如何创建（然后获取元素）数组/列表列表？

Question

How to create array or list of Lists in Java?

I have:

List<Item> rows = new ArrayList<Item>();

rows.add(new Item("a"));
rows.add(new Item("b"));

... and how to create array of lists like this above?

List<Item> lists[] = {};
lists[0] = rows;

---

Here is what I am trying to do:

public class MainActivity extends FragmentActivity {

    public static List<List<Item>> lists;
    String tabs[] = { "Tab 1", "Tab 2" };

    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        lists = new ArrayList<List<Item>>();

        for (int i = 0; i < tabs.length; i++)
        {
            lists.add(i, init());
        }
    }

    public List<Item> init()
    {
        List<Item> rows = new ArrayList<Item>();

        rows.add(new Item("test"));

        return rows;
    }

}

... and then I am trying to get that list by using:

MainActivity.lists.get(0);

Answer 1

You should not create an array of generics types, because it's not type safe . Here's a quote from that link

Parameterized types do not have exact runtime type information. As a consequence, the array store check does not work because it uses the dynamic type information regarding the array's (non-exact) component type for the array store check.

Emphasis mine.

Basically, array uses a store check at runtime, to check whether the elements stored in it, is compatible with the type of actual array. Now, since a generic type doesn't have actual type information at runtime due to type erasure , the array store check won't work.

Eg (Taken from the above link):

Pair<Integer,Integer>[] intPairArr = new Pair<Integer,Integer>[10]; // illegal
Object[] objArr = intPairArr; 
objArr[0] = new Pair<String,String>("",""); // should fail, but would succeed

It would work because Pair<Integer, Integer>[] , is actually Pair[] at runtime, and will successfully store Pair<String, String> , which is also a Pair at runtime.

But that would fail further in your code, once you try to fetch elements from array.

---

So, you should rather create a list of list :

List<List<Item>> itemList = new ArrayList<List<Item>>();
itemList.add(rows);

Answer 2

This is how you can create List of List

List<List<Item>> listOfList = new ArrayList<List<Item>>();
listOfList.add(new ArrayList<Item>());
listOfList.get(0).add(new Item());

and List of array

List<Item[]> listOfArray = new ArrayList<Item[]>();
listOfArray.add(new Item[] { item1, item2});