How to convert hash map keys into list?

Question

I have a hash map and I'm trying to convert the keys to a list. Here's the code:

List<ARecord> recs = new ArrayList<ARecord>();

HashMap<String, ARecord> uniqueRecs = new HashMap<String, ARecord>();
for(ARecord records:recs){
    if(!uniqueRecs.containsKey(records.getId())){
        uniqueRecs.put(records.getId(), records);
    }
}

When I try to do

List<ARecord> finalRecs = new ArrayList<ARecord>(uniqueRecs.keySet());

The error:

The constructor ArrayList(Set) is undefined".

How can I convert Hashmap keys to List<ARecord> finalRecs?

Solution 1

Your uniqueRecs has String type of the key. You have to do:

List<String> keys = new ArrayList<>(uniqueRecs.keySet());

or

List<ARecord> values = new ArrayList<>(uniqueRecs.values());

Solution 2

What worked for me that i wanted to modify an existing list was:

list.addAll(map.keySet());

Solution 3

I think the best way to do this 'nowadays' is:

List<Integer> result = map.keySet().stream().collect(Collectors.toList());

This will not throw any warnings and will get the List<Integer> generic correctly.

Comments

[Jay]

I get an error"The method addAll(Collection<? extends ARecord>) in the type List<ARecord> is not applicable for the arguments (Set<String>)"

[schtever]

@Jay revised the answer to populate the list with the Map's values not the keys.

[Jay]

would there be any way to convert List<String> keys into List<ARecord>?

[Jay]

I actually want the keys. Is there any way to get keys and convert to List<ARecord>?

[ka4eli]

ARecords in you map are not the keys, they are values! You have to decide list of what entities you want to get in the end: ARecords or Strings. If ARecords - then take values of a Map, if Strings - take keySet.

[Jay]

I want keySet. Once I get that, I pass into another method which takes in List<ARecord>. I can't figure out how to pass the keys of hash map as a List<ARecord>

[Louis Wasserman]

@Jay: That statement makes no sense. The keys are Strings. "how to pass the keys of hash map as a List<ARecord>" makes no sense, because the keys are not ARecords. That's the values.

[Jay]

it's an interface: public interface ARecord {public String gettId()}

[Mick Mnemonic]

@Jay, if the records in uniqueRecs are indeed unique as the name implies, meaning that all ARecords in the map have unique ids, then you can simply use new ArrayList<>(uniqueRecs.values()) as ka4eli is suggesting here.

[Mark]

Thank you - to elaborate/explain further keySet() returns a Set and this extends the Collection class. values() returns a Collection. This : public ArrayList(Collection<? extends E> c) from ArrayList constructor makes it valid.