#128 Raising an error in case an enum constant is mapped more than once

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

@@ -26,6 +26,7 @@ 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.Element;
 import javax.lang.model.element.ExecutableElement;
@@ -770,7 +771,20 @@ public class MapperCreationProcessor implements ModelElementProcessor<List<Sourc
                 enumMappings.add( new EnumMapping( enumConstant, mappedConstants.iterator().next().getTargetName() ) );
             }
             else {
-                //TODO Raise error
+                List<String> targetConstants = new ArrayList<String>( mappedConstants.size() );
+                for ( Mapping mapping : mappedConstants ) {
+                    targetConstants.add( mapping.getTargetName() );
+                }
+                messager.printMessage(
+                    Kind.ERROR,
+                    String.format(
+                        "One enum constant must not be mapped to more than one target constant, but constant %s is " +
+                                "mapped to %s.",
+                        enumConstant,
+                        Strings.join( targetConstants, ", " )
+                    ),
+                    method.getExecutable()
+                );
             }
 
         }

processor/src/test/java/org/mapstruct/ap/test/enums/EnumMappingTest.java

@@ -18,9 +18,14 @@
  */
 package org.mapstruct.ap.test.enums;
 
+import javax.tools.Diagnostic.Kind;
+
 import org.mapstruct.ap.testutil.IssueKey;
 import org.mapstruct.ap.testutil.MapperTestBase;
 import org.mapstruct.ap.testutil.WithClasses;
+import org.mapstruct.ap.testutil.compilation.annotation.CompilationResult;
+import org.mapstruct.ap.testutil.compilation.annotation.Diagnostic;
+import org.mapstruct.ap.testutil.compilation.annotation.ExpectedCompilationOutcome;
 import org.testng.annotations.Test;
 
 import static org.fest.assertions.Assertions.assertThat;
@@ -64,4 +69,20 @@ public class EnumMappingTest extends MapperTestBase {
         assertThat( orderDto ).isNotNull();
         assertThat( orderDto.getOrderType() ).isEqualTo( ExternalOrderType.SPECIAL );
     }
+
+    @Test
+    @WithClasses(ErroneousOrderMapperMappingSameConstantTwice.class)
+    @ExpectedCompilationOutcome(
+        value = CompilationResult.FAILED,
+        diagnostics = {
+            @Diagnostic(type = ErroneousOrderMapperMappingSameConstantTwice.class,
+                kind = Kind.ERROR,
+                line = 39,
+                messageRegExp = "One enum constant must not be mapped to more than one target constant, but " +
+                    "constant EXTRA is mapped to SPECIAL, DEFAULT\\.")
+        }
+    )
+    public void shouldRaiseErrorIfSameSourceEnumConstantIsMappedTwice() {
+
+    }
 }

processor/src/test/java/org/mapstruct/ap/test/enums/ErroneousOrderMapperMappingSameConstantTwice.java

@@ -0,0 +1,41 @@
+/**
+ *  Copyright 2012-2014 Gunnar Morling (http://www.gunnarmorling.de/)
+ *  and/or other contributors as indicated by the @authors tag. See the
+ *  copyright.txt file in the distribution for a full listing of all
+ *  contributors.
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+package org.mapstruct.ap.test.enums;
+
+import org.mapstruct.Mapper;
+import org.mapstruct.Mapping;
+import org.mapstruct.Mappings;
+import org.mapstruct.factory.Mappers;
+
+/**
+ * @author Gunnar Morling
+ */
+@Mapper
+public interface ErroneousOrderMapperMappingSameConstantTwice {
+
+    ErroneousOrderMapperMappingSameConstantTwice INSTANCE = Mappers.getMapper(
+        ErroneousOrderMapperMappingSameConstantTwice.class
+    );
+
+    @Mappings({
+        @Mapping(source = "EXTRA", target = "SPECIAL"),
+        @Mapping(source = "EXTRA", target = "DEFAULT"),
+    })
+    ExternalOrderType orderTypeToExternalOrderType(OrderType orderType);
+}