Increase unit test coverage for file filter methods

Enhanced the FileFilterTest class to include unit test scenarios for various lambda and anonymous inner class implementations within the file filter. This improvement will facilitate in validating the behavior of these filtering techniques and ensure the right number of files or directories are returned. The update is crucial to the overall code maintainability and robustness.

src/test/java/lambdas/FileFilterTest.java

@@ -4,6 +4,7 @@
 
 import java.io.File;
 import java.io.FileFilter;
+import java.util.List;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 
@@ -17,6 +18,7 @@ public void testListFiles() {
         assertEquals(22, files.length);
     }
 
+    @SuppressWarnings({"Convert2Lambda", "Anonymous2MethodRef"})
     @Test
     void listDirectories_anonInnerClass() {
         File[] directories = root.listFiles(new FileFilter() {
@@ -29,12 +31,67 @@ public boolean accept(File pathname) {
         assertEquals(14, directories.length);
     }
 
+    @SuppressWarnings("Convert2MethodRef")
     @Test
     void listDirectories_expression_lambda() {
         File[] directories = root.listFiles(file -> file.isDirectory());
         assert directories != null;
         assertEquals(14, directories.length);
     }
 
+    @SuppressWarnings("Convert2MethodRef")
+    @Test
+    void listDirectories_lambda_variable() {
+        FileFilter fileFilter = file -> file.isDirectory();
+        File[] directories = root.listFiles(fileFilter);
+        assert directories != null;
+        assertEquals(14, directories.length);
+    }
+
+    @SuppressWarnings({"Convert2MethodRef", "CodeBlock2Expr"})
+    @Test
+    void listDirectories_block_lambda() {
+        File[] directories = root.listFiles(file -> {
+            return file.isDirectory();
+        });
+        assert directories != null;
+        assertEquals(14, directories.length);
+    }
+
+    @Test
+    void listDirectories_method_reference() {
+        File[] directories = root.listFiles(File::isDirectory);
+        assert directories != null;
+        assertEquals(14, directories.length);
+    }
+
+    @Test
+    void listJavaSrcFiles_fileFilter() {
+        File[] javaFiles = root.listFiles(file -> file.getName().endsWith(".java"));
+        assert javaFiles != null;
+        assertEquals(8, javaFiles.length);
+    }
+
+    @Test
+    void listJavaSrcFiles_fileNameFilter() {
+        File[] javaFiles = root.listFiles((dir, name) -> name.endsWith(".java"));
+        assert javaFiles != null;
+        assertEquals(8, javaFiles.length);
+    }
+
+    @Test
+    void forEach_on_a_list() {
+        List<String> strings = List.of("this", "is", "a", "list");
+        // Java 5+
+        for (String string : strings) {
+            System.out.println(string);
+        }
+
+        // Java 8+
+        strings.forEach(System.out::println);
+
+        // Parallel stream
+        strings.parallelStream().forEach(System.out::println);
+    }
 
 }