Retrieving Elements from Collection in Java (For-each, Iterator, ListIterator & EnumerationIterator)
Prerequisite: Collection in Java
Following are the 4 ways to retrieve any elements from a collection object:
For each loop is meant for traversing items in a collection.
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// Iterating over collection 'c' using for-each for (Element e: c) System.out.println(e);
We read the ‘:’ used in for-each loop as “in”. So loop reads as “for each element e in elements”, here elements are the collection which stores Element type items.
Note:In Java 8 using lambda expressions we can simply replace for-each loop with
elements.forEach (e -> System.out.println(e) );
Cursor is an interface and it is used to retrieve data from collection object, one by one. Cursor has 3 types, which are given below:
Iterator Interface: Iterator is an interface provided by collection framework to traverse a collection and for a sequential access of items in the collection.
// Iterating over collection 'c' using iterator for (Iterator i = c.iterator(); i.hasNext(); ) System.out.println(i.next());
- It has 3 methods:
- boolean hasNext(): This method returns true if the iterator has more elements.
- elements next(): This method returns the next elements in the iterator.
- void remove(): This method removes from the collection the last elements returned by the iterator.
- ListIterator Interface: It is an interface that contains methods to retrieve the elements from a collection object, both in forward and reverse directions. This iterator is for list based collections.
It has following important methods:
- booleanhasNext(): This returns true if the ListIterator has more elements when traversing the list in the forward direction.
- booleanhasPrevious(): This returns true if the ListIterator has more elements when traversing the list in the reverse direction.
- element next(): This returns the next element in the list.
- element previous():This returns the previous element in the list.
- void remove(): This removes from the list the last elements that was returned by the next() or previous() methods.
- int nextIndex() Returns the index of the element that would be returned by a subsequent call to next(). (Returns list size if the list iterator is at the end of the list.)
- int previousIndex() Returns the index of the element that would be returned by a subsequent call to previous(). (Returns -1 if the list iterator is at the beginning of the list.)
- EnumerationIterator Interface: The interface is useful to retrieve one by one the element. This iterator is based on data from Enumeration and has methods:
- booleanhasMoreElements(): This method tests if the Enumeration has any more elements or not .
- element nextElement(): This returns the next element that is available in elements that is available in Enumeration
Iterators in Java
Iterator vs Foreach In Java
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