Restored changes for MemoizeFunction.

Author: javadev

src/main/java/com/github/underscore/MemoizeFunction.java

@@ -9,8 +9,9 @@ public abstract class MemoizeFunction<F, T> implements Function<F, T> {
     public abstract T calc(final F n);
 
     public T apply(final F key) {
-        final T value = calc(key);
-        cache.putIfAbsent(key, value);
-        return value;
+        if (!cache.containsKey(key)) {
+            cache.put(key, calc(key));
+        }
+        return cache.get(key);
     }
 }