Java How to add values to Array List used as value in HashMap

Question

What I have is a HashMap lt String  ArrayList lt String gt  gt  called examList  What I want to use it for is to save grades of each course a person is attending  So key for this HashMap is couresID  and value is a ArrayList of all grades  exam attempts  this person has made   The problem is I know how to work with array lists and hashmaps normally  but I m not sure how to even begin with this example  So how would I  or example  add something to ArrayList inside HashMap

User · Answer

First create HashMap   HashMap  mapList   new HashMap     Get value from HashMap against your input key   ArrayList arrayList   mapList get key   Add value to arraylist   arrayList add addvalue   Then again put arraylist against that key value  mapList put key arrayList     It will work

User · Answer

Java 8  has computeIfAbsent  examList computeIfAbsent map get id   k - gt  new ArrayList lt  gt      map get id  add value     https   docs oracle com javase 8 docs api java util Map html computeIfAbsent-K-java util function Function-

User · Answer

First  you have to lookup the correct ArrayList in the HashMap   ArrayList lt String gt  myAList   theHashMap get courseID    Then  add the new grade to the ArrayList   myAList add newGrade

User · Answer

String courseID    Comp-101   List lt String gt  scores   new ArrayList lt String gt      scores add  100    scores add  90    scores add  80    scores add  97     Map lt String  ArrayList lt String gt  gt  myMap   new HashMap lt String  ArrayList lt String gt  gt     myMap put courseID  scores     Hope this helps

User · Answer

You could either use the Google Guava library  which has implementations for Multi-Value-Maps  Apache Commons Collections has also implementations  but without generics    However  if you don t want to use an external lib  then you would do something like this   if  map get id     null      gets the value for an id      map put id  new ArrayList lt String gt        no ArrayList assigned  create new ArrayList  map get id  add value     adds value to list

User · Answer

First you retreieve the value  given a key  and then you add a new element to it      ArrayList lt String gt  grades   examList get courseId       grades add aGrade

[java] Java, How to add values to Array List used as value in HashMap

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