import java.util.*;
import java.util.function.*;
import java.util.stream.*;
public class PowerSet {
public static final Collection> of(Set s) {
List src = new ArrayList<>(s);
if (src.size() > 30) {
throw new IllegalArgumentException("Set too big " + s);
}
return new AbstractCollection>() {
@Override
public int size() {
return 1 << src.size();
}
@Override
public boolean contains(Object o) {
return o instanceof Set && src.containsAll((Set) o);
}
@Override
public Iterator> iterator() {
return new Iterator>() {
private int index = 0;
private int end = size();
@Override
public boolean hasNext() {
return index < end;
}
@Override
public Set next() {
Set result = new HashSet<>();
for (int i = 0, j = index; j != 0; i++, j >>= 1) {
if ((j & 1) == 1) {
result.add(src.get(i));
}
}
index++;
return result;
}
};
}
};
}
public static void main(String[] args) {
Set set = Set.of('a', 'b', 'c');
for (Set s : PowerSet.of(set)) {
System.out.println(s);
}
}
}