Java 8 compiles map <String, List <Object>>

Question

I have two objects. First:

public final class Object1 { private String a; private String b; // constructor getter and setter } 

Second:

 public class Object2 { private BigDecimal value1; private BigDecimal value2; // constructor getter and setter } 

I have a Map<Object1, Object2> :

  Object1{a="15", b="XXX"}, Object2{value1=12.1, value2=32.3} Object1{a="15", b="YYY"}, Object2{value1=21.1, value2=24.3} Object1{a="16", b="AAA"}, Object2{value1=34.1, value2=45.3} Object1{a="15", b="BBB"}, Object2{value1=23.1, value2=65.3} Object1{a="15", b="DDD"}, Object2{value1=23.1, value2=67.3} Object1{a="17", b="CCC"}, Object2{value1=78.1, value2=2.3} ........ 

I want to group this map with the same a in the Object2 list, like:

 a="15", {{value1=12.1, value2=32.3}, {value1=21.1, value2=24.3}, {value1=23.1, value2=65.3}, {value1=23.1, value2=67.3}}, a="16", {{value1=34.1, value2=45.3}} ... 

I am trying something like this:

 Map<String, List<Object2>> map1 = map.entrySet() .stream() .collect(Collectors.toMap(e -> e.getKey().getA(), list of object with this key); 

Answer 1

 yourMap.entrySet() .stream() .collect(Collectors.groupingBy(e -> e.getKey().getA(), Collectors.mapping(Entry::getValue, Collectors.toList())))