I am currently working with a List<Map<String, Object>> where I am trying to group across various keys within the map.
This seems to work nicely using the Java 8 Streams:
Map<Object, Map<Object, List<Map<String, Object>>>> collect =
list
.stream()
.collect(Collectors.groupingBy(
item -> item.get("key1"),
Collectors.groupingBy(item -> item.get("key2"))
));
As expected this give me a Map<Object, Map<Object, List<Map<String, Object>>>> which works well where the possible grouped results are greater than 1.
I have various examples where the grouping being done will always result in a single item in the lowest level list, for example.
List of Rows
{
[reference="PersonX", firstname="Person", dob="test", lastname="x"],
[reference="JohnBartlett", firstname="John", dob="test", lastname="Bartlett"]
}
Grouped by reference
Currently - grouped into a list with 1 Map<String,Object>
[PersonX, { [reference="PersonX", firstname="Person", dob="test", lastname="x"]}],
[JohnBartlett, { [reference="JohnBartlett", firstname="John", dob="test", lastname="Bartlett"]}]
Preference - No List just a single Map<String,Object>
[PersonX, [reference="PersonX", firstname="Person", dob="test", lastname="x"]],
[JohnBartlett, [reference="JohnBartlett", firstname="John", dob="test", lastname="Bartlett"]]
is there a way within the streams to force the output for these instances to be Map<Object, Map<Object, Map<String, Object>>> - so a single Map<String,Object> rather than a List of them?
Any help would be greatly appreciated.
If I understood correctly, then for the cases where you are sure that there is a single item, you should just replace:
.collect(Collectors.groupingBy(
item -> item.get("key1"),
Collectors.toMap(item -> item.get("key2"), Function.identity())
));
You can even provide a third argument as a BinaryOperator to merge your same entries (in case you need to)
Collectors.toMap() does exactly what you're looking to do.
Map<Object, Map<String, Object>> collect = maps.stream()
.collect(Collectors.toMap(p -> p.get("reference"), Function.identity()));
{
PersonX={firstname=Person, reference=PersonX, lastname=x, dob=test},
JohnBartlett={firstname=John, reference=JohnBartlett, lastname=Bartlett, dob=test}
}
This will throw an IllegalStateException if you have a duplicate key, which is probably exactly what you want when you never expect there to be a duplicate record in your data:
Exception in thread "main" java.lang.IllegalStateException: Duplicate key JohnBartlett (attempted merging values {dob=test, lastname=Bartlett, reference=JohnBartlett, firstname=John} and {dob=test 2, lastname=Bartlett, reference=JohnBartlett, firstname=John})
at java.base/java.util.stream.Collectors.duplicateKeyException(Collectors.java:133)
at java.base/java.util.stream.Collectors.lambda$uniqKeysMapAccumulator$1(Collectors.java:180)
at java.base/java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
at java.base/java.util.AbstractList$RandomAccessSpliterator.forEachRemaining(AbstractList.java:720)
at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:484)
at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:474)
at java.base/java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:913)
at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
at java.base/java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:578)
at so.UniqueKeyStreamExample.main(UniqueKeyStreamExample.java:22)