#87 Supporting iterable mapping methods where the target element type is the same as or a super-type of the source element type; Raising an error if iterable or map mapping methods can't be generated

processor/src/main/java/org/mapstruct/ap/model/Type.java

@@ -188,6 +188,8 @@ public class Type extends AbstractModelElement implements Comparable<Type> {
      *
      * @return {@code true} if and only if this type is assignable to the given other type.
      */
+    // TODO This doesn't yet take wild card types into account; e.g. ? extends Integer wouldn't be assignable to Number
+    // atm.
     public boolean isAssignableTo(Type other) {
         if ( equals( other ) ) {
             return true;

processor/src/main/java/org/mapstruct/ap/processor/MapperCreationProcessor.java

@@ -27,6 +27,7 @@ import java.util.LinkedList;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
+
 import javax.annotation.processing.Messager;
 import javax.lang.model.element.ExecutableElement;
 import javax.lang.model.element.TypeElement;
@@ -490,9 +491,25 @@ public class MapperCreationProcessor implements ModelElementProcessor<List<Metho
             )
         );
 
+        MappingMethodReference elementMappingMethod =
+            getMappingMethodReference( methods, sourceElementType, targetElementType );
+
+        if ( !sourceElementType.isAssignableTo( targetElementType ) && conversion == null &&
+            elementMappingMethod == null ) {
+            messager.printMessage(
+                Kind.ERROR,
+                String.format(
+                    "Can't create implementation of method %s. Found no method nor built-in conversion for mapping "
+                    + "source element type into target element type.",
+                    method
+                ),
+                method.getExecutable()
+            );
+        }
+
         return new IterableMappingMethod(
             method,
-            getMappingMethodReference( methods, sourceElementType, targetElementType ),
+            elementMappingMethod,
             conversion
         );
     }
@@ -524,6 +541,31 @@ public class MapperCreationProcessor implements ModelElementProcessor<List<Metho
             targetValueType
         );
 
+        if ( !sourceKeyType.isAssignableTo( targetKeyType ) && keyConversion == null && keyMappingMethod == null ) {
+            messager.printMessage(
+                Kind.ERROR,
+                String.format(
+                    "Can't create implementation of method %s. Found no method nor built-in conversion for mapping "
+                    + "source key type to target key type.",
+                    method
+                ),
+                method.getExecutable()
+            );
+        }
+
+        if ( !sourceValueType.isAssignableTo( targetValueType ) && valueConversion == null &&
+            valueMappingMethod == null ) {
+            messager.printMessage(
+                Kind.ERROR,
+                String.format(
+                    "Can't create implementation of method %s. Found no method nor built-in conversion for mapping "
+                    + "source value type to target value type.",
+                    method
+                ),
+                method.getExecutable()
+            );
+        }
+
         return new MapMappingMethod( method, keyMappingMethod, keyConversion, valueMappingMethod, valueConversion );
     }
 

processor/src/main/resources/org.mapstruct.ap.model.IterableMappingMethod.ftl

@@ -34,7 +34,7 @@ public <@includeModel object=returnType/> ${name}(<#list parameters as param><@i
     for ( <@includeModel object=sourceParameter.type.typeParameters[0]/> ${loopVariableName} : ${sourceParameter.name} ) {
         <#if elementMappingMethod??>
         ${resultName}.add( <@includeModel object=elementMappingMethod input="${loopVariableName}"/> );
-        <#else>
+        <#elseif conversion??>
             <#if (conversion.exceptionTypes?size == 0) >
         ${resultName}.add( <@includeModel object=conversion/> );
             <#else>
@@ -47,6 +47,8 @@ public <@includeModel object=returnType/> ${name}(<#list parameters as param><@i
         }
                 </#list>
             </#if>
+        <#else>
+            ${resultName}.add( ${loopVariableName} );
         </#if>
     }
     <#if returnType.name != "void">

processor/src/test/java/org/mapstruct/ap/test/collection/CollectionMappingTest.java

@@ -24,6 +24,7 @@ import java.util.EnumSet;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
+import java.util.Set;
 
 import org.mapstruct.ap.testutil.IssueKey;
 import org.mapstruct.ap.testutil.MapperTestBase;
@@ -287,4 +288,14 @@ public class CollectionMappingTest extends MapperTestBase {
 
         assertThat( source.getStringLongMap() ).hasSize( 2 );
     }
+
+    @Test
+    @IssueKey("87")
+    public void shouldMapIntegerSetToNumberSet() {
+        Set<Number> numbers = SourceTargetMapper.INSTANCE
+            .integerSetToNumberSet( new HashSet<Integer>( Arrays.asList( 123, 456 ) ) );
+
+        assertThat( numbers ).isNotNull();
+        assertThat( numbers ).containsOnly( 123, 456 );
+    }
 }

processor/src/test/java/org/mapstruct/ap/test/collection/SourceTargetMapper.java

@@ -46,4 +46,6 @@ public interface SourceTargetMapper {
     Set<String> colourSetToStringSet(Set<Colour> colours);
 
     Set<Colour> stringSetToColourSet(Set<String> colours);
+
+    Set<Number> integerSetToNumberSet(Set<Integer> integers);
 }

processor/src/test/java/org/mapstruct/ap/test/collection/map/MapMappingTest.java

@@ -147,4 +147,19 @@ public class MapMappingTest extends MapperTestBase {
             entry( 121L, new GregorianCalendar( 2013, 6, 20 ).getTime() )
         );
     }
+
+    @Test
+    @IssueKey("87")
+    public void shouldCreateMapMethodImplementationWithoutConversionOrElementMappingMethod() {
+        Map<String, String> values = createStringStringMap();
+
+        Map<Object, Object> target = SourceTargetMapper.INSTANCE.stringStringMapToObjectObjectMap( values );
+
+        assertThat( target ).isNotNull();
+        assertThat( target ).hasSize( 2 );
+        assertThat( target ).includes(
+            entry( "42", "01.01.1980" ),
+            entry( "121", "20.07.2013" )
+        );
+    }
 }

processor/src/test/java/org/mapstruct/ap/test/collection/map/SourceTargetMapper.java

@@ -47,4 +47,6 @@ public interface SourceTargetMapper {
     Target sourceToTarget(Source source);
 
     Source targetToSource(Target target);
+
+    Map<Object, Object> stringStringMapToObjectObjectMap(Map<String, String> source);
 }