Using Arrays.asList() in Java for 2d Arrays

Question

I'm having trouble figuring outa few things about the following code:

public final String[][] pockets = {{"STATUS CHANGERS", "RUNES", "KEY ITEMS", "TROPHIES"},
        {"POTION", "SUPER POTION", "FULL HEAL"},{"ARMOR+","ATTACK+","EXP+","HEALTH+", "DISPELL+"},
        {"QUEST ITEMS","STORY ITEMS","JOURNAL"},{TROPHIES}};


public final List<String[]> Pockets = Arrays.asList(pockets); //why couldn't I define it in the method, I wonder?

I am trying to figure out why I could not define the 2d Array in the asList() method like you might a normal Array. Also, I know there has to be a Java subclass to import or a more semantic approach.

Please clarify _like you might a normal Array_. – Sotirios Delimanolis Dec 04 '15 at 19:41 — Sotirios Delimanolis, Dec 04 '15 at 19:41

randers · Accepted Answer · 2015-12-12T17:48:09.003

In order to transform a two-dimensional array E[][] into a two-dimensional List<List<E>>, you can use the following code (look below for an explanation):

public final String[][] pocketArray = {{"STATUS CHANGERS", "RUNES", "KEY ITEMS", "TROPHIES"},
        {"POTION", "SUPER POTION", "FULL HEAL"}, {"ARMOR+", "ATTACK+", "EXP+", "HEALTH+", "DISPELL+"},
        {"QUEST ITEMS", "STORY ITEMS", "JOURNAL"}, {"TROPHIES"}};


public final List<List<String>> pocketList;

public Test()
{
    pocketList = new ArrayList<>(pocketArray.length);
    for (String[] pocket : pocketArray)
    {
        List<String> currentSubList = new ArrayList<>(pocket.length);
        // Nice and fast way instead of iterating over the whole array and adding the strings to the list.
        Collections.addAll(currentSubList, pocket);
        // instead, you could do something like this:
        //
        // for (String currentString : pocket)
        // {
        //     currentSubList.add(currentString);
        // }
        //
        pocketList.add(currentSubList);

        // Alternatively, you could use something like this:
        // pocketList.add(Arrays.asList(pocket));
    }
}

Basically, this code is creating a new ArrayList<List<String>> for holding all the sub-lists later. Then it iterates over the array, and saves all the sub-arrays into the list.

By the way, names like pockets versus Pockets are prone to causing confusion later, I recommend you to start field names with lowercase letters and name them uniquely, by their purpose and possibly their type, like I did above.

score 2 · Answer 2 · answered Dec 04 '15 at 19:51

Is this what you are trying to do? Notice the use of new String[][]

 public final List<String[]> Pockets = Arrays.asList(new String[][]{{"STATUS CHANGERS", "RUNES", "KEY ITEMS", "TROPHIES"},
        {"POTION", "SUPER POTION", "FULL HEAL"},{"ARMOR+","ATTACK+","EXP+","HEALTH+", "DISPELL+"},
        {"QUEST ITEMS","STORY ITEMS","JOURNAL"},{"TROPHIES"}});

PS: Pockets is not a standard naming convention, I suggest pocketList

score 0 · Answer 3 · answered Dec 04 '15 at 20:03

I am trying to figure out why I could not define the 2d Array in the asList() method like you might a normal Array.

We cannot answer why what you tried did not work when you do not tell us what you tried. I suppose what you really want to know, however, is how to generate a List<String[]> via Arrays.asList() by passing the individual elements, instead of first constructing a 2D array. That would be this:

public final List<String[]> pockets = Arrays.asList(
        new String[]{"STATUS CHANGERS", "RUNES", "KEY ITEMS", "TROPHIES"},
        new String[]{"POTION", "SUPER POTION", "FULL HEAL"},
        new String[]{"ARMOR+","ATTACK+","EXP+","HEALTH+", "DISPELL+"},
        new String[]{"QUEST ITEMS","STORY ITEMS","JOURNAL"},
        new String[]{"TROPHIES"}
    );

Note in particular that Java has no array literals, only array initializers (which can be associated with array declarations), and array constructors, such as in the above code. I speculate that in your attempt(s), you omitted the new String[] bits that make those constructors. Those bits are necessary to specify the element type of the arrays, if for no other reason.

Also, I know there has to be a Java subclass to import or a more semantic approach.

I've no idea what you mean. You're already using class java.util.Array to perform the construction, and you're building a java.util.List. I guess for most purposes I'd recommend using a List of Lists (List<List<String>>) and avoiding arrays altogether, but I can't be sure whether that would be suitable for your purposes.

Using Arrays.asList() in Java for 2d Arrays

3 Answers3