Support getting a value or invoking a supplier if the value is not set

Author: hduelme

api/src/main/java/org/mapstruct/tools/gem/GemValue.java

@@ -8,6 +8,7 @@
 import java.util.ArrayList;
 import java.util.List;
 import java.util.function.Function;
+import java.util.function.Supplier;
 import javax.lang.model.element.AnnotationMirror;
 import javax.lang.model.element.AnnotationValue;
 import javax.lang.model.element.AnnotationValueVisitor;
@@ -113,6 +114,16 @@ public T getValue() {
         return value;
     }
 
+    /**
+     * Returns the value set by the user, if present, otherwise invoke other and return the result of that invocation.
+     *
+     * @param other a Supplier whose result is returned if no value is present
+     * @return the value  set by the user, if present, otherwise the result of {@code other.get()}
+     */
+    public T getValueOrElseGet(Supplier<T> other) {
+        return value != null ? value : other.get();
+    }
+
     /**
      * The default value, as declared in the annotation
      *