Java - arraylist of multidimensional arrays or arraylist of multidimensional arraylists?

Question

I'm learning Java and am trying to wrap my head around these data structures. I'm coming from Python/PHP so I'm used to dynamically sizing arrays on the fly and being able to store multiple data types in one array.

How would I best store this data? Say I don't know how many rows I'll have, but I do know that each row will hold 2 columns of data. One column being a string, and the other column being a double.

Example in pseudo-ish code if I had 3 rows:

array(array("description1", 10.00),
      array("description2", 12.00),
      array("description3", 14.00));

Then, I want to loop through the array to process the datawith something like:

foreach(rows as row){
    myStringVal = row[0]; //on first iteration would be "description1"
    myIntVal = row[1];    //on first iteration would be 10.00
    ... do something with the values ...
}

I'm thinking I need to create an arraylist that holds an array, but I can't store both strings and doubles in a java array, so what do i do? Do I use a map instead, and treat it as if it were an array? For example, do I create a map where the first element is a numeric ID for each row, the second element is the string value, the 3rd element is the double value, and then I use a loop to increase a counter and grab each row from the map that using the numeric ID>

Really confused as to how this will work. Any suggestions? Thanks!

Answer 1

You're not storing "different kind of values", you're storing semantic data that encodes "a tuple" but using the non-java concept of just sticking that in an array. Don't do that, instead use the C/C++/Java concept of encoding linked data as a struct/object:

// our "struct"esque class for the linked data we're handling
class MyObject {
  String description;
  float value;
  Public MyObject(String description, float value) {
    this.description = description;
    this.value = value;
  }
}

// build our list of objects
ArrayList<MyObject> mylist = new ArrayList<MyObject>();
mylist.add(new MyObject("description1", 10));
mylist.add(new MyObject("description2", 12));
mylist.add(new MyObject("description3", 14));
...

// and then we iterate
String d; float v;
for(MyObject m: mylist) {
  d = m.description;
  v = m.value;
  ...
}

Answer 2

It looks like a List<Map<String, BigDecimal>> to me.

• The List will give you the ability to enter more than one element continuously.

• The Map will give you the association between the String and floating-point value, which I've chosen BigDecimal to represent. You could use Double if you wanted.

   List<Map<String, BigDecimal>> elements = new ArrayList<>(); elements.add(new HashMap<String, BigDecimal>()); elements.get(0).put("description1", BigDecimal.valueOf("10.00")); 

Answer 3

Use a class instead of an array.

public class X
{
    private final String description;
    private final double value;

    public X(final String desc,
             final double val)
    {
        decription = desc;
        value      = val;
    }

    // getters
}

If you want to be able to change the description and value then don't make them final.

You could make the description and value variables public, but I would avoid that temptation.