CODEWITHSUNDEEP
javaarraylistinstantiation
Xephos
Xephos

Reputation: 63

Filling a List<List<String>> with values

So I've been running into a few errors. When I try to add an event to a date in a month, I try to add an event into the date ArrayList but the list doesn't have that number available. So I want to know how to instantiate a list that will have the amount of days in it.

I know you can instantiate how many months there are by using

public static List<List<String>> events = new ArrayList<List<String>>(12);

which 12 being the number of months, but is there a way to make all 12 of the arrays have 31 slots open for days?

Upvotes: 0

Views: 1569

Answers (4)

AJMansfield
AJMansfield

Reputation: 4129

While @Adam's answer is probably the best way to handle this particular problem, the general question of how to initialize a multidimensional list structure should also be addressed.

The standard idiom for constructing and initializing a multi-dimensional data structure like the one you describe is to use a set of for loops, like this:

int[][] myArray = new int[][myFirstDimension];
for(int i=0; i<a.length; i++){
    int [] a = new int[mySecondDimension];
    for(int j=0; j<b.length; j++){
        a[j] = myInitialValue;
    }
    myArray[i] = a;
}

Nested ArrayLists are constructed in a similar way to arrays, but with one important difference: The parameter passed to the ArrayList is only an initial capacity, not the initial size. In fact, ArrayList actually makes no guarantees about how much space it allocates, although in practice, IIRC, it typically allocates the next power of two. Attempting to index into the ArrayList before you have initialized its contents will throw an IndexOutOfBoundsException.

Thus, when initializing nested ArrayLists, we typically do it like this:

List<List<String>> myList = new ArrayList<>();
for(int i=0; i<myFirstDimension; i++){
    List<String> a = new ArrayList<>();
    for(int i=0; i<mySecondDimension; i++){
        a.add(""); // or `null`, if that is needed
    }
   myList.add(a);
}

Upvotes: 0

Adam
Adam

Reputation: 36703

I'd recommend completely changing your design.

  • Use a map of date against event list - to deal with multiple events on same date
  • Lazy initialize map. Create a method that returns list for given day/month, if it isn't there, add it.

Code

private class Events {
    Map<Date, List<String>> events = new HashMap<Date, List<String>>();
    public List<String> getEvents(int month, int day) {
        Calendar calendar = GregorianCalendar.getInstance();
        calendar.set(0, month, date, 0, 0, 0);
        Date date = calendar.getTime();
        List<String> list = events.get(date);
        if (list == null) {
            list = new ArrayList<String>();
            events.put(date, list);
        }
        return list;
    }
}

Usage 

Events events = new Events();
events.getEvents(11, 25).add("Christmas");
events.getEvents(0, 1).add("New years day");

Upvotes: 2

muasif80
muasif80

Reputation: 6006

Why not use

List<Map<String, Integer>> = new List<HashMap<String, Integer>>();

Upvotes: 0

engineercoding
engineercoding

Reputation: 832

I think you should change your design plan. Make a class Month with an ArrayLisy<String>(31) which you create 12 times in another class called year or something. For myself I would do this and it makes your code more readable!

Also, when it is a fixed number of slots (and you don't want it to go over that limit), use basic arrays

Upvotes: 1

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