Add unit tests for FileFilter functionality

FileFilterTest.java is a new file containing unit tests for FileFilter functions. It includes tests for listing files, directories, Java source files, use of anonymous inner classes, lambda expressions, and block lambda in file filtering. It aims to ensure the functionality of the system's file filtering operations.

src/test/java/lambdas/FileFilterTest.java

@@ -0,0 +1,79 @@
+package lambdas;
+
+import org.junit.jupiter.api.Test;
+
+import java.io.File;
+import java.io.FileFilter;
+import java.util.List;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class FileFilterTest {
+    private final File root = new File("src/main/java");
+
+    @Test
+    public void testListFiles() {
+        File[] files = root.listFiles();
+        assert files != null;
+        for (File file : files) {
+            System.out.println(file);
+        }
+        assertThat(files.length).isEqualTo(22);
+    }
+
+    @Test
+    void listDirectories_anonInnerClass() {
+        File[] directories = root.listFiles(new FileFilter() {
+            @Override
+            public boolean accept(File file) {
+                return file.isDirectory();
+            }
+        });
+        assert directories != null;
+        assertThat(directories.length).isEqualTo(14);
+    }
+
+    @Test
+    void listDirectories_expressionLambda() {
+        File[] directories = root.listFiles(file -> file.isDirectory());
+        assert directories != null;
+        assertThat(directories.length).isEqualTo(14);
+    }
+
+    @Test
+    void listDirectories_blockLambda() {
+        File[] directories = root.listFiles(file -> {
+            System.out.println("Checking " + file + " to see if it's a directory.");
+            return file.isDirectory();
+        });
+        assert directories != null;
+        assertThat(directories.length).isEqualTo(14);
+    }
+
+    @Test
+    void listJavaSrcFiles_fileFilter() {
+        File[] javaFiles = root.listFiles(file -> file.getName().endsWith(".java"));
+        assert javaFiles != null;
+        assertThat(javaFiles.length).isEqualTo(8);
+    }
+
+    @Test
+    void listJavaSrcFiles_fileNameFilter() {
+        File[] javaFiles = root.listFiles((dir, name) -> name.endsWith(".java"));
+        assert javaFiles != null;
+        assertThat(javaFiles.length).isEqualTo(8);
+    }
+
+    @Test
+    void forEachOnList() {
+        List<String> strings = List.of("this", "is", "a", "list", "of", "strings");
+
+        // Using the forEach(Consumer) method
+        strings.forEach(s -> System.out.println("The next string is " + s));
+
+        // For-each loop
+        for (String s : strings) {
+            System.out.println("The next string is " + s);
+        }
+    }
+}