Cover restoring the tree expansion state and the viewer element map

tests/org.eclipse.jface.tests/src/org/eclipse/jface/tests/viewers/AllViewersTests.java

@@ -32,7 +32,8 @@
 		Bug256889TableViewerTest.class, Bug287765Test.class, Bug242231Test.class,
 		EditingSupportValueMismatchTest.class, StyledStringBuilderTest.class,
 		TreeViewerWithLimitTest.class, TreeViewerWithLimitCompatibilityTest.class, TableViewerWithLimitTest.class,
-		TableViewerWithLimitCompatibilityTest.class })
+		TableViewerWithLimitCompatibilityTest.class, TreeViewerExpansionStateTest.class,
+		StructuredViewerElementMapTest.class })
 public class AllViewersTests {
 
 }

tests/org.eclipse.jface.tests/src/org/eclipse/jface/tests/viewers/StructuredViewerElementMapTest.java

@@ -0,0 +1,205 @@
+/*******************************************************************************
+ * Copyright (c) 2026 Vogella GmbH and others.
+ *
+ * This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License 2.0
+ * which accompanies this distribution, and is available at
+ * https://www.eclipse.org/legal/epl-2.0/
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *
+ * Contributors:
+ *     Lars Vogel <Lars.Vogel@vogella.com> - initial API and implementation
+ *******************************************************************************/
+package org.eclipse.jface.tests.viewers;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertSame;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+import org.eclipse.jface.viewers.ArrayContentProvider;
+import org.eclipse.jface.viewers.IElementComparer;
+import org.eclipse.jface.viewers.LabelProvider;
+import org.eclipse.jface.viewers.TableViewer;
+import org.eclipse.swt.widgets.Shell;
+import org.eclipse.swt.widgets.Widget;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Tests the hash table a viewer keeps to map elements to their items, reached
+ * through the comparer, which is the only way a client controls how elements
+ * hash.
+ */
+public class StructuredViewerElementMapTest {
+
+	/**
+	 * The capacity a viewer's element map starts with. Slots are the hash modulo
+	 * this, so hashes differing by a multiple of it collide until the table grows.
+	 */
+	private static final int INITIAL_CAPACITY = 13;
+
+	private Shell shell;
+
+	private TableViewer viewer;
+
+	private static class Element {
+		final String name;
+		final int hash;
+
+		Element(String name, int hash) {
+			this.name = name;
+			this.hash = hash;
+		}
+
+		@Override
+		public String toString() {
+			return name;
+		}
+	}
+
+	/**
+	 * Takes the hash from the element, so a test can choose its slot, and compares
+	 * by name, so equal but distinct elements are recognized.
+	 */
+	private static final IElementComparer COMPARER = new IElementComparer() {
+
+		@Override
+		public boolean equals(Object a, Object b) {
+			if (a instanceof Element first && b instanceof Element second) {
+				return first.name.equals(second.name);
+			}
+			return a == null ? b == null : a.equals(b);
+		}
+
+		@Override
+		public int hashCode(Object element) {
+			return element instanceof Element typed ? typed.hash : element.hashCode();
+		}
+	};
+
+	@AfterEach
+	public void tearDown() {
+		if (shell != null) {
+			shell.dispose();
+			shell = null;
+		}
+		viewer = null;
+	}
+
+	private void createViewer(List<Element> input) {
+		shell = new Shell();
+		shell.setSize(500, 500);
+		viewer = new TableViewer(shell);
+		viewer.setComparer(COMPARER);
+		viewer.setUseHashlookup(true);
+		viewer.setContentProvider(ArrayContentProvider.getInstance());
+		viewer.setLabelProvider(new LabelProvider());
+		viewer.setInput(input);
+		shell.open();
+	}
+
+	private void assertMapsToItemFor(Element element) {
+		Widget item = viewer.testFindItem(element);
+		assertNotNull(item, "no item mapped for " + element);
+		assertSame(element, item.getData(), "wrong item mapped for " + element);
+	}
+
+	/**
+	 * Keys with identical hashes share a slot and have to be told apart by the
+	 * comparer.
+	 */
+	@Test
+	public void testFindsElementsWithIdenticalHashes() {
+		List<Element> input = List.of(new Element("a", 7), new Element("b", 7), new Element("c", 7));
+		createViewer(input);
+
+		for (Element element : input) {
+			assertMapsToItemFor(element);
+		}
+		assertNull(viewer.testFindItem(new Element("absent", 7)), "an absent element was found");
+	}
+
+	/**
+	 * Keys whose hashes differ but share a slot must not be confused.
+	 */
+	@Test
+	public void testFindsElementsWithCollidingSlots() {
+		List<Element> input = List.of(new Element("a", 1), new Element("b", 1 + INITIAL_CAPACITY),
+				new Element("c", 1 + 2 * INITIAL_CAPACITY));
+		createViewer(input);
+
+		for (Element element : input) {
+			assertMapsToItemFor(element);
+		}
+		// Same slot, no matching hash, and no matching name.
+		assertNull(viewer.testFindItem(new Element("absent", 1 + 3 * INITIAL_CAPACITY)),
+				"an absent element sharing a slot was found");
+	}
+
+	/**
+	 * Growing past the initial capacity redistributes every key, and all of them
+	 * have to stay findable.
+	 */
+	@Test
+	public void testFindsElementsAfterTableGrowth() {
+		List<Element> input = new ArrayList<>();
+		for (int i = 0; i < 200; i++) {
+			// Spread over the slots and force several rounds of growth.
+			input.add(new Element("e" + i, i * 31));
+		}
+		createViewer(input);
+
+		for (Element element : input) {
+			assertMapsToItemFor(element);
+		}
+	}
+
+	/**
+	 * After a refresh hands the viewer equal but distinct objects, the map has to
+	 * find them and the items have to hold the new ones.
+	 */
+	@Test
+	public void testFindsEqualButDistinctElementsAfterRefresh() {
+		List<Element> input = new ArrayList<>(
+				Arrays.asList(new Element("a", 3), new Element("b", 3), new Element("c", 9)));
+		createViewer(input);
+
+		List<Element> replacements = new ArrayList<>();
+		for (Element element : input) {
+			replacements.add(new Element(element.name, element.hash));
+		}
+		input.clear();
+		input.addAll(replacements);
+		viewer.refresh();
+
+		for (Element replacement : replacements) {
+			assertMapsToItemFor(replacement);
+		}
+		assertEquals(replacements.size(), viewer.getTable().getItemCount());
+	}
+
+	/**
+	 * A removed element must not be reachable through the map any more.
+	 */
+	@Test
+	public void testRemovedElementIsNoLongerFound() {
+		List<Element> input = new ArrayList<>(
+				Arrays.asList(new Element("a", 5), new Element("b", 5), new Element("c", 5)));
+		createViewer(input);
+
+		Element removed = input.remove(1);
+		viewer.remove(removed);
+
+		assertNull(viewer.testFindItem(removed), "the removed element is still mapped");
+		for (Element element : input) {
+			assertMapsToItemFor(element);
+		}
+	}
+
+}