Add FileFilterTest unit tests

Introduce unit tests to validate file and directory listing using FileFilter. Tests include anonymous inner class, expression lambda, block lambda, and a pre-assigned variable for filtering directories. Ensure the tests cover multiple approach for better code reliability.

src/test/java/lambdas/FileFilterTest.java

@@ -0,0 +1,55 @@
+package lambdas;
+
+import org.junit.jupiter.api.Test;
+
+import java.io.File;
+import java.io.FileFilter;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class FileFilterTest {
+    private final File root = new File("src/main/java");
+
+    @Test
+    void listFiles() {
+        File[] files = root.listFiles();
+        assert files != null;
+        for (File file : files) {
+            System.out.println(file);
+        }
+        assertEquals(22, files.length);
+    }
+
+    @Test
+    void listDirectories_anonInnerClass() {
+        File[] directories = root.listFiles(new FileFilter() {
+            @Override
+            public boolean accept(File file) {
+                return file.isDirectory();
+            }
+        });
+        assertEquals(14, directories.length);
+    }
+
+    @Test
+    void listDirectories_expressionLambda() {
+        File[] directories = root.listFiles(file -> file.isDirectory());
+        assertEquals(14, directories.length);
+    }
+
+    @Test
+    void listDirectories_blockLambda() {
+        File[] directories = root.listFiles(file -> {
+            System.out.println("Checking: " + file);
+            return file.isDirectory();
+        });
+        assertEquals(14, directories.length);
+    }
+
+    @Test
+    void listDirectories_assignedToVariable() {
+        FileFilter filter = (File file) -> file.isDirectory();
+        File[] directories = root.listFiles(filter);
+        assertEquals(14, directories.length);
+    }
+}