Update Person class with variant constructors; add LazyErrorMessage t…

…est class

Multiple constructors were added to the Person class to enhance flexibility and allow for different initialization methods. Comments were added to describe each constructor's usage - default, single-arg, copy, and varargs. Additionally, a new test class, 'LazyErrorMessage.java', was introduced under the 'lambdas' package. It contains tests to underline the differences in producing eager and lazy error messages, thereby improving our understanding of the concept.

src/main/java/lambdas/Person.java

@@ -5,16 +5,20 @@
 public class Person {
     private String name;
 
+    // Default constructor
     public Person() {}
 
+    // Single-arg constructor
     public Person(String name) {
         this.name = name;
     }
 
+    // Copy constructor
     public Person(Person other) {
         this.name = other.name;
     }
 
+    // Varargs constructor
     public Person(String... names) {
         this.name = String.join(" ", names);
     }

src/test/java/lambdas/FileFilterTest.java

@@ -4,10 +4,11 @@
 
 import java.io.File;
 import java.io.FileFilter;
-import java.io.FilenameFilter;
+import java.util.List;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 
+@SuppressWarnings({"Convert2MethodRef", "Convert2Lambda", "Anonymous2MethodRef"})
 public class FileFilterTest {
     private final File root = new File("src/main/java");
 
@@ -56,7 +57,7 @@ void listDirectories_methodReference() {
 
     @Test
     void listDirectories_assignToVariable() {
-        FileFilter filter = path -> path.isDirectory();
+        FileFilter filter = File::isDirectory;
         File[] directories = root.listFiles(filter);
         assert directories != null;
         assertEquals(14, directories.length);
@@ -75,4 +76,11 @@ void listJavaSrcFiles_fileNameFilter() {
         assert javaFiles != null;
         assertEquals(8, javaFiles.length);
     }
+
+    @Test
+    void forEach_list() {
+        List<String> strings = List.of("one", "two", "three");
+        strings.forEach(s -> System.out.println(s));
+        strings.forEach(System.out::println);
+    }
 }

src/test/java/lambdas/LazyErrorMessage.java

@@ -0,0 +1,31 @@
+package lambdas;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.logging.Logger;
+
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+@SuppressWarnings("Convert2MethodRef")
+public class LazyErrorMessage {
+    private final Logger logger = Logger.getLogger(LazyErrorMessage.class.getName());
+
+    private String getErrorMessage() {
+        System.out.println("Calculating error message...");
+        return "This is the error message";
+    }
+
+    @Test
+    void eagerErrorMessage() {
+        boolean x = true;
+        logger.fine(getErrorMessage());
+        assertTrue(x, getErrorMessage());  // eagerly creates the error message
+    }
+
+    @Test
+    void lazyErrorMessage() {
+        boolean x = true;
+        logger.fine(() -> getErrorMessage());  // Supplier<String>
+        assertTrue(x, () -> getErrorMessage()); // does NOT create the error message
+    }
+}