diff --git a/javatests/com/google/gerrit/server/patch/DiffOperationsTest.java b/javatests/com/google/gerrit/server/patch/DiffOperationsTest.java
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+++ b/javatests/com/google/gerrit/server/patch/DiffOperationsTest.java
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+// Copyright (C) 2021 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.server.patch;
+
+import static com.google.common.truth.Truth.assertThat;
+import static java.nio.charset.StandardCharsets.UTF_8;
+
+import com.google.common.collect.ImmutableMap;
+import com.google.gerrit.entities.Project;
+import com.google.gerrit.server.git.GitRepositoryManager;
+import com.google.gerrit.server.patch.filediff.FileDiffOutput;
+import com.google.gerrit.server.util.time.TimeUtil;
+import com.google.gerrit.testing.InMemoryModule;
+import com.google.inject.Guice;
+import com.google.inject.Inject;
+import com.google.inject.Injector;
+import java.io.IOException;
+import java.util.Map;
+import org.eclipse.jgit.lib.CommitBuilder;
+import org.eclipse.jgit.lib.Constants;
+import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.ObjectInserter;
+import org.eclipse.jgit.lib.ObjectReader;
+import org.eclipse.jgit.lib.PersonIdent;
+import org.eclipse.jgit.lib.Repository;
+import org.eclipse.jgit.lib.TreeFormatter;
+import org.eclipse.jgit.revwalk.RevWalk;
+import org.junit.Before;
+import org.junit.Test;
+
+/** Test class for diff related logic of {@link DiffOperations}. */
+public class DiffOperationsTest {
+  @Inject private GitRepositoryManager repoManager;
+  @Inject private DiffOperations diffOperations;
+
+  private static final Project.NameKey testProjectName = Project.nameKey("test-project");
+  private Repository repo;
+
+  private final String fileName1 = "file_1.txt";
+  private final String fileContent1 = "File content 1";
+  private final String fileName2 = "file_2.txt";
+  private final String fileContent2 = "File content 2";
+
+  @Before
+  public void setUpInjector() throws Exception {
+    Injector injector = Guice.createInjector(new InMemoryModule());
+    injector.injectMembers(this);
+    repo = repoManager.createRepository(testProjectName);
+  }
+
+  @Test
+  public void diffModifiedFileAgainstParent() throws Exception {
+    ImmutableMap<String, String> oldFiles =
+        ImmutableMap.of(fileName1, fileContent1, fileName2, fileContent2);
+    ObjectId oldCommitId = createCommit(repo, null, oldFiles);
+
+    ImmutableMap<String, String> newFiles =
+        ImmutableMap.of(fileName1, fileContent1, fileName2, fileContent2 + "\nnew line here");
+    ObjectId newCommitId = createCommit(repo, oldCommitId, newFiles);
+
+    FileDiffOutput diffOutput =
+        diffOperations.getModifiedFileAgainstParent(
+            testProjectName, newCommitId, /* parentNum=*/ null, fileName2, /* whitespace=*/ null);
+
+    assertThat(diffOutput.oldCommitId()).isEqualTo(oldCommitId);
+    assertThat(diffOutput.newCommitId()).isEqualTo(newCommitId);
+    assertThat(diffOutput.comparisonType().isAgainstParent()).isTrue();
+    assertThat(diffOutput.edits()).hasSize(1);
+  }
+
+  private ObjectId createCommit(
+      Repository repo, ObjectId parentCommit, ImmutableMap<String, String> fileNameToContent)
+      throws IOException {
+    ObjectId treeId = createTree(repo, fileNameToContent);
+    return createCommitInRepo(repo, treeId, parentCommit);
+  }
+
+  private static ObjectId createCommitInRepo(
+      Repository repo, ObjectId treeId, ObjectId parentCommit) throws IOException {
+    try (ObjectInserter oi = repo.newObjectInserter()) {
+      PersonIdent committer =
+          new PersonIdent(new PersonIdent("Foo Bar", "foo.bar@baz.com"), TimeUtil.nowTs());
+      CommitBuilder cb = new CommitBuilder();
+      cb.setTreeId(treeId);
+      cb.setCommitter(committer);
+      cb.setAuthor(committer);
+      cb.setMessage("Test commit");
+      if (parentCommit != null) {
+        cb.setParentIds(parentCommit);
+      }
+      ObjectId commitId = oi.insert(cb);
+      oi.flush();
+      oi.close();
+      return commitId;
+    }
+  }
+
+  private static ObjectId createTree(
+      Repository repo, ImmutableMap<String, String> fileNameToContent) throws IOException {
+    try (ObjectInserter oi = repo.newObjectInserter();
+        ObjectReader reader = repo.newObjectReader();
+        RevWalk rw = new RevWalk(reader); ) {
+      TreeFormatter formatter = new TreeFormatter();
+      for (Map.Entry<String, String> entry : fileNameToContent.entrySet()) {
+        String fileName = entry.getKey();
+        String fileContent = entry.getValue();
+        ObjectId fileObjId = createBlob(repo, fileContent);
+        formatter.append(fileName, rw.lookupBlob(fileObjId));
+      }
+      ObjectId treeId = oi.insert(formatter);
+      oi.flush();
+      oi.close();
+      return treeId;
+    }
+  }
+
+  private static ObjectId createBlob(Repository repo, String content) throws IOException {
+    try (ObjectInserter oi = repo.newObjectInserter()) {
+      ObjectId blobId = oi.insert(Constants.OBJ_BLOB, content.getBytes(UTF_8));
+      oi.flush();
+      oi.close();
+      return blobId;
+    }
+  }
+}