeet
    2. COHow many levels do you have? Just 1?
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      1. POHow to implement iterator on nested collection in Java?
      1. USAdam Arold
    3. COWell, `return null;` does not seem right, I agree. But what is the quesiton? `I tried to implement` Please try harder, as just adding some fields, but otherwise leaving the generated methods untouched as they are is not much of "implementing"...
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      1. POHow to implement iterator on nested collection in Java?
      1. USppeterka
 

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