Add items to list in core-java-collections (#4841)

core-java-collections/src/main/java/com/baeldung/java/list/Flower.java

@@ -0,0 +1,28 @@
+package com.baeldung.java.list;
+
+public class Flower {
+
+    private String name;
+    private int petals;
+
+    public Flower(String name, int petals) {
+        this.name = name;
+        this.petals = petals;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+
+    public int getPetals() {
+        return petals;
+    }
+
+    public void setPetals(int petals) {
+        this.petals = petals;
+    }
+}

core-java-collections/src/test/java/com/baeldung/list/listoflist/AddElementsToListUnitTest.java

@@ -0,0 +1,69 @@
+package com.baeldung.list.listoflist;
+
+import com.baeldung.java.list.Flower;
+import org.junit.Assert;
+import org.junit.Before;
+import org.junit.Test;
+import java.util.*;
+import static org.junit.Assert.*;
+
+public class AddElementsToListUnitTest {
+
+    List<Flower> flowers;
+
+    @Before
+    public void init() {
+        this.flowers = new ArrayList<>(Arrays.asList(
+                new Flower("Poppy", 12),
+                new Flower("Anemone", 8),
+                new Flower("Catmint", 12)));
+    }
+    @Test
+    public void givenAList_whenTargetListIsEmpty_thenReturnTargetListWithNewItems() {
+        List<Flower> anotherList = new ArrayList<>();
+        anotherList.addAll(flowers);
+        assertEquals(anotherList.size(), flowers.size());
+        Assert.assertTrue(anotherList.containsAll(flowers));
+    }
+    @Test
+    public void givenAList_whenTargetListIsEmpty_thenReturnTargetListWithOneModifiedElementByConstructor() {
+        List<Flower> anotherList = new ArrayList<>();
+        anotherList.addAll(flowers);
+        Flower flower = anotherList.get(0);
+        flower.setPetals(flowers.get(0).getPetals() * 3);
+        assertEquals(anotherList.size(), flowers.size());
+        Assert.assertTrue(anotherList.containsAll(flowers));
+    }
+    @Test
+    public void givenAListAndElements_whenUseCollectionsAddAll_thenAddElementsToTargetList() {
+        List<Flower> target = new ArrayList<>();
+        Collections.addAll(target, flowers.get(0), flowers.get(1), flowers.get(2), flowers.get(0));
+        assertEquals(target.size(), 4);
+    }
+    @Test
+    public void givenTwoList_whenSourceListDoesNotHaveNullElements_thenAddElementsToTargetListSkipFirstElementByStreamProcess() {
+        List<Flower> flowerVase = new ArrayList<>();
+        flowers.stream()
+                .skip(1)
+                .forEachOrdered(flowerVase::add);
+        assertEquals(flowerVase.size() + 1, flowers.size());
+        assertFalse(flowerVase.containsAll(flowers));
+    }
+    @Test
+    public void givenTwoList_whenSourceListDoesNotHaveNullElements_thenAddElementsToTargetListFilteringElementsByStreamProcess() {
+        List<Flower> flowerVase = new ArrayList<>();
+        flowers.stream()
+                .filter(f -> f.getPetals() > 10)
+                .forEachOrdered(flowerVase::add);
+        assertEquals(flowerVase.size() + 1, flowers.size());
+        assertFalse(flowerVase.containsAll(flowers));
+    }
+    @Test
+    public void givenAList_whenListIsNotNull_thenAddElementsToListByStreamProcessWihtOptional() {
+        List<Flower> target = new ArrayList<>();
+        Optional.ofNullable(flowers)
+                .ifPresent(target::addAll);
+        assertNotNull(target);
+        assertEquals(target.size(), 3);
+    }
+}