Java和数据结构（Map，Hashmap，List，Arraylist等）

Question

I'm new to Java and Android development and I'm stuck on following problem. My app gets some data from a database which I would like to structure in a following manner:

DATA = 
{ 
  RESULT[0] = 
  {
    ARRAY[0]=
    {
      VALUE[0], ..., VALUE[X]
    },
    ARRAY[1] = 
    {
      VALUE[0], ..., VALUE[Y]
    },
    ...,
    ARRAY[N] = 
    {
      VALUE[0], ..., VALUE[Z]
    }
  },
  RESULT[1] =
  {
    /* Same as above */
  },
  ...,
  RESULT[M] =
  {
    /* Same as above */
  }
}

where X, Y, ..., Z can be different in each ARRAY of every RESULT but N is always same for all RESULT . I would like to access DATA so that I could read ARRAY s from above structure as

ListArray<String> array = readData(indexOfResult,indexOfArray);

where indexOfResult is 0...M and indexOfArray is 0...N.

I have tried this

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

but maybe something simpler could do the job?

Answer 1

You're better of wrapping each level of your object structure in a class that represents it. For example:

• The class Data contains an array (or arraylist) of Result classes.
• Result contains an array (or arraylist) of ValueCollection , which contains the values.

It's just a raw suggestion. When facing this kind of situations always try to define classes that represent the different levels of the big picture, starting from the main container and going further to the tiny bits of data that can be represented with collections or fields.

Think of a design yourself (I always recommend pen & paper if you can't come up with one directly on code) and try it out. It's the best way to learn.

Answer 2

you should probably create some classes that represent what you are trying to display instead of parametrizising everything.

eg

class Example implements Map<Integer, List<String>> {
...
}

Or if you want to do it your way:

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

List<Map<Integer,ArrayList<String>>> list1 = new Arraylist<Map<Integer,ArrayList<String>>>();
Map<Integer,ArrayList<String>> map1 = new HashMap<Integer,ArrayList<String>>();
ArrayList<String>> list2 = new ArrayList<String>>();

list2.add("test");
map1.add(1, list2);
list1.add(map1);

data.put(1, list1);

Then you have to write a method where you get this data out of your data structure. But you should always try to code as simple as possible and structure everything so it is easy to understand.

Answer 3

you have some options:

Map<Integer,List<String[]> result;

or

List<List<String[]> result;

or

List<List<List<String>>> result;

or even

String[][][] result;

...depends on your usage and what you prefer.

Answer 4

I got my problem solved with

SparseArray<SparseArray<ArrayList<String>>>

However, I encountered another issue which I just can't understand. I do following

/* MAIN */

Data data;

String s = "";
ArrayList<String> arr_N = new ArrayList<String>();
arr_N.add(s);
data.addResult(arr_N);

and in class Data I have

/* CLASS DEFINITION */

class Data {

  private final static int N_A = 0;
  private SparseArray<SparseArray<ArrayList<String>>> results;
  private int n_results;

  public Data() {
    this.results = new SparseArray<SparseArray<ArrayList<String>>>();
    this.n_results = 0;
  }

  public void addResult(ArrayList<String> a) {
    SparseArray<ArrayList<String>> result = new SparseArray<ArrayList<String>>(1);
    result.put(N_A,a);
    this.results.put(this.n_results,result);
    this.n_results++;
  }

  public ArrayList<String> getA_N(int i) {
    return( this.get_value(i,N_A) );
  }

  private ArrayList<String> get_value(int i, int j) {
    SparseArray<ArrayList<String>> result = this.results.valueAt(results.keyAt(i));
    ArrayList<String> list = result.valueAt(result.keyAt(j));
    return( list );
  }
}

Now, if I do this

/* MAIN */

int i = 0; /* Something that is < n_results in class Data */
ArrayList<String> r = data.getA_N(i);
int n_r = r.size();
System.out.println("Size of a_n is " + r.size() );

I get "Size of a_n is 1".

Q1: In the beginning I set s = ""; but is it so that this is not treated as an empty string?

Q2: If I do r.isEmpty() I get that r is not an empty string (although I set s = ""; ). Why is that?

Q3: If I set s = null in the beginning, I still get that the size of r above (In MAIN) is 1, even though r.get(i) (where 0 <= i < n_results ) gives me Index out of bounds exception. Why is that?

Ok, I think I should not put a into result in addResult if it is "" , null or something funny.