1 files changed, 23 insertions, 0 deletions
diff --git a/indexinglanguage/src/main/java/com/yahoo/vespa/indexinglanguage/expressions/OutputExpression.java b/indexinglanguage/src/main/java/com/yahoo/vespa/indexinglanguage/expressions/OutputExpression.java
index 398c2751bd8..267fb6fc51b 100644
--- a/indexinglanguage/src/main/java/com/yahoo/vespa/indexinglanguage/expressions/OutputExpression.java
+++ b/indexinglanguage/src/main/java/com/yahoo/vespa/indexinglanguage/expressions/OutputExpression.java
@@ -2,6 +2,11 @@
 package com.yahoo.vespa.indexinglanguage.expressions;
 
 import com.yahoo.document.DataType;
+import com.yahoo.vespa.objects.ObjectOperation;
+import com.yahoo.vespa.objects.ObjectPredicate;
+
+import java.util.ArrayList;
+import java.util.List;
 
 /**
  * @author Simon Thoresen Hult
@@ -58,4 +63,22 @@ public abstract class OutputExpression extends Expression {
         return getClass().hashCode() + (fieldName != null ? fieldName.hashCode() : 0);
     }
 
+    public static class OutputFieldNameExtractor implements ObjectOperation, ObjectPredicate {
+
+        private final List<String> outputFieldNames = new ArrayList<>(1);
+
+        public List<String> getOutputFieldNames() { return outputFieldNames; }
+
+        @Override
+        public void execute(Object obj) {
+            outputFieldNames.add(((OutputExpression) obj).getFieldName());
+        }
+
+        @Override
+        public boolean check(Object obj) {
+            return obj instanceof OutputExpression;
+        }
+
+    }
+
 }