How to combine Multiple Maps with multiple same keys and lists as values

I am new to Java and I am trying to merge multiple maps with string as key and list as values to produce a new Map.

public class Student{    private String name;    private String country;    //Setters and Getters  }  

Now I have an util class to add students to the list based on their country.

  public class MapAdder {      static Map<String,List<Student>> studentMap = new LinkedHashMap<String,List<Student>>();            public static void addToMap(String key,Student student) {            studentMap.computeIfAbsent( key , k->new LinkedList<Student>()).add(student);          }            public static Map<String, List<Student>> getStudentMap() {          return studentMap;      }             public static  void  clearStudentMap() {            studentMap.clear();      }                 }  

Main Method

public static void main(String[] args) {      Map<String,List<Student>> studentMap1   ;      Map<String,List<Student>> studentMap2  ;      Map<String,List<Student>> studentMap3 ;             MapAdder.addToMap("India", new Student("Mounish","India") );      MapAdder.addToMap("USA", new Student("Zen","USA") );      MapAdder.addToMap("India", new Student("Ram","India") );      MapAdder.addToMap("USA", new Student("Ronon","USA") );      MapAdder.addToMap("UK", new Student("Tony","UK") );             studentMap1 = MapAdder.getStudentMap();      MapAdder.clearStudentMap();            MapAdder.addToMap("India", new Student("Rivar","India") );      MapAdder.addToMap("UK", new Student("Loki","UK") );      MapAdder.addToMap("UK", new Student("Imran","UK") );      MapAdder.addToMap("USA", new Student("ryan","USA") );            studentMap2= MapAdder.getStudentMap();      MapAdder.clearStudentMap();            Map<String,List<Student>>map3  = Stream.of(studentMap1, studentMap2)                .flatMap(map -> map.entrySet().stream())                .collect(Collectors.toMap(                   Entry::getKey,                   Entry::getValue                   ));  

But when I try to merge both the maps I am getting empty map. Actually, I need to have a map with three keys (India, UK, USA) and their values that are list from multiple maps to be merged w.r.t keys.

https://stackoverflow.com/questions/65428163/how-to-combine-multiple-maps-with-multiple-same-keys-and-lists-as-values December 24, 2020 at 01:08AM