Java groupingBy is always changing order [duplicate]

Question

i have List with simple object:

private String unit; private Double value;

List looks like that:

f, 1.0; ml, 15.0; g, 9.0

I have created a simple function where I want to group this values and put them to the map with unit as a key, and list of objects as value, but I want to save the order like in my original list. This is my current solution:

myList.stream() .collect(groupingBy(MyObject::getUnit));

But after that my map is sorted alphabetically this way: f, g, ml instead of f, ml, g. Is there any alternative for groupingBy to fix it?

The "alphabetic" order is coincidence, the hashcodes of "f" and "g" indeed being ordered (same length). — Joop Eggen
– Joop Eggen, Commented Nov 14, 2019 at 13:31

Andy Turner · Accepted Answer · 2019-11-14 14:03:11Z

The issue is that groupingBy puts things into some map, probably a HashMap, which has no guaranteed ordering.

Use the groupingBy overload which takes a Supplier<M> mapFactory, to get a map with an ordering guarantee, e.g.

myList.stream() .collect(groupingBy(MyObject::getUnit, LinkedHashMap::new, Collectors.toList()));

Andy Turner · Accepted Answer · 2019-11-14 13:54:18Z

The javadoc for groupingBy() says:

There are no guarantees on the type, mutability, serializability, or thread-safety of the Map or List objects returned.

And then, the Map interface itself doesn't give any guarantee about the order of keys!

Thus: you either have to use another grouping method (that allows you to force a LinkedHashMap or a TreeMap), or to step back, and to rethink your whole approach. You could for example sort your list after it was pulled together.

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