Issue #162 Adjustments

- Modified fMethodDescriptions to be final
- Changed from put to putIfAbsent

Author: Corey Vaillancourt

src/main/java/org/junit/runners/BlockJUnit4ClassRunner.java

@@ -49,7 +49,7 @@
  * @since 4.5
  */
 public class BlockJUnit4ClassRunner extends ParentRunner<FrameworkMethod> {
-    private ConcurrentHashMap<FrameworkMethod, Description> fMethodDescriptions = new ConcurrentHashMap<FrameworkMethod, Description>();
+    private final ConcurrentHashMap<FrameworkMethod, Description> fMethodDescriptions = new ConcurrentHashMap<FrameworkMethod, Description>();
     /**
      * Creates a BlockJUnit4ClassRunner to run {@code klass}
      *
@@ -80,7 +80,7 @@ protected Description describeChild(FrameworkMethod method) {
         if (description == null) {
             description = Description.createTestDescription(getTestClass().getJavaClass(),
                     testName(method), method.getAnnotations());
-            fMethodDescriptions.put(method, description);
+            fMethodDescriptions.putIfAbsent(method, description);
         }
 
         return description;