Java: Arrays & Vectors

Question

Hi guys,

I'm used to working with PHP but lately I've been working with Java and I'm having a headache trying to figure this out. I want to save this representation in Java:

Array ( 
        ["col_name_1"] => Array ( 
                               1 => ["col_value_1"], 
                               2 => ["col_value_2"], 
                               ... , 
                               n => ["col_value_n"] 
                          ),
        ["col_name_n"] => Array ( 
                               1 => ["col_value_1"], 
                               2 => ["col_value_2"], 
                               ... , 
                               n => ["col_value_n"] 
                          )
)

Is there a clean way (i.e. no dirty code) to save this thing in Java? Note; I would like to use Strings as array indexes (in the first dimension) and I don't know the definite size of the arrays..

Answer 1

You want a Map, which are keyed by just about anything. HashMaps work in most cases.

Something like this.

List<String> col1Vals = new java.util.ArrayList<String>();
col1Vals.add("col_value_1");
col1Vals.add("col_value_2");
Map<String, List<String>> map = new HashMap<String, List<String>>();
map.put("col_name_1", col1Vals);

If you want something simpler, the commons-lang library has a MultiMap.

Answer 2

Try using a Map<String, List<String>>. This will allow you to use Strings as keys / indices into the outer map and get a result being a list of Strings as values. You'll probably want to use a HashMap for the outer map and ArrayList's for the inner lists.

If you want some clean code that is similar to the PHP you gave to initialize it, you can do something like this:

Map<String, List<String>> columns = new HashMap<String, List<String>>() {{
    put("col_name_1", Arrays.asList("col_val_1", "col_val_2", "col_val_n"));
    put("col_name_2", Arrays.asList("col_val_1", "col_val_2", "col_val_n"));
    put("col_name_n", Arrays.asList("col_val_1", "col_val_2", "col_val_n"));
}};

Answer 3

You can use a Map and a List (these both are interfaces implemented in more than one way for you to choose the most adequate in your case).

For more information check the tutorials for Map and List and maybe you should start with the Collections tutorial.

An example:

import java.util.*;

public class Foo {
    public static void main(String[] args) {
        Map<String, List<String>> m = new HashMap<String, List<String>>();
        List<String> l = new LinkedList<String>();
        l.add("col_value_1");
        l.add("col_value_2");
        //and so on
        m.put("col_name_1",l); //repeat for the rest of the colnames

       //then, to get it you do

       List<String> rl = m.get("col_name_1");

    }
}

Answer 4

Be forewarned that the Vector is legacy code for the Collections framework. It synchronizes access to its elements which slows down performance. Most use cases for using List don't need this kind of thread safety, and even if you did, I would be more inclined to use the CopyOnWriteArrayList.