Commited fix and test from cm-rudolph for JAVAMONEY-78.

Author: atsticks

src/main/java/org/javamoney/moneta/internal/DefaultMonetaryAmountsSingletonSpi.java

@@ -101,7 +101,9 @@ private static int getServicePriority(Object service){
      * @return the comparison result.
      */
     public static <T> int comparePriority(T service1, T service2){
-        return getServicePriority(service2) - getServicePriority(service1);
+        int servicePriority1 = getServicePriority(service1);
+        int servicePriority2 = getServicePriority(service2);
+        return servicePriority1 == servicePriority2 ? 0 : servicePriority1 < servicePriority2 ? 1 : -1;
     }
 
     /**

src/test/java/org/javamoney/moneta/internal/DefaultMonetaryAmountsSingletonSpiTest.java

@@ -0,0 +1,56 @@
+/**
+ * Copyright (c) 2012, 2014, Credit Suisse (Anatole Tresch), Werner Keil and others by the @author tag.
+ *
+ * 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.javamoney.moneta.internal;
+
+import org.javamoney.moneta.spi.ServicePriority;
+import org.testng.annotations.Test;
+
+import static org.testng.Assert.assertTrue;
+
+/**
+ * Tests for DefaultMonetaryAmountsSingletonSpi.
+ *
+ * @author cm-rudolph
+ */
+public class DefaultMonetaryAmountsSingletonSpiTest{
+
+    /**
+     * Test method for
+     * {@link org.javamoney.moneta.internal.DefaultMonetaryAmountsSingletonSpi#comparePriority(Object, Object)}
+     * .
+     */
+    @Test
+    public void testComparePriority(){
+        Object service1 = new Service1();
+        Object service2 = new Service2();
+        assertTrue(DefaultMonetaryAmountsSingletonSpi.comparePriority(service1, service2) > 0);
+        assertTrue(DefaultMonetaryAmountsSingletonSpi.comparePriority(service1, service1) == 0);
+        assertTrue(DefaultMonetaryAmountsSingletonSpi.comparePriority(service2, service1) < 0);
+        assertTrue(DefaultMonetaryAmountsSingletonSpi.comparePriority(null, service2) > 0);
+    }
+
+    /**
+     * Test class used, with prio 1.
+     */
+    @ServicePriority(1)
+    private static class Service1{}
+
+    /**
+     * Test class used with prio 2.
+     */
+    @ServicePriority(2)
+    private static class Service2{}
+}