Hashmap with Streams in Java 8 Streams to collect value of Map

Question

Let consider a hashmap  Map lt Integer  List gt  id1   new HashMap lt Integer List gt       I inserted some values into both hashmap   For Example       List lt String gt  list1   new ArrayList lt String gt          list1 add  r1        list1 add  r4         List lt String gt  list2   new ArrayList lt String gt         list2 add  r2        list2 add  r5         List lt String gt  list3   new ArrayList lt String gt         list3 add  r3        list3 add  r6         id1 put 1 list1       id1 put 2 list2       id1 put 3 list3       id1 put 10 list2       id1 put 15 list3     Q1     Now I want to apply a filter condition on the key in hashmap and retrieve the corresponding value List    Eg  Here My query is key 1  and output should be  list1   I wrote   id1 entrySet   stream   filter  e - gt  e getKey      1     But I don t know how to retrieve as a list as output of this stream operation   Q2         Again I want to apply a filter condition on the key in hashmap and retrieve the corresponding list of lists   Eg   Here My query is key 1  i e key can be 1 10 15   and output should be  list1   list2   list3  list of lists

User · Answer

What you need to do is create a Stream out of the Map s  entrySet        Map lt K  V gt  -- gt  Set lt Map Entry lt K  V gt  gt  -- gt  Stream lt Map Entry lt K  V gt  gt  map entrySet   stream     From the on  you can  filter   over these entries  For instance      Stream lt Map Entry lt K  V gt  gt  -- gt  Stream lt Map Entry lt K  V gt  gt   filter entry - gt  entry getKey      1    And to obtain the values from it you  map        Stream lt Map Entry lt K  V gt  gt  -- gt  Stream lt V gt   map Map Entry  getValue    Finally  you need to collect into a List      Stream lt V gt  -- gt  List lt V gt   collect Collectors toList      If you have only one entry  use this instead  NOTE  this code assumes that there is a value  otherwise  use  orElse    see the javadoc of Optional for more details       Stream lt V gt  -- gt  Optional lt V gt  -- gt  V  findFirst   get

User · Answer

Maybe the sample is oversimplified  but you don t need the Java stream API here  Just use the Map directly   List lt String gt  list1   id1 get 1      this will return the list from your map

User · Answer

If you are sure you are going to get at most a single element that passed the filter  which is guaranteed by your filter   you can use findFirst    Optional lt List gt  o   id1 entrySet                          stream                          filter  e - gt  e getKey      1                         map Map Entry  getValue                         findFirst      In the general case  if the filter may match multiple Lists  you can collect them to a List of Lists    List lt List gt  list   id1 entrySet                         stream                         filter    some predicate                           map Map Entry  getValue                        collect Collectors toList

User · Answer

Using keySet-  id1 keySet   stream            filter x - gt  x    1           map x - gt  id1 get x            collect Collectors toList

User · Answer

For your Q2  there are already answers to your question  For your Q1  and more generally when you know that the key s filtering should give a unique value  there s no need to use Streams at all    Just use get or getOrDefault  i e   List lt String gt  list1   id1 getOrDefault 1  Collections emptyList

User · Answer

You can also do it like this  public Map lt Boolean  List lt Student gt  gt  getpartitionMap List lt Student gt  studentsList         List lt Predicate lt Student gt  gt  allPredicates   getAllPredicates        Predicate lt Student gt  compositePredicate    allPredicates stream                                 reduce w - gt  true  Predicate  and       Map lt Boolean  List lt Student gt  gt  studentsMap  studentsList                  stream                    collect Collectors partitioningBy compositePredicate        return studentsMap     public List lt Student gt  getValidStudentsList Map lt Boolean  List lt Student gt  gt  studentsMap  throws Exception        List lt Student gt  validStudentsList    studentsMap entrySet                 stream                 filter p - gt  p getKey      Boolean TRUE                flatMap p - gt  p getValue   stream                  collect Collectors toList          return validStudentsList     public List lt Student gt  getInValidStudentsList Map lt Boolean  List lt Student gt  gt  studentsMap  throws Exception        List lt Student gt  invalidStudentsList                 partionedByPredicate entrySet                 stream                 filter p - gt  p getKey      Boolean FALSE                flatMap p - gt  p getValue   stream                  collect Collectors toList          return invalidStudentsList       With flatMap you will get just List lt Student gt  instead of List lt List lt Student gt  gt    Thanks

[java] Hashmap with Streams in Java 8 Streams to collect value of Map

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