Add random number generation and map iteration tests

Extended SummarizingDemo with random number generation and statistics computation. Introduced a new test in FileFilterTest to demonstrate iteration over a map using both pre-Java 8 and Java 8+ approaches.

src/main/java/SummarizingDemo.java

@@ -13,5 +13,19 @@ public static void main(String[] args) {
         System.out.println(stats.getAverage());
         System.out.println(stats.getMax());
         System.out.println(stats.getSum());
+
+        System.out.println();
+        DoubleStream.generate(Math::random)
+                .limit(10)
+                .forEach(System.out::println);
+
+        System.out.println();
+        stats = DoubleStream.generate(Math::random)
+                .limit(10)
+                .peek(x -> System.out.println("before mapping " + x))
+                .map(x -> x * 100)
+                .peek(x -> System.out.println("after mapping " + x))
+                .summaryStatistics();
+        System.out.println(stats);
     }
 }

src/test/java/lambdas/FileFilterTest.java

@@ -4,6 +4,7 @@
 
 import java.io.File;
 import java.io.FileFilter;
+import java.util.Map;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 
@@ -79,4 +80,15 @@ void listJavaSrcFiles_filenameFilter() {
         assert javaFiles != null;
         assertEquals(8, javaFiles.length);
     }
+
+    @Test
+    void iterateOverMap() {
+        var map = Map.of("a", 1, "b", 2, "c", 3);
+        // Before Java 8:
+        for (Map.Entry<String, Integer> entry : map.entrySet()) {
+            System.out.println(entry.getKey() + " -> " + entry.getValue());
+        }
+
+        map.forEach((k, v) -> System.out.println(k + " -> " + v));
+    }
 }