support for common collection base type

Author: Jorg Heymans

pom.xml

@@ -158,6 +158,12 @@
 			<version>2.0M6</version>
 			<scope>test</scope>
 		</dependency>
+    <dependency>
+      <groupId>com.google.guava</groupId>
+      <artifactId>guava</artifactId>
+      <version>13.0.1</version>
+      <scope>test</scope>
+    </dependency>
 	</dependencies>
 
 	<distributionManagement>

src/main/java/de/danielbechler/diff/Instances.java

@@ -157,7 +157,20 @@ public Class<?> getType()
 		if (types.size() == 1)
 		{
 			return Collections.firstElementOf(types);
-		}
+		} 
+        if (types.size() > 1)
+        {
+            // check for common Collection base type
+            boolean areAllCollectionTypes = true;
+            for (Class<?> type : types) {
+                areAllCollectionTypes = Collection.class.isAssignableFrom(type);
+            }
+            if (areAllCollectionTypes)
+            {
+                return Collection.class;
+            }
+        }
+
 		throw new IllegalStateException("Detected instances of different types " + types + ". " +
 				"Instances must either be null or have the exact same type.");
 		// NOTE It would be nice to be able to define a least common denominator like Map or Collection to allow mixed types

src/test/java/de/danielbechler/diff/CollectionDifferTest.java

@@ -16,6 +16,8 @@
 
 package de.danielbechler.diff;
 
+import com.google.common.collect.ImmutableSet;
+import com.google.common.collect.Sets;
 import de.danielbechler.diff.mock.*;
 import de.danielbechler.diff.node.*;
 import de.danielbechler.diff.path.*;
@@ -127,4 +129,12 @@ public void testCompareWithChangedItem() throws Exception
 		final Node child = node.getChild(propertyPath);
 		assertThat(child.getState(), is(Node.State.CHANGED));
 	}
+    
+    @Test
+    public void testCollectionBaseType() throws Exception {
+        Set base = Sets.newHashSet("one", "two");
+        Set working = ImmutableSet.copyOf(Sets.newHashSet("one", "three"));
+        final CollectionNode node = differ.compare(base, working);
+        assertThat(node.hasChanges(), is(true));
+    }
 }