diff --git a/.jules/bolt.md b/.jules/bolt.md
index 39f32f6e..3340bbeb 100644
--- a/.jules/bolt.md
+++ b/.jules/bolt.md
@@ -1,3 +1,7 @@
 ## 2024-06-21 - Regex Compilation in Loops
 **Learning:** In Kotlin, compiling regular expressions (`.toRegex()`) inside a loop over files is a significant O(N * M) performance bottleneck when processing ignore files (N files * M rules).
 **Action:** Always map string rules to compiled `Regex` objects outside of the file iteration loop (O(M) compilation) to avoid unnecessary regex re-compilations.
+
+## 2024-06-28 - String Concatenation in Loops
+**Learning:** In `html4tree`, using `+=` string concatenation inside a loop over directory files causes unnecessary intermediate object creation and memory overhead, resulting in O(N^2) complexity. The Jacoco plugin configuration treats the implicit null check on `.listFiles()?.toMutableList()` as a partially missed branch, causing test coverage issues.
+**Action:** Use `StringBuilder` for loop string building (O(N)), and split `curr_dir.listFiles()` into a clear `if (files != null)` check to satisfy Jacoco branch coverage to maintain 100%.
diff --git a/src/main/kotlin/html4tree/main.kt b/src/main/kotlin/html4tree/main.kt
index 50b2680d..8f58bc43 100644
--- a/src/main/kotlin/html4tree/main.kt
+++ b/src/main/kotlin/html4tree/main.kt
@@ -124,18 +124,20 @@ fun process_dir(curr_dir: File){
 """ 
 
     val index_middle = fun():String{ 
-        var l=""
+        // 성능 향상: 반복문 내 문자열 연결(concatenation)을 StringBuilder로 변경하여 메모리 할당 및 복사 오버헤드 감소 (O(N^2) -> O(N))
+        val sb = StringBuilder()
 
-        val dir_files: MutableList<File> = curr_dir.listFiles()?.toMutableList() ?: mutableListOf()
+        val files = curr_dir.listFiles()
+        val dir_files: MutableList<File> = if (files != null) files.toMutableList() else mutableListOf()
         dir_files.sortWith(compareBy ({it.name}) )
         dir_files.forEach {
            val isLinkedDirectory = it.isDirectory() && !java.nio.file.Files.isSymbolicLink(it.toPath())
            if((it.getName() !in exclude) && (isLinkedDirectory || !it.isDirectory())) {
-              l += """          <li><a style="display:block; width:100%" href="${if (isLinkedDirectory) { "./${it.getName().urlEncodePath()}/" } else { "./${it.getName().urlEncodePath()}" }}">${if (isLinkedDirectory) { "&#128193;" } else { "&rtrif;" }} ${it.getName().escapeHtml()}</a></li>"""+"\n"
+              sb.append("""          <li><a style="display:block; width:100%" href="${if (isLinkedDirectory) { "./${it.getName().urlEncodePath()}/" } else { "./${it.getName().urlEncodePath()}" }}">${if (isLinkedDirectory) { "&#128193;" } else { "&rtrif;" }} ${it.getName().escapeHtml()}</a></li>""").append("\n")
            }
         }
 
-        return l;
+        return sb.toString()
      } 
 
    val index_bottom="""
diff --git a/src/test/kotlin/html4tree/MainTest2.kt b/src/test/kotlin/html4tree/MainTest2.kt
new file mode 100644
index 00000000..6cb016ec
--- /dev/null
+++ b/src/test/kotlin/html4tree/MainTest2.kt
@@ -0,0 +1,40 @@
+package html4tree
+
+import org.junit.After
+import org.junit.Before
+import org.junit.Test
+import java.io.File
+import java.nio.file.Files
+import kotlin.test.assertTrue
+
+class MainTest2 {
+    private lateinit var tempDir: File
+
+    @Before
+    fun setup() {
+        tempDir = Files.createTempDirectory("html4tree-test2-").toFile()
+    }
+
+    @After
+    fun teardown() {
+        if (tempDir.exists()) {
+            tempDir.deleteRecursively()
+        }
+    }
+
+    @Test
+    fun testProcessDirListFilesNull() {
+        val unreadableDir = File(tempDir, "unreadable2")
+        unreadableDir.mkdir()
+        unreadableDir.setReadable(false, false)
+        try {
+            process_dir(unreadableDir)
+            val indexFile = File(unreadableDir, "index.html")
+            assertTrue(indexFile.exists())
+        } finally {
+            unreadableDir.setReadable(true, false)
+            unreadableDir.setWritable(true, false)
+            unreadableDir.setExecutable(true, false)
+        }
+    }
+}