Add FileFilterTest class and remove redundant annotation

A new test class FileFilterTest.java to test various scenarios of file filtering has been added in the lambdas package. Additionally, a redundant "@SuppressWarnings("DuplicatedCode")" annotation has been removed from ProcessDictionary.java.

src/main/java/io/ProcessDictionary.java

@@ -15,7 +15,6 @@
 import static java.util.stream.Collectors.counting;
 import static java.util.stream.Collectors.groupingBy;
 
-@SuppressWarnings("DuplicatedCode")
 public class ProcessDictionary {
     private final Path dictionary = Paths.get("/usr/share/dict/words");
 

src/test/java/lambdas/FileFilterTest.java

@@ -0,0 +1,56 @@
+package lambdas;
+
+import org.junit.jupiter.api.Test;
+
+import java.io.File;
+import java.io.FileFilter;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class FileFilterTest {
+    private final File root = new File("src/main/java");
+
+    @Test
+    void listFiles() {
+        File[] files = root.listFiles();
+        assertThat(files)
+                .isNotNull()
+                .isNotEmpty()
+                .hasSize(22);
+    }
+
+    @Test
+    void listDirectories_anonInnerClass() {
+        File[] directories = root.listFiles(new FileFilter() {
+            @Override
+            public boolean accept(File pathname) {
+                return pathname.isDirectory();
+            }
+        });
+        assertThat(directories)
+                .isNotNull()
+                .isNotEmpty()
+                .hasSize(14);
+    }
+
+    @Test
+    void listDirectories_expressionLambda() {
+        File[] directories = root.listFiles(pathname -> pathname.isDirectory());
+        assertThat(directories)
+                .isNotNull()
+                .isNotEmpty()
+                .hasSize(14);
+    }
+
+    @Test
+    void listDirectories_blockLambda() {
+        File[] directories = root.listFiles(pathname -> {
+            System.out.println("Checking " + pathname);
+            return pathname.isDirectory();
+        });
+        assertThat(directories)
+                .isNotNull()
+                .isNotEmpty()
+                .hasSize(14);
+    }
+}