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javadictionaryjava-8java-streammultimap

Converting List of List and String to Map with Java Stream

I have a class like this:

public class A {
    private List<String> stringList;
    private String someString;
}

I have a list of these objects like so:

List<A> list = //some method to generate list

I want to conver this to a Map<String, String> where each string in the stringList maps to the same someString value (like a multimap). How can I do this using java 8 stream?

I could convert this to a flat map like so:

list.stream.flatMap(....

But I'm not sure where to go from there.

Solution

  • Try this.

    Map<String, String> r = list.stream()
    .flatMap(a -> a.stringList.stream()
        .map(k -> new AbstractMap.SimpleEntry<>(k, a.someString)))
    .collect(Collectors.toMap(e -> e.getKey(), e -> e.getValue()));