Map接口的注意点

• Map是一种把键对象和值对象进行关联的容器,而一个值对象又可以是一个Map,依次类推，这样就可形成一个多级映射
• 键对象不允许重复！Map没有继承Collection接口！一个Map中不能包含相同的key，每个key只能映射一个value
• Map接口提供3种集合，Map的内容可以被当作一组key集合，一组value集合，或者一组key-value映射

可以理解为key相当于“学号”，value相当于对应的“姓名”，“姓名”之后对应的“年龄”依次类推。

常用方法：（实现类常用HashMap）

• put(k,v)

HashMap map = new HashMap();
        map.put( "罗","20" );

• get(k)

HashMap map = new HashMap();
        map.put( "罗","20" );
        System.out.println( map.get( "罗" ) );

• remove(k)

HashMap map = new HashMap();
        map.put( "罗","20" );
        map.remove("罗");

• containskey(k)

 HashMap map = new HashMap();
        map.put( "罗","20" );
        System.out.println( map.containskey("罗"));

• containsvalue(v)

HashMap map = new HashMap();
        map.put( "罗","20" );
        System.out.println( map.containsvalue("20") )

• isEmpty()

HashMap map = new HashMap();
        map.put( "罗","20" );
        System.out.println(map.isEmpty())

• 等等~~

public static void main(String[] args) {
        Map map = new HashMap();
        map.put( "001","罗同学" );
        map.put( "002","李同学" );
        map.put( "003","莫同学" );
        Set set = map.keySet();
        Iterator iterator = set.iterator();
        System.out.println("key元素中的元素：");
        while (iterator.hasNext()){
            System.out.println( iterator.next() );
        }
        Collection coll = map.values();
        iterator = coll.iterator();
        System.out.println("value集合中的元素：");
        while (iterator.hasNext()){
            System.out.println( iterator.next() );
        }
    }

public static void main(String[] args) {
        Map map = new HashMap();
        Emp emp1 = new Emp( "001","罗焕钊" );
        Emp emp2 = new Emp( "003","小钊" );
        Emp emp3 = new Emp( "005","小罗" );
        Emp emp4 = new Emp( "006","小焕" );
        Emp emp5 = new Emp( "008","小焕钊" );

        map.put( emp1.getE_id(),emp1.getE_name() );
        map.put( emp2.getE_id(),emp2.getE_name() );
        map.put( emp3.getE_id(),emp3.getE_name() );
        map.put( emp4.getE_id(),emp4.getE_name() );
        map.put( emp5.getE_id(),emp5.getE_name() );

        Set set = map.keySet();
        Iterator it = set.iterator();
        System.out.println("HashMap实现的集合：");
        while (it.hasNext()){
            String str = (String) it.next();
            String name = (String) map.get( str );
            System.out.println(str+"  "+name);
        }
        TreeMap treeMap = new TreeMap();
        treeMap.putAll( map );
        Set set1 = treeMap.keySet();
        Iterator it1 = set.iterator();
        System.out.println("treemap实现的集合：");
        while (it1.hasNext()){
            String str1 = (String) it1.next();
            String name1 = (String) treeMap.get( str1 );
            System.out.println(str1+"  "+name1);
        }
    }