Bael-667 Flatten Nested Collections (#1330)

* article Bael-667 initial commit.

* Switch to use logging framework for output.

core-java/src/main/java/com/baeldung/list/flattennestedlist/FlattenNestedList.java

@@ -0,0 +1,17 @@
+package com.baeldung.list.flattennestedlist;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class FlattenNestedList {
+
+    public List<String> flattenListOfLists(List<List<String>> lol) {
+
+        // flatten the list
+        List<String> ls = new ArrayList<>();
+        lol.forEach((k) -> ls.addAll(k));
+
+        return ls;
+    }
+
+}

core-java/src/test/java/com/baeldung/list/flattennestedlist/FlattenNestedListTest.java

@@ -0,0 +1,52 @@
+package com.baeldung.list.flattennestedlist;
+
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertTrue;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+import org.junit.Before;
+import org.junit.Test;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class FlattenNestedListTest {
+
+    private static final Logger LOGGER = LoggerFactory.getLogger(FlattenNestedListTest.class);
+    private FlattenNestedList flol;
+    
+    @Before
+    public void setup() {
+        flol = new FlattenNestedList();
+    }
+    
+    @Test
+    public void givenListOfListOfString_flattenNestedList() {
+
+        // create the list to flatten
+        List<String> ls1 = Arrays.asList("one:one", "one:two", "one:three");
+        List<String> ls2 = Arrays.asList("two:one", "two:two", "two:three");
+        List<String> ls3 = Arrays.asList("three:one", "three:two", "three:three");
+
+        List<List<String>> lol = new ArrayList<>();
+        lol.addAll(Arrays.asList(ls1, ls2, ls3));
+        
+        // show nested list
+        LOGGER.debug("\nNested list: ");
+        lol.forEach((nl) -> LOGGER.debug(nl + ""));
+
+        // flatten it
+        List<String> ls = flol.flattenListOfLists(lol);
+
+        assertNotNull(ls);
+        assertTrue(ls.size() == 9);
+        
+        // show flattened list
+        LOGGER.debug("\nFlattened list:");
+        ls.forEach((l) -> LOGGER.debug(l));
+
+    }
+
+}