java----List ，Set ，Map

1，怎么用，用好？

2，主要在于练习量，孰能生巧；

3，Map：在集合中应用多于 Set ，和collection 无关， 独立接口；

             元素：俩个对象( key--value：键值对，key 是不可以重复的，value 是可以重复的，

                                       通过key 找 value )；

             特点：key--value：键值对，key 是不可以重复的，value 是可以重复的；

             方法：put(Object k,Object v) ：添加(或是覆盖已存在键值对) 键值对；

                       get(Object k)：返回Map中k对应的v值；

                       remove(Object k)：删除k所在的键值对；

                       size()：返回键值对的数量；

                       containsKey(Object k)：判断k这个键是否存在；

                       containsValue(Object v)：判断v这个值是否存在；

              例子：

                        Map map=new HashMap();     //创建一个map，实现类是HashMap；

                         map.put("Italy","Germany");

                         map.put("USA","Brazil");

                         string s=(string)map.get("USA");

                         System.out.println("美国世界杯的冠军是"+s);

              遍历：1，值遍历：用 Map 中的values()方法：返回Map中所有值得集合：Collection

                                           static void printValues(Map map){

                                                 collection vs=map.values();

                                                 Iterator it=vs.iterator();

                                                  while(it.hasNext()){

                                                   Object v= it.next();

                                                   system.out.println(v);

                                                 }

                                            }

                        2，键遍历：keySet()：返回Map中所有键得集合：Set

                                           static void printKeys(Map map){

                                                 Set ks=map.keySet();

                                                 Iterator it=ks.iterator();

                                                  while(it.hasNext()){

                                                   Object k= it.next();

                                                   Object v=map.get(key);

                                                   system.out.println(k+"...."+v);

                                                 }

                                            }

                        3，键值对遍历：entrySet() ：返回Map中所有键值对的集合：Map.Entry对象

                                                                       封装了一个key和一个value；

                                         static void printKeysValues(Map map){

                                                  Set es=map.entrySet();

                                                  Iterator it=es.iterator();

                                                  while(it.hasNext()){

                                                   Map.Entry me=(Map.Entry)it.next();  

                                                   Object k=me.getKey();

                                                   Object v=me.getValue();

                                                   system.out.println(k+"...."+v);

                                                 }

                                          }

                        实现类：1，HashMap：1.2，允许用null作为key或者value

                                      2，Hashtable：1.0  不允许用null作为key或者value

                                      3，Properties：Hashtable的子类 key和value 都是string

                                                               一般用于配置文件的读取

                                      4，TreeMap：实现了SortedMap(Map的子接口) 自动对key排序