Java 8 - List conversion to List

OK,

So we all know how we would do this in Java 6 right... something similar to:

public class ConvertListType1 {

 public static void main(String[] args) {
  List<String> items1 = Arrays.asList("1111", "2222", "3333", "4444", "5555", "6666");
  List<Long> items2 = new ArrayList<Long>();
  for (String string : items1) {
   items2.add(Long.parseLong(string));
  }
 }
}

Now lets break it down a bit:

public class ConvertListType2 {

 public static void main(String[] args) {
  List<String> items1 = Arrays.asList("1111", "2222", "3333", "4444", "5555", "6666");
  List<Long> items2 = collectConvertedItems(items1);
 }

 protected static long function(String string) {
  return Long.parseLong(string);
 }

 protected static List<Long> collectConvertedItems(List<String> items1) {
  List<Long> items2 = new ArrayList<Long>();
  for (String string : items1) {
   items2.add(function(string));
  }
  return items2;
 }
}

Now this seems to make no sense in Java-6, but have great meaning with the usage of Streams in Java-8.

public class ConvertListType3 {

 public static void main(String[] args) {
  Function<String, Long> function = new Function<String ,Long>() {
   @Override
   public Long apply(String string) {
    return Long.parseLong(string);
   }
  };
  List<Long> items2 = Arrays.asList("1111", "2222", "3333", "4444", "5555", "6666").stream().map(function).collect(Collectors.toList());
 }
}

So what is going on?

Stream is a new entity in the later Java version, that allows functional manipulation of a collection of items(it does not make coffee just yet), so we take the list of strings and create a stream of it for further manipulation.

NOTE that function is now an anonymous type, assigned as a local variable that is passed to the mapping, which returns a new stream with the new generic type defined by the return type of the map Function... This is where the conversion really happens, they called it map (which also makes some sense, mapping from one type to another).

Later there is a call to collect, which in the most abstract description, wraps your new stream in a new wrapper, in our simple case just returns a list with the new type Long.
NOTE, collect can do much more than just make a list.

That is it. I was trying to keep it simple as I recall it was hard for me to get my head around the transition from Java-6 to Java-8, Hope I saved you a few minutes :).