on Oct 23rd, 2009Tip: Computing the difference of two collections

Sometimes you have two collections and want to know how they differ. It would also be useful to have a series of steps that transform collection ‘A’ into collection ‘B’ (or the reverse).

private static List list1 = Arrays.asList("a", "b", "c");
private static List list2 = Arrays.asList("a", "c", "d");
 
// Diff of list1 vs list2:
//  removed 'b' at 1
//  added 'd' at 2

With a little help from the class Diffs (found in the org.eclipse.core.databinding.observable bundle / package), it only takes a few lines, as shown in the snippet below. Thank you, Eclipse Databinding!

If you don’t see the snippet click here.

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Tags: databinding, tips

4 responses so far Elias Volanakis syndicate

4 Responses to “Tip: Computing the difference of two collections”

1. Boris Bokowski says:

   October 23, 2009 at 2:34 am

   And, thank you EMF, where the code for computing the difference originally came from!

   Boris
2. ekke says:

   October 23, 2009 at 7:22 am

   so: thanks EMF

   ekke
3. Matthew Hall says:

   October 23, 2009 at 7:35 pm

   You should also check out ListDiff.accept(ListDiffVisitor) which interprets composite changes (i.e. related adds and removes) as moves or replacements.

   ListDiff diff = Diffs.computeListDiff(before, after);
   diff.accept(new ListDiffVisitor() {
       public void handleAdd(int index, Object element) {
         System.out.println(String.format("\tadded '%s' at %d", element, index));
       }
       public void handleRemove(int index, Object element) {
         System.out.println(String.format("\tremoved '%s' at %d", element, index));
       }
       public void handleMove(int oldIndex, int newIndex, Object element) {
         System.out.println(String.format("\tmoved '%s' from %d to %d", element, oldIndex, newIndex));
       }
       public void handleReplace(int index, Object oldElement, Object newElement) {
         System.out.println(String.format("\treplaced '%s' with '%s' at %d", oldElement, newElement, index));
       }
     });
   
4. Elias Volanakis says:

   October 27, 2009 at 12:58 am

   Thanks everybody for the comments and snippet.