How get a Map<String, List<Object>> from a Stream in Java 8

Question

I have the next class:

public class ExampleClass {
    
    private String title;
    private String codeResponse;
    private String fileName;
}

I need to generate a map in which the key is the filename, and the value is the list of objects that contain that filename. I did the following:

Map<String, List<ExampleClass>> mapValues = items
            .stream()
            .collect(Collectors.groupingBy(
                item -> item.getFileName(), 
                Collectors.mapping(item -> item, Collectors.toList())
            ));

But in this case I am saving in each fileName the total list of objects, including those that do not apply.

remove Collectors.mapping. groupingBy + toList will do your task — Vasily Liaskovsky
– Vasily Liaskovsky, Commented Oct 12, 2021 at 21:19
How come in this case I am saving in each fileName the total list of objects, including those that do not apply? This does not seem to be reproducible and a list of ExampleClass instances with the same fileName is returned. — Nowhere Man
– Nowhere Man, Commented Oct 12, 2021 at 22:29

WJS · Accepted Answer · 2021-10-12 21:29:16Z

4

All you need is Collectors.groupingBy. The instances will be placed in a list by default.

Map<String, List<ExampleClass>> mapValues = items.stream()
                .collect(Collectors.groupingBy(item->item.getFileName()));

You could also use ExampleClass::getFileName in place of the lambda. But that is a matter of personal preference.

answered Oct 12, 2021 at 21:29

WJS

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1 Comment

Nahuel Giani Over a year ago

Thank you so much!

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