Add Record class and FileFilter Tests; Update UsePerson

Introduced a Java record, PersonRecord, in the Person.java file to provide an immutable data holder functionality. Inclusion of 'record class' also brings auto-generation of equals(), hashCode() and toString() methods. Further, added FileFilter Tests in FileFilterTest.java, ensuring that different filtering scenarios work as expected. Modifications were also made to UsePerson.java file to improve code readability and performance via using parallel streams and the addition of useful comments for better understanding of the code.

src/main/java/lambdas/Person.java

@@ -2,6 +2,13 @@
 
 import java.util.Objects;
 
+// Records:
+// - are immutable data holders
+// - have "canonical" or "primary" constructors BEFORE the braces
+// - autogenerate equals(), hashCode(), and toString()
+// - have "getters" whose names match the field names, e.g., name()
+record PersonRecord(String name) {}
+
 public class Person {
     private String name;
 

src/main/java/lambdas/UsePerson.java

@@ -7,6 +7,7 @@
 import java.util.stream.Collectors;
 
 public class UsePerson {
+    @SuppressWarnings("Convert2MethodRef")
     public static void main(String[] args) {
         List<String> names = Arrays.asList("John", "Paul", "George", "Ringo");
 
@@ -17,21 +18,23 @@ public static void main(String[] args) {
         }
         System.out.println(beatles);
 
+        // Functional approach:
         List<Person> people = names.stream()    // Stream<String>
                 .map(name -> new Person(name))  // Stream<Person>
                 .collect(Collectors.toList());  // Converts Stream<Person> to List<Person>
         System.out.println(people);
 
         people = names.stream()
-                .map(Person::new) // uses the Person(String) ctr
+                .map(Person::new)    // uses the Person(String) ctr --> Stream<Person>
                 // .map(Person::new) // uses the Person(Person) ctr
                 .collect(Collectors.toList());
         System.out.println(people);
 
         Person[] peopleArray = names.stream()
                 .map(Person::new)
-                .toArray(Person[]::new);
-                //.toArray(value -> new Person[value]);
+                .toArray(Person[]::new);  // dimension an array of Person whose size
+                // is the same as the number of elements in the stream
+                //.toArray(value -> new Person[value]);  // lambda equivalent of the constructor reference
         System.out.println(Arrays.toString(peopleArray));
 
         List<String> fullNames = Arrays.asList(
@@ -44,27 +47,33 @@ public static void main(String[] args) {
         System.out.println(people.getClass().getName());
 
         // p1..p5 | p6..p10 | p11..p15 | p16..p20  // say you have 4 cores and run in parallel
-        //   l1       l2         l3         l4
+        //   l1       l2         l3         l4     // Java uses a ForkJoinPool by default
         //                 list
         LinkedList<Person> linkedPersons = names.stream()
                 .map(Person::new)
                 .collect(
-                        () -> new LinkedList<Person>(),          // Supplier<LinkedList>
+                        () -> new LinkedList<>(),                // Supplier<LinkedList>
                         (list, person) -> list.add(person),      // BiConsumer<LinkedList, Person>
                         (list1, list2) -> list1.addAll(list2));  // BiConsumer<LinkedList, LinkedList>
         System.out.println(linkedPersons);
 
-        linkedPersons = names.stream()
+        linkedPersons = names.parallelStream()
                 .map(Person::new)
                 .collect(
                         LinkedList::new,      // Supplier<LinkedList>
                         LinkedList::add,      // BiConsumer<LinkedList, Person>
                         LinkedList::addAll);  // BiConsumer<LinkedList, LinkedList>
         System.out.println(linkedPersons);
 
-        linkedPersons = names.stream()
+        linkedPersons = names.parallelStream()
                 .map(Person::new)
-                .collect(Collectors.toCollection(LinkedList::new));
+                .collect(Collectors.toCollection(LinkedList::new)); // collect preserves order
         System.out.println(linkedPersons);
+
+        List<Person> persons = new ArrayList<>();
+        names.parallelStream()
+                .map(Person::new)
+                .forEach(persons::add);  // forEach does not preserve order
+        System.out.println(persons);
     }
 }

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 java.io.FilenameFilter;
+
+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);
+    }
+
+    @SuppressWarnings({"Convert2Lambda", "Anonymous2MethodRef"})
+    @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);
+    }
+
+    @SuppressWarnings("Convert2MethodRef")
+    @Test
+    void listDirectories_expressionLambda() {
+        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() {
+        File[] directories = root.listFiles(pathname -> {
+            System.out.println("Checking " + pathname);
+            return pathname.isDirectory();
+        });
+        assert directories != null;
+        assertEquals(14, directories.length);
+    }
+
+    @Test
+    void listJavaSrcFiles_fileFilter() {
+        File[] javaSrcFiles = root.listFiles(pathname -> pathname.getName().endsWith(".java"));
+        assert javaSrcFiles != null;
+        assertEquals(8, javaSrcFiles.length);
+    }
+
+    @Test
+    void listJavaSrcFiles_fileNameFilter() {
+        File[] javaSrcFiles = root.listFiles((dir, name) -> name.endsWith(".java"));
+        assert javaSrcFiles != null;
+        assertEquals(8, javaSrcFiles.length);
+    }
+
+    @Test
+    void listJavaSrcFiles_assignToVariable() {
+        FilenameFilter filter = (File dir, String name) -> name.endsWith(".java");
+        File[] javaSrcFiles = root.listFiles(filter);
+        assert javaSrcFiles != null;
+        assertEquals(8, javaSrcFiles.length);
+    }
+}