CODEWITHSUNDEEP
javahashmap
onkar
onkar

Reputation: 4547

retrieve HashMap list into another list , one key having many values

I am saving values into an Hashmap<String,Records> Sample value 1,objRecord1,2,objRecord2,1,objRecord3 so on... I need to retrieve values of all records from hashmap whose string is 1

something like (I am messing here itself) arrayList myArraylist=Hashmap.get(1);

Upvotes: 1

Views: 1410

Answers (6)

Suresh Atta
Suresh Atta

Reputation: 121998

arrayList myArraylist=Hashmap.get("1");

To satisfy that your Hashmap declaration must be 

Map<String,List<Record>> myMap = new HashMap<>();

While adding add to list

Example for Adding in to map:

  if(myMap.containsKey("1")){
         myMap.put("1", myMap.get("1").add(new Record()));//record obj
      }else{
         List<Record> list = new ArrayList<>();
         list.add(record);
         myMap.put("1", list));
     }

Getting back

 List<Record> list = myMap.get("1");

Upvotes: 0

Ishan Rastogi
Ishan Rastogi

Reputation: 830

If you want mulitple records per key then better to use Set of records for the values

Map<String, Set<Record>>

Upvotes: 0

lol
lol

Reputation: 3390

Better will be to use List as values in hashmap.

Something like: HashMap<String, List<Record>>

When you want to put record in map, you can do something like:

void insertRecord(String key, Record record){
    List<Record> records = map.get(key);
    if(records == null){
        records = new ArrayList<Record>();
        map.put(key, records);
    }
    records.add(record);
}

Upvotes: 0

Brian Agnew
Brian Agnew

Reputation: 272297

Do your multiple records per key need to be ordered ? If so, something like:

Map<String, List<Record>>

is suitable. Otherwise a Guava MultiMap would be suitable.

Upvotes: 0

Kanagavelu Sugumar
Kanagavelu Sugumar

Reputation: 19260

Use List or Set as Values, Then keep update that list or Set.

 Hashmap<String, ArrayList<Records>>

Upvotes: 0

sandymatt
sandymatt

Reputation: 5612

Using a Multimap would be easiest, in my opinion.

http://google-collections.googlecode.com/svn/trunk/javadoc/com/google/common/collect/Multimap.html

That is if you're willing to add a dependency on google commons :)

NOTE: this is not the same as MultiMap (notice the case difference) from apache commons.

Upvotes: 2

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