From 30900cb53c30c570593e805c3d24b3c56de648c7 Mon Sep 17 00:00:00 2001
From: Sergey Vinogradov <mr.vursen@gmail.com>
Date: Mon, 22 Sep 2025 15:30:06 +0400
Subject: [PATCH] refactor: throw when data provider returns more items than
 requested

---
 .../HierarchicalDataCommunicator.java         | 30 ++++++++++++-------
 1 file changed, 19 insertions(+), 11 deletions(-)

diff --git a/flow-data/src/main/java/com/vaadin/flow/data/provider/hierarchy/HierarchicalDataCommunicator.java b/flow-data/src/main/java/com/vaadin/flow/data/provider/hierarchy/HierarchicalDataCommunicator.java
index 1256b032da8..6b536110913 100644
--- a/flow-data/src/main/java/com/vaadin/flow/data/provider/hierarchy/HierarchicalDataCommunicator.java
+++ b/flow-data/src/main/java/com/vaadin/flow/data/provider/hierarchy/HierarchicalDataCommunicator.java
@@ -259,7 +259,8 @@ public void refresh(T item, boolean refreshChildren) {
 
     @Override
     public Stream<T> fetchFromProvider(int offset, int limit) {
-        return fetchDataProviderChildren(null, Range.withLength(offset, limit));
+        return fetchDataProviderChildren(null, Range.withLength(offset, limit))
+                .stream();
     }
 
     @Override
@@ -516,8 +517,7 @@ && isExpanded(item)) {
     private void preloadRange(Cache<T> cache, int start, int length) {
         var range = Range.withLength(start, length)
                 .restrictTo(Range.withLength(0, cache.getSize()));
-        var items = fetchDataProviderChildren(cache.getParentItem(), range)
-                .toList();
+        var items = fetchDataProviderChildren(cache.getParentItem(), range);
         cache.setItems(range.getStart(), items);
     }
 
@@ -751,16 +751,24 @@ public HierarchicalQuery<T, Object> buildQuery(T parent, int offset,
     }
 
     @SuppressWarnings("unchecked")
-    private Stream<T> fetchDataProviderChildren(T parent, Range range) {
+    private List<T> fetchDataProviderChildren(T parent, Range range) {
         var query = buildQuery(parent, range.getStart(), range.length());
 
-        return ((HierarchicalDataProvider<T, Object>) getDataProvider())
-                .fetchChildren(query).peek((item) -> {
-                    if (item == null) {
-                        throw new IllegalStateException(
-                                "Data provider returned a null item. Null values are not supported");
-                    }
-                });
+        var items = ((HierarchicalDataProvider<T, Object>) getDataProvider())
+                .fetchChildren(query).toList();
+        if (items.size() > range.length()) {
+            throw new IllegalStateException(
+                    """
+                            Data provider returned more items than requested. Requested %d but got %d. \
+                            Make sure that your data provider respects the offset and limit in HierarchicalQuery.
+                            """
+                            .formatted(range.length(), items.size()));
+        }
+        if (items.contains(null)) {
+            throw new IllegalStateException(
+                    "Data provider returned a null item. Null values are not supported");
+        }
+        return items;
     }
 
     @SuppressWarnings("unchecked")