How to populate a HashMap using a Lambda Expression

Question

There is one class (SomeOrders), which has few fields like Id, Summary, Amount, etc...

The requirement is to collect Id as key and Summary as value to a HashMap from an input List of SomeOrder objects.

Code in Before java 8:

List<SomeOrder> orders = getOrders();
Map<String, String> map = new HashMap<>();
for (SomeOrder order : orders) {
    map.put(order.getId(), order.getSummary());
}

How to achieve the same with Lambda expression in Java 8?

possible duplicate of [Java: How to convert List to Map](http://stackoverflow.com/questions/4138364/java-how-to-convert-list-to-map) — Alexis C., Mar 23 '15 at 11:06
Yes, almost same. http://stackoverflow.com/questions/4138364/java-how-to-convert-list-to-map But above is just asked in general, from that one answer can find with Lambda expression. Anyhow thanks for notifying me. — Paramesh Korrakuti, Mar 23 '15 at 11:09
Don't get confuse among *lambda expression* and *stream functional operations*. I think you would like to get answer for *functional operations*, as the answer below. — Alex, Oct 04 '17 at 03:11

score 37 · Accepted Answer · edited Oct 04 '17 at 03:05

37

Use Collectors.toMap :

orders.stream().collect(Collectors.toMap(SomeOrder::getID, SomeOrder::getSummary));

or

orders.stream().collect(Collectors.toMap(o -> o.getID(), o -> o.getSummary()));

edited Oct 04 '17 at 03:05

buræquete

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answered Mar 23 '15 at 10:21

Eran

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3

What if the orders list has duplicates and the map should have a List to collect the order summaries ? List<> to Map> – prime Feb 02 '18 at 11:42
How to populate an EnumMap like this ? – insanely_sin Aug 27 '19 at 21:34

How to populate a HashMap using a Lambda Expression

1 Answers1