Add test cases for lambdas functionality

Three new unit test files have been added: FileFilterTest, LazySupplierTest, and PredicateTest. These tests will validate various features and behaviors related to Java lambda expressions. Specifically, testing file filtering, the use of suppliers and predicates, and functionality with both expression and block lambda syntax.

src/test/java/lambdas/FileFilterTest.java

@@ -0,0 +1,72 @@
+package lambdas;
+
+import org.junit.jupiter.api.Test;
+
+import java.io.File;
+import java.io.FileFilter;
+
+import static java.util.Map.*;
+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();
+        assert files != null;
+        assertThat(files.length).isEqualTo(22);
+    }
+
+    @Test
+    void listDirectories_anonInnerClass() {
+        File[] directories = root.listFiles(new java.io.FileFilter() {
+            @Override
+            public boolean accept(File file) {
+                return file.isDirectory();
+            }
+        });
+        assert directories != null;
+        assertThat(directories.length).isEqualTo(14);
+    }
+
+    @Test
+    void listDirectories_expressionLambda() {
+        File[] directories = root.listFiles(file -> file.isDirectory());
+        assert directories != null;
+        assertThat(directories.length).isEqualTo(14);
+    }
+
+    @Test
+    void listJavaSrcFiles_fileFilter() {
+        File[] javaSrcFiles = root.listFiles(file -> file.getName()
+                .endsWith(".java"));
+        assert javaSrcFiles != null;
+        assertThat(javaSrcFiles.length).isEqualTo(8);
+    }
+
+    @Test
+    void listJavaSrcFiles_fileNameFilter() {
+        File[] javaSrcFiles = root.listFiles((File dir, String name) -> name.endsWith(".java"));
+        assert javaSrcFiles != null;
+        assertThat(javaSrcFiles.length).isEqualTo(8);
+    }
+
+    @Test
+    void listJavaSrcFiles_fileFilter_variable() {
+        FileFilter filter = (File file) -> file.getName()
+                .endsWith(".java");
+        File[] javaSrcFiles = root.listFiles(filter);
+        assert javaSrcFiles != null;
+        assertThat(javaSrcFiles.length).isEqualTo(8);
+    }
+
+    @Test
+    void forEachMap() {
+        ofEntries(
+                entry("a", 1),
+                entry("b", 2),
+                entry("c", 2))
+                .forEach((k, v) -> System.out.println(k + " -> " + v));
+    }
+}

src/test/java/lambdas/LazySupplierTest.java

@@ -0,0 +1,35 @@
+package lambdas;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.logging.Logger;
+
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class LazySupplierTest {
+    private final Logger logger = Logger.getLogger(LazySupplierTest.class.getName());
+
+    private String getErrorMessage() {
+        System.out.println("calling getErrorMessage()");
+        return "An error occurred";
+    }
+
+    private String getLogMessage() {
+        System.out.println("calling getLogMessage()");
+        return "log message";
+    }
+
+    @Test
+    void eagerMessages() {
+        boolean condition = true;
+        assertTrue(condition, getErrorMessage());
+        logger.fine(getLogMessage());
+    }
+
+    @Test
+    void lazyMessages() {
+        boolean condition = true;
+        assertTrue(condition, () -> getErrorMessage());
+        logger.fine(() -> getLogMessage());
+    }
+}

src/test/java/lambdas/PredicateTest.java

@@ -0,0 +1,47 @@
+package lambdas;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.function.Predicate;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class PredicateTest {
+
+    @Test
+    void implementPredicateAsAnonInnerClass() {
+        Predicate<String> p = new Predicate<String>() {
+            @Override
+            public boolean test(String s) {
+                return s.length() > 5;
+            }
+        };
+        assertFalse(p.test("Hello"));
+        assertTrue(p.test("Hello World"));
+    }
+
+    @Test
+    void implementPredicateAsExpressionLambda() {
+        Predicate<String> p = s -> s.length() > 5;
+        assertFalse(p.test("Hello"));
+        assertTrue(p.test("Hello World"));
+    }
+
+    @Test
+    void implementPredicateAsBlockLambda() {
+        Predicate<String> p = s -> {
+            System.out.println("Evaluating whether s length > 5: " + s);
+            return s.length() > 5;
+        };
+        assertFalse(p.test("Hello"));
+        assertTrue(p.test("Hello World"));
+    }
+
+    @Test
+    void testRunnable() {
+        Runnable r = () -> System.out.println("Hello World");
+        Thread thread = new Thread(r);
+        thread.start();
+    }
+}