重要的方法

public static void main(String[] args) {//如果key相同,把原先的数据覆盖HashMap<String,String> hm = new HashMap<String,String>();hm.put("111", "222");hm.put("111", "333");String valueStr = hm.get("111").toString();System.out.println(valueStr);//把ArrayList的聚合转换成数组ArrayList al = new ArrayList();al.add("aaa");al.add("bbb");al.add("ccc");String str[] = (String[])al.toArray(new String[0]);for(int i=0; i<str.length; i++){System.out.println(str[i]);}//把一个指定数组的大小转换成列表String[] ttt = new String[3];ttt[0] = "a";ttt[1] = "b";ttt[2] = "c";List as = java.util.Arrays.asList(ttt);for(int i=0; i<as.size(); i++){System.out.println(as.get(i).toString());}}