连接多个arraylists引用

Question

So I have 3 arraylists called for example books , newspapers , magazines and one arraylist called resources .

If you do something like : resources = books; then resources will point to the same memory location as books.

But how can you add to this resources at the end the start of the other two arraylists so the resources will contain all arraylists? Something like concatenation of arraylists memories references.

Answer 1

1. If you want the elements of book, newspaper and magazine ArrayLists will appear in one ArrayList resources , then you can make use of ArrayList.addAll(Collection) function: Appends all of the elements in the specified collection to the end of this list, in the order that they are returned by the specified collection's Iterator.

    resources.addAll(book); resources.addAll(newspaper); resource.addAll(magazine); 

2. You want to create an ArrayList which will contain the ArrayLists as element:

     ArrayList<ArrayList<String>>resources = new ArrayList<>(); resources.add(book); resources.add(newspaper); resources.add(magazine); 

Assumed that the type of book, magazine and newspaper ArrayLists is ArrayList<String> . However, if second one is your target then i think it is better you declare resources as an HashMap<String, ArrayList<String>> .

    HashMap<String, ArrayList<String>>resources = new HashMap<>();
            // replace String with the type of the elements of book\newspaper\magazine
    resources.put("book", book);
    resources.put("newspaper", newspaper);
    resources.put("magazine", magazine);