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()

Answer 1

For lists you can use Arrays.asList like this:

List<String> stringList = Arrays.asList("one", "two");
List<Integer> intList = Arrays.asList(1, 2);

For Maps you could use this:

public static <K, V> Map<K, V> mapOf(Object... keyValues) {
    Map<K, V> map = new HashMap<>();

    K key = null;
    for (int index = 0; index < keyValues.length; index++) {
        if (index % 2 == 0) {
            key = (K)keyValues[index];
        }
        else {
            map.put(key, (V)keyValues[index]);
        }
    }

    return map;
}

Map<Integer, String> map1 = mapOf(1, "value1", 2, "value2");
Map<String, String> map2 = mapOf("key1", "value1", "key2", "value2");

Note: in Java 9 you can use Map.of
Note2: Double Brace Initialization for creating HashMaps as suggested in other answers has it caveats