Java 求集合的所有子集

递归方法调用，求解集合的所有子集。

package ch01;

import java.util.HashSet;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.Set;

public class QuerySubSet {

    public static List<Set<Object>> Query(Set<Object>   target)
    {
        if(target.size() == 0)
        {
            List<Set<Object>> list= new LinkedList<Set<Object>>();
            list.add(target);
            return list;
        }else if(target.size() == 1)
        {
            List<Set<Object>> list= new LinkedList<Set<Object>>();
            list.add(target);
            Set<Object> nullset=new HashSet<Object>();
            list.add(nullset);
            return list;
        }else
        {
            Iterator<Object> iter=target.iterator();
            Object elem=iter.next();
            target.remove(elem);
            Set<Object> newSet=new HashSet<Object>();
            newSet.add(elem);

            List<Set<Object>> list1=Query(newSet);
            List<Set<Object>> list2=Query(target);
            return merge(list1, list2);
        }
    }

    public static   List<Set<Object>> merge(List<Set<Object>> list1, List<Set<Object>> list2)
    {
         List<Set<Object>> mlist=new LinkedList<Set<Object>>();
         for(Set<Object> set1: list1 )
             for(Set<Object> set2: list2)
             {
                 Set<Object>  newSet=new HashSet();
                 newSet.addAll(set1);
                 newSet.addAll(set2);
                 mlist.add(newSet);
             }
         return mlist;
    }

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Set<Object>     targ=new HashSet<Object>();
        for(int i=0; i<10; i++)
        {
            targ.add(Integer.toString(i));
        }

        List<Set<Object>> allsubset= QuerySubSet.Query(targ);

        boolean f1=true;
        for(Set<Object> set1: allsubset)
        {
            if(!f1)
                System.out.print(",");
            f1=false;

            System.out.print("{");

            boolean f2=true;
            for(Object obj: set1)
            {
                if(!f2)
                    System.out.print(",");
                f2=false;
                System.out.print(obj.toString());
            }
            System.out.println("}");
        }

    }

}