From f3d95e908bdf9b5c7a2280db422a518e73fae4c8 Mon Sep 17 00:00:00 2001
From: qibinhang <qibh@nus.edu.sg>
Date: Fri, 8 Aug 2025 13:20:08 +0800
Subject: [PATCH] Add tests for covering additional, important scenarios of
 `FileUtil`.

---
 .../java/cn/hutool/core/io/FileUtilTest.java  | 38 +++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/hutool-core/src/test/java/cn/hutool/core/io/FileUtilTest.java b/hutool-core/src/test/java/cn/hutool/core/io/FileUtilTest.java
index 056a209b3..8405af6ca 100644
--- a/hutool-core/src/test/java/cn/hutool/core/io/FileUtilTest.java
+++ b/hutool-core/src/test/java/cn/hutool/core/io/FileUtilTest.java
@@ -528,6 +528,44 @@ public class FileUtilTest {
 		assertTrue(FileUtil.isSub(file, file2));
 	}
 
+	@Test
+	public void isSub_SubIsAncestorOfParentTest() {
+		File parent = new File("d:/home/user/docs/notes");
+		File sub = new File("d:/home/user/docs");
+		assertFalse(FileUtil.isSub(parent, sub));
+	}
+
+	@Test
+	public void isSub_SamePathTest() {
+		File parent = new File("d:/home/user/docs");
+		File sub = new File("d:/home/user/docs");
+		assertTrue(FileUtil.isSub(parent, sub));
+	}
+
+	@Test
+	public void isSub_NonexistentPathsTest() {
+		File parent = new File("d:/unlikely/to/exist/parent");
+		File sub = new File("d:/unlikely/to/exist/parent/child/file.txt");
+		assertTrue(FileUtil.isSub(parent, sub));
+
+		File nonchild = new File("d:/also/unlikely/path.txt");
+		assertFalse(FileUtil.isSub(parent, nonchild));
+	}
+
+	@Test
+	public void isSub_NullParentTest() {
+		assertThrows(IllegalArgumentException.class, () -> {
+			FileUtil.isSub(null, new java.io.File("d:/any/path"));
+		});
+	}
+
+	@Test
+	public void isSub_NullSubTest() {
+		assertThrows(IllegalArgumentException.class, () -> {
+			FileUtil.isSub(new java.io.File("d:/any/path"), null);
+		});
+	}
+
 	@Test
 	@Disabled
 	public void appendLinesTest(){