Merge pull request #16980 from vespa-engine/mpolden/grouping-by

Add groupingBy to AbstractFilteringList

1 files changed, 11 insertions, 0 deletions

diff --git a/vespajlib/src/main/java/com/yahoo/collections/AbstractFilteringList.java b/vespajlib/src/main/java/com/yahoo/collections/AbstractFilteringList.java
index 4a24cdcc7bf..ec85cb6de56 100644
--- a/vespajlib/src/main/java/com/yahoo/collections/AbstractFilteringList.java
+++ b/vespajlib/src/main/java/com/yahoo/collections/AbstractFilteringList.java
@@ -5,15 +5,18 @@ import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.Comparator;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Iterator;
 import java.util.List;
+import java.util.Map;
 import java.util.Optional;
 import java.util.Random;
 import java.util.Set;
 import java.util.function.BiFunction;
 import java.util.function.Function;
 import java.util.function.Predicate;
+import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
 import static java.util.stream.Collectors.toUnmodifiableList;
@@ -86,6 +89,14 @@ public abstract class AbstractFilteringList<Type, ListType extends AbstractFilte
         return constructor.apply(items.stream().sorted(comparator).collect(toUnmodifiableList()), false);
     }
 
+    /** Returns the items grouped by the given classifier. */
+    public final <OtherType> Map<OtherType, ListType> groupingBy(Function<Type, OtherType> classifier) {
+        return items.stream().collect(Collectors.groupingBy(classifier,
+                                                            HashMap::new,
+                                                            Collectors.collectingAndThen(toUnmodifiableList(),
+                                                                                         (list) -> constructor.apply(list, false))));
+    }
+
     public final boolean isEmpty() { return items.isEmpty(); }
 
     public final int size() { return items.size(); }