Make sure that we handle methods with less than 3 characters correctly

Fixes #10

src/main/java/org/mapstruct/intellij/util/MapstructUtil.java

@@ -116,7 +116,14 @@ public static String getPropertyName(@NotNull PsiMethod method) {
     @NotNull
     @NonNls
     public static String getPropertyName(@NotNull String methodName) {
-        return Introspector.decapitalize( methodName.substring( methodName.startsWith( "is" ) ? 2 : 3 ) );
+        String name = "";
+        if ( methodName.startsWith( "is" ) ) {
+            name = methodName.substring( 2 );
+        }
+        else if ( methodName.length() > 2 ) {
+            name = methodName.substring( 3 );
+        }
+        return Introspector.decapitalize( name );
     }
 
     /**

src/test/java/org/mapstruct/intellij/MapstructMethodUsagesSearcherTest.java

@@ -72,6 +72,12 @@ public void testFindUsagesTargetReferenceMethod() {
             } );
     }
 
+    public void testIssue10Mapper() {
+        myFixture.configureByFiles( getTestName( false ) + ".java" );
+        Collection<UsageInfo> usages = myFixture.findUsages( myFixture.getElementAtCaret() );
+        assertThat( usages ).isEmpty();
+    }
+
     public void testFindUsagesForOnlyGetMethodOnSource() {
         myFixture.configureByFiles( "OnlyGetMethodOnSource.java" );
         Collection<UsageInfo> usages = myFixture.findUsages( myFixture.getElementAtCaret() );

testData/usages/Issue10Mapper.java

@@ -0,0 +1,59 @@
+/*
+ *  Copyright 2017 the MapStruct authors (http://www.mapstruct.org/)
+ *  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.intellij.test.examples;
+
+import org.mapstruct.Mapper;
+import org.mapstruct.Mapping;
+
+@Mapper
+public interface SimpleMapper {
+
+    @Mapping(source = "", target = "testName")
+    Target map(Source source);
+
+    class Source {
+
+        private String name;
+
+        public String getName() {
+            return name;
+        }
+
+        public void setName(String name) {
+            this.name = name;
+        }
+
+        public String ge<caret>() {
+            return null;
+        }
+    }
+
+    class Target {
+
+        private String testName;
+
+        public String getTestName() {
+            return testName;
+        }
+
+        public void setTestName(String testName) {
+            this.testName = testName;
+        }
+    }
+}