Java Unique List - Revision history

Brianegge at 22:42, 10 December 2011

2011-12-10T22:42:02Z

← Older revision		Revision as of 22:42, 10 December 2011
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	The Java collections bundled with the JDK do not provide a built in method to get the unique items from a list. Here's an easy an efficient way to get the unique items. Simply put your '''list''' into a '''set''' and then create a '''list''' again.		The Java collections bundled with the JDK do not provide a built in method to get the unique items from a list. Here's an easy an efficient way to get the unique items. Simply put your '''list''' into a '''set''' and then create a '''list''' again.

	<~~geshi~~ lang="java5">		<syntaxhighlight lang="java5">
	ArrayList list = new ArrayList();		ArrayList list = new ArrayList();
	list.add("A");		list.add("A");
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	ArrayList b = new ArrayList(set);		ArrayList b = new ArrayList(set);
	assertEquals(2, b.size());		assertEquals(2, b.size());
	</~~geshi~~>		</syntaxhighlight>

	Why does this work? The HashSet constructor takes in a collection, and after creating setting up a map, adds all of the elements.		Why does this work? The HashSet constructor takes in a collection, and after creating setting up a map, adds all of the elements.
	<~~geshi~~ lang="java5">		<syntaxhighlight lang="java5">
	public HashSet(Collection<? extends E> c) {		public HashSet(Collection<? extends E> c) {
	map = new HashMap<E,Object>(Math.max((int) (c.size()/.75f) + 1, 16));		map = new HashMap<E,Object>(Math.max((int) (c.size()/.75f) + 1, 16));
	addAll(c);		addAll(c);
	}		}
	</~~geshi~~>		</syntaxhighlight>

	'''addAll''' in turn attempts to add each item to the map. If the item already exists, it will be ignored. This requires the objects you are adding to have proper '''equals''' and '''hashCode''' implementations.		'''addAll''' in turn attempts to add each item to the map. If the item already exists, it will be ignored. This requires the objects you are adding to have proper '''equals''' and '''hashCode''' implementations.

Brianegge at 11:45, 13 July 2009

2009-07-13T11:45:59Z

← Older revision		Revision as of 11:45, 13 July 2009
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	Similarly, the ArrayList constructor takes in a collection, and adds the items it it's internal array. The downside of this method is that the order of the items in the unique list will not be the same as those in the original. If maintaining order is important, simply use a '''LinkedHashSet''' instead of a HashSet, or if you want the items sorted, use a '''TreeSet''' instead of the HashSet.		Similarly, the ArrayList constructor takes in a collection, and adds the items it it's internal array. The downside of this method is that the order of the items in the unique list will not be the same as those in the original. If maintaining order is important, simply use a '''LinkedHashSet''' instead of a HashSet, or if you want the items sorted, use a '''TreeSet''' instead of the HashSet.

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	[[Category:Java\|unique list, Java]]		[[Category:Java\|unique list, Java]]

Egge at 23:05, 2 September 2008

2008-09-02T23:05:11Z

@@ Line 10: / Line 10: @@
 assertEquals(2, b.size());
 </geshi>
 [[Category:Java|unique list, Java]]

Egge at 03:53, 1 September 2008

2008-09-01T03:53:26Z

← Older revision		Revision as of 03:53, 1 September 2008
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	The ~~built in~~ Java collections do not provide a built in method to get the unique items from a list. Here's an easy an efficient way to get the unique items. Simply put your '''list''' into a '''set''' and then create a '''list''' again.		The Java collections bundled with the JDK do not provide a built in method to get the unique items from a list. Here's an easy an efficient way to get the unique items. Simply put your '''list''' into a '''set''' and then create a '''list''' again.

	<geshi lang="java5">		<geshi lang="java5">

Egge: New page: The built in Java collections do not provide a built in method to get the unique items from a list. Here's an easy an efficient way to get the unique items. Simply put your '''list''' in...

2007-10-03T07:32:27Z

New page: The built in Java collections do not provide a built in method to get the unique items from a list. Here's an easy an efficient way to get the unique items. Simply put your '''list''' in...

New page

The built in Java collections do not provide a built in method to get the unique items from a list. Here's an easy an efficient way to get the unique items. Simply put your '''list''' into a '''set''' and then create a '''list''' again.

<geshi lang="java5">
ArrayList list = new ArrayList();
list.add("A");
list.add("B");
list.add("A");
Set set = new HashSet(list);
ArrayList b = new ArrayList(set);
assertEquals(2, b.size());
</geshi>

[[Category:Java|unique list, Java]]