底层是用HashMap来实现

HashMap map; private static final int VALUE=10; public test() { map=new HashMap(); }public void add(Object obs) //将输入的值作为HashMap的键，从而达到不重复{ map.put(obs, VALUE);}public int size(){ return map.size();}public String toString(){ StringBuilder ss=new StringBuilder(); for(Object s:map.keySet()) { ss.append(s+","); } ss.setCharAt(ss.length()-1, ' '); return ss.toString();}