import java.util.List;
import java.util.Set;
import java.util.Arrays;
import java.util.stream.Collectors;
 
class Ideone {
	public static void main (String[] args) {
		List<String> list = Arrays.asList("a", "b", "c");
 
		List<String> combinations =
            list.stream()
                .flatMap(str1 -> list.stream().map(str2 -> str1 + str2))
                .collect(Collectors.toList());
 
        System.out.println(combinations);
	}
}

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