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

Question

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);

docs.oracle.com/javase/8/docs/api/java/util/stream/…

JB Nizet
– JB Nizet

2018-01-19 09:41:30 +00:00
Commented Jan 19, 2018 at 9:41 — JB Nizet
– JB Nizet, Commented Jan 19, 2018 at 9:41

Eugene · Accepted Answer · 2018-01-23 09:00:53Z

5

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

edited Jan 23, 2018 at 9:00

answered Jan 19, 2018 at 9:45

Eugene

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6 Comments

emoleumassi Over a year ago

How can i group this map with the same 'a' and the mean value of all value1 in the list of object2 in a collect statement? Something like a="15", mean value of Object2.value1, a="16", mean value of Object2.value1

Eugene Over a year ago

@emoleumassi so basically u want a Map<String, BigDecimal> where value is the min from all those value1?

emoleumassi Over a year ago

yes. a Map<String, BigDecimal> where value is the min from all those value1

Eugene Over a year ago

@emoleumassi

map.entrySet()                 .stream()                 .collect(Collectors.groupingBy(x -> x.getKey().getA(),                         Collectors.mapping(x -> x.getValue().getValue1(),                                 Collectors.collectingAndThen(                                         Collectors.minBy(                                                 Comparator.naturalOrder()),                                         Optional::get))))

fps Over a year ago

@Eugene or just

map.entrySet().stream().collect(Collectors.toMap(x -> x.getKey().getA(), x -> x.getValue().getValue1(), BigDecimal::min))

, which is quite shorter

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