Java ArrayList and HashMap on-the-fly

Question

Can someone please provide an example of creating a Java ArrayList and HashMap on the fly? So instead of doing an add() or put(), actually supplying the seed data for the array/hash at the class instantiation?

To provide an example, something similar to PHP for instance:

$array = array (3, 1, 2);
$assoc_array = array( 'key' => 'value' );

Answer 1

You mean like this?

public List<String> buildList(String first, String second)
{
     List<String> ret = new ArrayList<String>();
     ret.add(first);
     ret.add(second);
     return ret;
}

...

List<String> names = buildList("Jon", "Marc");

Or are you interested in the ArrayList constructor which takes a Collection<? extends E>? For example:

String[] items = new String[] { "First", "Second", "Third" };
// Here's one way of creating a List...
Collection<String> itemCollection = Arrays.asList(items);
// And here's another
ArrayList<String> itemList = new ArrayList<String>(itemCollection);

Answer 2

A nice way of doing this is using Google Collections:

List<String> list = ImmutableList.of("A", "B", "C");

Map<Integer, String> map = ImmutableMap.of(
  1, "A",
  2, "B",
  3, "C");

Answer 3

ArrayLists are straightforward:

List<String> al = Arrays.asList("vote", "for", "me"); //pandering

Taken from comments, note that this returns a fixed size Array$ArrayList, not an ArrayList

Bruno's approach works best and could be considered on the fly for maps. I prefer the other method for lists though (seen above):

Map<String,String> map = new HashMap<String,String>() {
 {
    put("key1", "value1");
    put("key2", "value2");
 }
};

Answer 4

Use a nice anonymous initializer:

List<String> list = new ArrayList<String>() {{
    add("a");
    add("b");
}};

Same goes for a Map:

Map<String, String> map = new HashMap<String, String>() {{
    put("a", "a");
    put("b", "b");
}};

I find this the most elegant and readable.

Other methods demand creating an array first, then converting it to a List - too expensive in my taste, and less readable.

Answer 5

List<String> list = new ArrayList<String>() {
 {
    add("value1");
    add("value2");
 }
};

Map<String,String> map = new HashMap<String,String>() {
 {
    put("key1", "value1");
    put("key2", "value2");
 }
};

Answer 6

for short lists:

    List<String> ab = Arrays.asList("a","b");