Added HasSelf util class

aksw-commons-utils-parent/aksw-commons-utils/src/main/java/org/aksw/commons/util/obj/HasSelf.java

@@ -0,0 +1,20 @@
+package org.aksw.commons.util.obj;
+
+/** This interface is useful for builders of the form below, in order to hide
+ * the uncheck cast warning of {@code return (X)this;}.
+ *
+ * <pre>{@code
+ * interface MyBuilderMixin<X extends MyBuilderMixin<X>> extends HasSelf<X> {
+ *   default X someMethod() {
+ *      // Do something
+ *      return self();
+ *   }
+ * }
+ * }</pre>
+ */
+public interface HasSelf<T> {
+    @SuppressWarnings("unchecked")
+    default T self() {
+        return (T)this;
+    }
+}