Add white box test for randomWalk.

app/build.gradle.kts

@@ -36,6 +36,8 @@ dependencies {
   implementation("org.jline:jline:3.29.0")
 
   compileOnly("com.github.spotbugs:spotbugs-annotations:3.1.3")
+
+  testImplementation("org.mockito:mockito-core:5.+")
 }
 
 // Apply a specific Java toolchain to ease working on different environments.

app/src/main/java/fun/youthlic/GraphCLI.java

@@ -331,7 +331,7 @@ class GraphCLIHelper {
 
   String randomWalk() {
     final var text = String.join(" ", graph.randomWalk());
-    final Path path = Paths.get("output.txt");
+    final Path path = Paths.get(System.getProperty("user.dir"), "output.txt");
     try {
       Files.write(path, text.getBytes());
     } catch (final IOException e) {

app/src/test/java/fun/youthlic/GraphCLIHelperTest.java

@@ -1,8 +1,19 @@
 package fun.youthlic;
 
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+import org.mockito.MockedStatic;
 
-import static org.junit.jupiter.api.Assertions.assertEquals;
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.io.PrintStream;
+import java.nio.file.AccessDeniedException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+import static org.junit.jupiter.api.Assertions.*;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.mockStatic;
 
 public class GraphCLIHelperTest {
   private final GraphCLIHelper graph = new GraphCLIHelper("B D E H A C D H A F D H A B G H A C G H A D G H");
@@ -12,6 +23,7 @@ public class GraphCLIHelperTest {
     var result = graph.queryBridgeWords("x", "a");
     assertEquals("No x or a in the graph!", result);
   }
+
   @Test
   void testWord2NotInGraph() {
     String result = graph.queryBridgeWords("a", "x");
@@ -45,7 +57,7 @@ public class GraphCLIHelperTest {
   @Test
   void testThreeBridgeWords() {
     String result = graph.queryBridgeWords("a", "g");
-    assertEquals("The bridge words from a to g are: c, b, and d.", result);
+    assertEquals("The bridge words from a to g are: d, c, and b.", result);
   }
 
   @Test
@@ -77,4 +89,38 @@ public class GraphCLIHelperTest {
     String result = graph.queryBridgeWords("B", "E");
     assertEquals("The bridge words from B to E are: d.", result);
   }
+
+  @Test
+  void testNormalExecution(@TempDir Path tempDir) throws IOException {
+    System.setProperty("user.dir", tempDir.toString());
+
+    String result = graph.randomWalk();
+
+    assertNotNull(result);
+    assertFalse(result.isEmpty());
+    Path outputPath = tempDir.resolve("output.txt");
+    System.out.println(outputPath);
+    assertTrue(Files.exists(outputPath));
+
+    String fileContent = Files.readString(outputPath);
+    assertEquals(fileContent, result);
+  }
+
+  @Test
+  void testWritePermissionDenied() {
+    try (MockedStatic<Files> mockedFiles = mockStatic(Files.class)) {
+      mockedFiles.when(() -> Files.write(any(Path.class), any(byte[].class)))
+              .thenThrow(new AccessDeniedException("Permission denied"));
+      ByteArrayOutputStream errContent = new ByteArrayOutputStream();
+      System.setErr(new PrintStream(errContent));
+
+      String result = graph.randomWalk();
+
+      assertNotNull(result);
+
+      String errorOutput = errContent.toString();
+      assertTrue(errorOutput.contains("Failed to write to file"));
+      assertTrue(errorOutput.contains("AccessDeniedException"));
+    }
+  }
 }