传统方法：new ArrayList() + addAll()

List<Integer> list1 = Arrays.asList(1, 2, 3);
List<Integer> list2 = Arrays.asList(4, 5);
List<Integer> list3 = Arrays.asList(6);

List<Integer> mergedList = new ArrayList<>();

mergedList.addAll(list1);
mergedList.addAll(list2);
mergedList.addAll(list3);

System.out.println(mergedList); // 输出：[1, 2, 3, 4, 5, 6]

Java 8 + Stream API

List<Integer> list1 = Arrays.asList(1, 2);
List<Integer> list2 = Arrays.asList(3, 4);
List<Integer> list3 = Arrays.asList(4, 5);

// 1. 基础合并（不去重）
List<Integer> mergedList = Stream.of(list1, list2, list3) // 包装为 Stream<List<Integer>>
                             .flatMap(Collection::stream) // 扁平化为 Stream<Integer>
                             .collect(Collectors.toList()); // 收集为 List

// 2. 合并+去重+排序
List<Integer> mergedDistinctSortedList = Stream.of(list1, list2, list3)
                                            .flatMap(Collection::stream)
                                            .distinct() // 去重
                                            .sorted() // 排序
                                            .collect(Collectors.toCollection(ArrayList::new)); // 指定 List 实现类

System.out.println(mergedList); // [1, 2, 3, 4, 4, 5]
System.out.println(mergedDistinctSortedList); // [1, 2, 3, 4, 5]

Java 16 + Stream.concat()

List<String> listA = List.of("a", "b");
List<String> listB = List.of("c");
List<String> listC = List.of("d", "e");

List<String> mergedList = Stream.concat(
                             listA.stream(),
                             listB.stream(),
                             listC.stream()
                       ).collect(Collectors.toList());

System.out.println(mergedList); // [a, b, c, d, e]