Java 8 collect to Map<String, List<Object>>

I have two object. The first one:

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

The second one:

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 a list of Object2 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 try something like this:

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