java 8 change list to map using instance of list

Question

I'm try to convert a list to a map using the Collectors.toMap call. The list consists of ActivityReconcile objects. I want to pass an instance for every entry in the list into the toMap call.

The code is below and where I need the instances is denoted by ??.

final List<ActivityReconcile> activePostedList = loader.loadActivePosted(accessToken);
Map<AccountTransactionKey, ActivityReconcile> postedActiveMap = 
activePostedList.stream().collect(
 Collectors.toMap(
 AccountTransactionKey.createNewAccountTransactionKeyFromActivityReconcileRecord(??),??));

Answer 1

If I understood you correctly, you will need something like

Map<AccountTransactionKey, ActivityReconcile> result = choices
              .stream()
              .collect(Collectors.toMap(
                      AccountTransactionKey::generate,
                      Function.identity()));

And the method (in AccountTransactionKey class) will look like

public static AccountTransactionKey generate(ActivityReconcile reconcile) {...}

---

I've replaced createNewAccountTransactionKeyFromActivityReconcileRec by generate for making the answer more readable and understandable.

Answer 2

To "fix" your code with the least changes, add a lambda parameter:

activePostedList.stream().collect(Collectors.toMap(
  ar -> AccountTransactionKey.createNewAccountTransactionKeyFromActivityReconcileRecord(ar)),
  o -> o));

or use a method reference:

activePostedList.stream().collect(Collectors.toMap(
  AccountTransactionKey::createNewAccountTransactionKeyFromActivityReconcileRecord, o -> o));

btw, I can't recall ever seeing a method name as long as createNewAccountTransactionKeyFromActivityReconcileRecord - for readability, consider reducing it to just create(), since the return type and parameter type are enough to distinguish it from other factory methods you may have.