Reflect collection changes on another

Tuesday, 18 September 2012 13:36 Written by Sébastien Dante Ursini

Composite Collection

Sometimes you need to reflect the changes you made on a collection in another collection providing a single unified view on a so called composite collection.
To achieve this result you can develop you own observer and replicate the changes on you target collection or you can use Apache Commons Collections .

Depending on you collection type you can use the following classes :

• CompositeCollection for Collection
• CompositeSet for Sets

Sample usage

Let's illustrate it using two sets called A and B and an unified collection called unifiedCollection containing the items added in A and B

public class CompositeCollectionTest {

	private final Integer ONE 	= 1;
	private final Integer TWO 	= 2;
	private final Integer THREE	= 3;
	
	@Test
	public void compositeCollection(){
		
		Set<Integer> setA = new HashSet<Integer>(); 
		Set<Integer> setB = new HashSet<Integer>();
				
		CompositeCollection unifiedSet = new CompositeCollection(new Set[]{setA,setB});
		
		setA.add(ONE);
		assertTrue (unifiedSet.contains(ONE));

		setA.add(TWO);
		assertTrue (unifiedSet.contains(TWO));
		
		setB.add(THREE);
		assertTrue (unifiedSet.contains(THREE));
	}
}

Apache Commons Collection is an essential effective and reliable library�for complex collection manipulation.

Sébastien Dante Ursini

Java/Finance Specialist
17 Years of experience in Java
22 Year in Banking/Finance
Based in Geneva/Switzerland