[jOOQ/jOOQ#12341] Add a Records::intoHierarchy Collector to turn Result into tree representations

jOOQ/src/main/java/org/jooq/Records.java

@@ -42,12 +42,16 @@ import static java.util.stream.Collectors.toCollection;
 
 import java.lang.reflect.Array;
 import java.util.ArrayList;
+import java.util.Collection;
 import java.util.LinkedHashMap;
 import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Map;
+import java.util.Map.Entry;
 import java.util.Set;
+import java.util.function.BiFunction;
 import java.util.function.Function;
+import java.util.function.Supplier;
 import java.util.stream.Collector;
 import java.util.stream.Collectors;
 
@@ -573,6 +577,109 @@ public final class Records {
         );
     }
 
+    /**
+     * Create a collector that can collect {@link Record} resulting from a
+     * {@link ResultQuery} into a hierarchy of custom data types.
+     * <p>
+     * For example:
+     * <p>
+     *
+     * <pre>
+     * <code>
+     * record File(String name, List&lt;File&gt; contents) {}
+     *
+     * List&lt;File&gt; files =
+     * ctx.select(FILE.ID, FILE.PARENT_ID, FILE.NAME)
+     *    .from(FILE)
+     *    .collect(intoHierarchy(
+     *        r -&gt; r.value1(),
+     *        r -&gt; r.value2(),
+     *        (r, l) -&gt; new File(r.value3(), l)
+     *    ));
+     * </code>
+     * </pre>
+     *
+     * @param <K> The key type (e.g. an <code>ID</code>)
+     * @param <E> The value type (e.g. a POJO)
+     * @param <R> The record type
+     * @param keyMapper A function that extract a key from a record
+     * @param parentKeyMapper A function that extracts the parent key from a
+     *            record.
+     * @param recordMapper A function that maps a record and a list of child
+     *            values to a new value type.
+     */
+    public static final <K, E, R extends Record> Collector<R, ?, List<E>> intoHierarchy(
+        Function<? super R, ? extends K> keyMapper,
+        Function<? super R, ? extends K> parentKeyMapper,
+        BiFunction<? super R, ? super List<E>, ? extends E> recordMapper
+    ) {
+        return intoHierarchy(keyMapper, parentKeyMapper, recordMapper, ArrayList::new);
+    }
+
+    /**
+     * Create a collector that can collect {@link Record} resulting from a
+     * {@link ResultQuery} into a hierarchy of custom data types.
+     * <p>
+     * For example:
+     * <p>
+     *
+     * <pre>
+     * <code>
+     * record File(String name, List&lt;File&gt; contents) {}
+     *
+     * List&lt;File&gt; files =
+     * ctx.select(FILE.ID, FILE.PARENT_ID, FILE.NAME)
+     *    .from(FILE)
+     *    .collect(intoHierarchy(
+     *        r -&gt; r.value1(),
+     *        r -&gt; r.value2(),
+     *        (r, l) -&gt; new File(r.value3(), l),
+     *        ArrayList::new
+     *    ));
+     * </code>
+     * </pre>
+     *
+     * @param <K> The key type (e.g. an <code>ID</code>)
+     * @param <E> The value type (e.g. a POJO)
+     * @param <R> The record type
+     * @param keyMapper A function that extract a key from a record
+     * @param parentKeyMapper A function that extracts the parent key from a
+     *            record.
+     * @param collectionFactory A supplier for new child value collections.
+     * @param recordMapper A function that maps a record and a list of child
+     *            values to a new value type.
+     */
+    public static final <K, E, C extends Collection<E>, R extends Record> Collector<R, ?, List<E>> intoHierarchy(
+        Function<? super R, ? extends K> keyMapper,
+        Function<? super R, ? extends K> parentKeyMapper,
+        BiFunction<? super R, ? super C, ? extends E> recordMapper,
+        Supplier<? extends C> collectionFactory
+    ) {
+        return collectingAndThen(
+            intoMap(keyMapper, r -> {
+                C e = collectionFactory.get();
+                return new Tuple3<R, C, E>(r, e, recordMapper.apply(r, e));
+            }),
+            m -> {
+                List<E> r = new ArrayList<>();
+
+                m.forEach((k, v) -> {
+                    K parent = parentKeyMapper.apply(v.t1());
+                    E child = v.t3();
+
+                    if (m.containsKey(parent))
+                        m.get(parent).t2().add(child);
+                    else
+                        r.add(child);
+                });
+
+                return r;
+            }
+        );
+    }
+
+    private static final record Tuple3<T1, T2, T3>(T1 t1, T2 t2, T3 t3) {}
+
 
 
     /**