Suseika Suseika - 27 days ago 14
Java Question

Is there a Collector that collects to an order-preserving Set?

Collectors.toSet()
does not preserve order. I could use Lists instead, but I want to indicate that the resulting collection does not allow element duplication, which is exactly what
Set
interface is for.

Answer

You can use toCollection and provide the concrete instance of the set you want. For example if you want to keep insertion order:

Set<MyClass> set = myStream.collect(Collectors.toCollection(LinkedHashSet::new));

For example:

public class Test {    
    public static final void main(String[] args) {
        List<String> list = Arrays.asList("b", "c", "a");

        Set<String> linkedSet = 
            list.stream().collect(Collectors.toCollection(LinkedHashSet::new));

        Set<String> collectorToSet = 
            list.stream().collect(Collectors.toSet());

        System.out.println(linkedSet); //[b, c, a]
        System.out.println(collectorToSet); //[a, b, c]
    }
}
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