将3个arraylist合并为一个

Question

我想在java中合并3个arraylist。有谁知道哪种方法最好？

Answer 1

使用ArrayList.addAll()。这样的东西应该工作（假设列表包含String对象;你应该相应地改变）。

List<String> combined = new ArrayList<String>();
combined.addAll(firstArrayList);
combined.addAll(secondArrayList);
combined.addAll(thirdArrayList);

更新

我可以通过您的评论看到您实际上可能正在尝试创建2D列表。如果是这样，以下代码应该起作用：

List<List<String>> combined2d = new ArrayList<List<String>>();
combined2d.add(firstArrayList);
combined2d.add(secondArrayList);
combined2d.add(thirdArrayList);

Answer 2

使用java.util.Arrays.asList来简化合并怎么样？

List<String> one = Arrays.asList("one","two","three");
List<String> two = Arrays.asList("four","five","six");
List<String> three = Arrays.asList("seven","eight","nine");

List<List<String>> merged = Arrays.asList(one, two, three);

Answer 3

使用Java 8 Streams：

清单清单

List<List<String>> listOfList = Stream.of(list1, list2, list3).collect(Collectors.toList());

字符串列表

List<String> list = Stream.of(list1, list2, list3).flatMap(Collection::stream).collect(Collectors.toList());

使用Java 9 List.of静态工厂方法（警告：此列表是不可变的，不允许为null）

List<List<String>> = List.of​(list1, list2, list3);

list1, list2, list3的类型为List<String>

Answer 4

好吧差不多一年了我知道但我最近遇到了这个问题，我有一个对象（JobList），它包含一个对象列表（作业）这个列表可以是任何大小我的解决方案需要X个JobLists并返回一个JobList，所以给出3个JobList对象，它将返回一个Job对象列表，如下所示

job1FromJobList1 job1FromJobList2 job1FromJobList3 job2FromJobList1 job2FromJobList2 job2FromJobList3 job3FromJobList1 job3FromJobList2 job3FromJobList3

这就是我的意思，如果我说合并，你可以忽略我创建对象的部分主要部分获得最大，然后将它们添加到列表，如果我<jobList.getJobList（）。size（）

public JobList merge(List<JobList> jobListList) {
    JobList mergedJobList = new JobList();
    int max = 0;
    ArrayList<Job> mergedList = new ArrayList<>();
    ArrayList<String> tags = new ArrayList<>();
    for (JobList jobList : jobListList){
        max = Math.max(max,ListConverter.jobListFromString(jobList.getJobList()).size());
        for (String tag : jobList.getJobListsTags()){
            tags.add(tag);
        }
    }
    for (int i = 0; i < max; i++){
        for (JobList jobList : jobListList){
            if (i < ListConverter.jobListFromString(jobList.getJobList()).size())
                mergedList.add(ListConverter.jobListFromString(jobList.getJobList()).get(i));
        }
    }
    mergedJobList.setJobListTitle("Title");
    mergedJobList.setJobListFavourite(false);
    mergedJobList.setJobListsTags((String[]) tags.toArray());
    mergedJobList.setJobListColour(this.getString(R.string.white));
    mergedJobList.setJobList(ListConverter.stringFromJobList(mergedList));
    return mergedJobList;
}