Add FileFilterTest for lambda expressions

Created a new test class `FileFilterTest` to validate the correct functionality of file filters using anonymous inner classes and lambda expressions. This includes tests for listing all files and directories, ensuring the improved readability and simplicity of lambda expressions does not compromise the functionality.

src/test/java/lambdas/FileFilterTest.java

@@ -0,0 +1,54 @@
+package lambdas;
+
+import org.junit.jupiter.api.Test;
+
+import java.io.File;
+import java.io.FileFilter;
+
+import static org.assertj.core.api.Assertions.assertThat;
+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;
+        assertEquals(22, files.length);
+        assertThat(files.length).isEqualTo(22);
+    }
+
+    @Test
+    void listDirectories_anonInnerClass() {
+        File[] directories = root.listFiles(new FileFilter() {
+            @Override
+            public boolean accept(File pathname) {
+                return pathname.isDirectory();
+            }
+        });
+        assert directories != null;
+        assertEquals(14, directories.length);
+        assertThat(directories.length).isEqualTo(14);
+    }
+
+    @Test
+    void listDirectories_expressionLambda() {
+        FileFilter filter = pathname -> pathname.isDirectory();
+        File[] directories = root.listFiles(filter);
+        assert directories != null;
+        assertEquals(14, directories.length);
+        assertThat(directories.length).isEqualTo(14);
+    }
+
+    @Test
+    void listDirectories_blockLambda() {
+        FileFilter filter = pathname -> {
+            return pathname.isDirectory();
+        };
+        File[] directories = root.listFiles(filter);
+        assert directories != null;
+        assertEquals(14, directories.length);
+        assertThat(directories.length).isEqualTo(14);
+    }
+}