Flattening a HashMap of (String → ArrayList) to an ArrayList

If it's really a list of entries that you want, then this is about as fast as it gets.

But it can be better:

• Use interface types instead of implementations, for example List instead of ArrayList and Map instead of HashMap
• Java 6 is no longer supported, and in Java 7 you should use the diamond operator to create new instances, for example: Map<String, List<String>> map1 = new HashMap<>();
• Use better, more natural names, for example:
  • entry is better than ent
  • list is better than ls
• Use unit tests to verify your implementation

Based on the above, here's an alternative implementation:

public static List<String> concatMapEntries(Map<String, List<String>> map) {
    List<String> finalList = new ArrayList<>();
    for (Map.Entry<String, List<String>> entry : map.entrySet()) {
        List<String> list = entry.getValue();
        for (String string : list) {
            finalList.add(entry.getKey() + " = " + string);
        }
    }
    return finalList;
}

And unit test:

@Test
public void testExample() {
    Map<String, List<String>> map1 = new HashMap<>();

    List<String> map11 = new ArrayList<>();
    map11.add("a");
    map11.add("b");
    map1.put("1", map11);

    List<String> map12 = new ArrayList<>();
    map12.add("c");
    map12.add("d");
    map1.put("2", map12);

    assertEquals("[1 = a, 1 = b, 2 = c, 2 = d]", concatMapEntries(map1).toString());
}