spr-7728: empty collection conversion can return value not assignable to targetType

org.springframework.core/src/main/java/org/springframework/core/convert/support/CollectionToCollectionConverter.java

@@ -62,9 +62,6 @@ final class CollectionToCollectionConverter implements ConditionalGenericConvert
 			return null;
 		}
 		Collection<?> sourceCollection = (Collection<?>) source;
-		if (sourceCollection.isEmpty()) {
-			return sourceCollection;
-		}
 		Collection target = CollectionFactory.createCollection(targetType.getType(), sourceCollection.size());
 		for (Object sourceElement : sourceCollection) {
 			Object targetElement = this.conversionService.convert(sourceElement, sourceType.getElementTypeDescriptor().applyIndexedObject(sourceElement), targetType.getElementTypeDescriptor());

org.springframework.core/src/main/java/org/springframework/core/convert/support/MapToMapConverter.java

@@ -67,9 +67,6 @@ final class MapToMapConverter implements ConditionalGenericConverter {
 			return null;
 		}
 		Map<?, ?> sourceMap = (Map<?, ?>) source;
-		if (sourceMap.isEmpty()) {
-			return sourceMap;
-		}
 		Map targetMap = CollectionFactory.createMap(targetType.getType(), sourceMap.size());
 		for (Object entry : sourceMap.entrySet()) {
 			Map.Entry sourceMapEntry = (Map.Entry) entry;

org.springframework.core/src/test/java/org/springframework/core/convert/support/GenericConversionServiceTests.java

@@ -399,6 +399,19 @@ public class GenericConversionServiceTests {
 
 	public List<Integer> emptyListTarget;
 
+	@Test
+	public void emptyListToListDifferentTargetType() throws Exception {
+		conversionService.addConverter(new CollectionToCollectionConverter(conversionService));
+		conversionService.addConverterFactory(new StringToNumberConverterFactory());
+		List<String> list = new ArrayList<String>();
+		TypeDescriptor sourceType = TypeDescriptor.forObject(list);
+		TypeDescriptor targetType = new TypeDescriptor(getClass().getField("emptyListDifferentTarget"));
+		assertTrue(conversionService.canConvert(sourceType, targetType));
+		assertEquals(LinkedList.class, conversionService.convert(list, sourceType, targetType).getClass());
+	}
+
+	public LinkedList<Integer> emptyListDifferentTarget;
+
 	@Test
 	public void emptyListToArray() throws Exception {
 		conversionService.addConverter(new CollectionToArrayConverter(conversionService));
@@ -434,6 +447,19 @@ public class GenericConversionServiceTests {
 
 	public Map<String, String> emptyMapTarget;
 
+	@Test
+	public void emptyMapToMapDifferentTargetType() throws Exception {
+		conversionService.addConverter(new MapToMapConverter(conversionService));
+		conversionService.addConverterFactory(new StringToNumberConverterFactory());
+		Map<String, String> map = new HashMap<String, String>();
+		TypeDescriptor sourceType = TypeDescriptor.forObject(map);
+		TypeDescriptor targetType = new TypeDescriptor(getClass().getField("emptyMapDifferentTarget"));
+		assertTrue(conversionService.canConvert(sourceType, targetType));
+		assertEquals(map, conversionService.convert(map, sourceType, targetType));
+	}
+
+	public Map<String, String> emptyMapDifferentTarget;
+	
 	private interface MyBaseInterface {
 
 	}

org.springframework.core/src/test/java/org/springframework/core/convert/support/Spr7728Tests.java

@@ -0,0 +1,52 @@
+package org.springframework.core.convert.support;
+
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Vector;
+
+import org.junit.Assert;
+import org.junit.Before;
+import org.junit.Test;
+import org.springframework.core.convert.TypeDescriptor;
+
+public class Spr7728Tests
+{
+    private CollectionToCollectionConverter theConverter;
+    private Vector<String> theSrcVector;
+    private TypeDescriptor theSrcType;
+    private TypeDescriptor theTargetType;
+
+    @Before
+    public void setup()
+    {
+        theSrcVector = new Vector<String>();
+        theSrcType = TypeDescriptor.forObject(theSrcVector);
+        theTargetType = TypeDescriptor.forObject(new ArrayList());
+        theConverter = new CollectionToCollectionConverter(new GenericConversionService());
+    }
+
+    @Test
+    public void convertEmptyVector_shouldReturnEmptyArrayList()
+        throws Exception
+    {
+        theSrcVector.add("Element");
+
+        testCollectionConversionToArrayList(theSrcVector);
+    }
+
+    @Test
+    public void convertNonEmptyVector_shouldReturnNonEmptyArrayList()
+        throws Exception
+    {
+        testCollectionConversionToArrayList(theSrcVector);
+    }
+
+    private void testCollectionConversionToArrayList(Collection<String> aSource)
+    {
+        Object myConverted = theConverter.convert(aSource, theSrcType, theTargetType);
+
+        Assert.assertTrue(myConverted instanceof ArrayList<?>);
+        Assert.assertEquals(aSource.size(), ((ArrayList<?>) myConverted).size());
+    }
+
+}