Add unit tests for collection operations with lambdas

This commit introduces a new test class, CollectionsTest, which includes unit tests for collection operations using lambda expressions. The tests cover functional approaches and highlight differences in order respect for parallel streams and the use of forEach. This addition will aid in verifying the behavior of collection processing methods more effectively.

src/test/java/lambdas/CollectionsTest.java

@@ -0,0 +1,39 @@
+package lambdas;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Map;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.stream.Collectors;
+
+public class CollectionsTest {
+
+    @Test
+    void forEach_with_collections() {
+        List<String> strings = List.of("this", "is", "a", "list", "of", "strings");
+        List<Integer> lengths = new ArrayList<>();
+
+        // Old school, but may have concurrency issues
+        for (String string : strings) {
+            lengths.add(string.length());
+        }
+        System.out.println(lengths);
+
+        // functional way (collect respects "order")
+        lengths = strings.parallelStream()
+                .peek(x -> System.out.println(x + "on thread " + Thread.currentThread().getName()))
+                .map(String::length)
+                .collect(Collectors.toList());
+        System.out.println(lengths);
+
+        // forEach does NOT respect order
+        List<Integer> stringLengths = new ArrayList<>();
+        strings.parallelStream().forEachOrdered( str -> stringLengths.add(str.length()));
+        System.out.println(stringLengths);
+
+        Map<String, Integer> map = Map.of("a", 1, "b", 2, "c", 2);
+        map.forEach((key, value) -> System.out.println(key + ": " + value));
+    }
+}