Add comments in RunnableDemo and create FileFilterTest

Lambda expressions in RunnableDemo were updated to include commentary for clarity. A new FileFilterTest.java file was added to the test directory, including various testing methods for file operations using different variants of lambda expressions.

src/main/java/lambdas/RunnableDemo.java

@@ -4,6 +4,7 @@
 import java.util.concurrent.Executors;
 
 public class RunnableDemo {
+    @SuppressWarnings("Convert2Lambda")
     public static void main(String[] args) {
         ExecutorService executorService = Executors.newFixedThreadPool(4);
 
@@ -15,8 +16,10 @@ public void run() {
             }
         });
 
+        // Expression lambda
         executorService.submit(() -> System.out.println("Inside expression lambda"));
 
+        // Block lambda
         executorService.submit(() -> {
             System.out.println(Thread.currentThread().getName());
             System.out.println("Inside block lambda");

src/test/java/lambdas/FileFilterTest.java

@@ -0,0 +1,81 @@
+package lambdas;
+
+import org.junit.jupiter.api.Test;
+
+import java.io.File;
+import java.io.FileFilter;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+@SuppressWarnings({"Convert2MethodRef", "Convert2Lambda", "Anonymous2MethodRef"})
+public class FileFilterTest {
+    private final File root = new File("src/main/java");
+
+    @Test
+    public void listFiles() {
+        File[] files = root.listFiles();
+        assert files != null;
+        assertEquals(22, files.length);
+    }
+
+    @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);
+    }
+
+    @Test
+    void listDirectories_expressionLambda() {
+        // Single argument to lambda, without data type --> no parens necessary
+        File[] directories = root.listFiles(pathname -> pathname.isDirectory());
+        assert directories != null;
+        assertEquals(14, directories.length);
+    }
+
+    @Test
+    void listDirectories_methodReference() {
+        File[] directories = root.listFiles(File::isDirectory);
+        assert directories != null;
+        assertEquals(14, directories.length);
+    }
+
+    @Test
+    void listDirectories_blockLambda() {
+        // Single argument to lambda, without data type --> no parens necessary
+        File[] directories = root.listFiles(pathname -> {
+            System.out.println("pathname = " + pathname);
+            return pathname.isDirectory();
+        });
+        assert directories != null;
+        assertEquals(14, directories.length);
+    }
+
+    @Test
+    void listDirectories_assignToVariable() {
+        FileFilter filter = pathname -> pathname.isDirectory();
+        File[] directories = root.listFiles(filter);
+        assert directories != null;
+        assertEquals(14, directories.length);
+    }
+
+    @Test
+    void listJavaSrcFiles_fileFilter() {
+        File[] javaFiles = root.listFiles(pathname -> pathname.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);
+    }
+
+}