Utils.java 文件源码

/** Creates the power set of the items in elements.
 *
 * This version of the method will modify the elements list
 * as the power set is built.
 *
 * @param <ElementType> Type of elements.
 * @param elements Items to create power set over.
 * @return Returns power set of elements.
 */
private static <ElementType> Set<Set<ElementType>> getPowerSetDestructive(Set<ElementType> elements) {
    if ( elements.isEmpty() ) {
        return Collections.EMPTY_SET;
    }
    else if ( elements.size() == 1 ) {
        HashSet<Set<ElementType>> s = new HashSet<Set<ElementType>>();
        s.add(Collections.EMPTY_SET);
        s.add(Collections.singleton(elements.iterator().next()));
        return s;
    }
    else {
        ElementType e = elements.iterator().next();  // Next is gaurantee to work, since we check isEmpty
        elements.remove(e);

        Set<Set<ElementType>> pT = getPowerSetDestructive(elements);
        Set<Set<ElementType>> cross = getCrossProduct(pT, e);
        pT.addAll(cross);

        return pT;
    }
}