addAll 函数正在复制 List 中的对象

Question

My goal is to join two lists in one list, but the problem is that the objects of the first list duplicates twice, this the code below:

List<Image> unionList = new ArrayList<Image>();

      unionList.addAll(fromImageList);

       unionList.addAll(fromTagList);

so that the objects of fromImageList are joined twice.

Answer 1

If you want aggregate only unique images, then you should use Set

Set<Image> uniqueImages = new HashSet<>();

uniqueImages.addAll(fromImageList);

uniqueImages.addAll(fromTagList);

List<Image> unionList = new ArrayList<>(uniqueImages)

Answer 2

When List<Image> unionList = new ArrayList<Image>(); is excecuted, an empty arrayList is created.

When unionList.addAll(fromImageList); is excecuted, all the images in fromImageList are added to unionList.

When unionList.addAll(fromTagList); is excecuted, all the images in fromTagList are added to unionList.

ArrayList doen't prevent duplication. If you want to prevent duplication, you have to use Set implementation like HashSet.

You can do like the following

Set<Image> uniqueImages = new HashSet<>();
uniqueImages.addAll(fromImageList);
uniqueImages.addAll(fromTagList);

List<Image> unionList = new ArrayList<>(uniqueImages)