Java-Map集合的应用-一对多的映射…

一对多：

一个教室对应多个学生.

学校里边不同的班级有不同的学生，但是可能ID相同.

//优化过后的代码，更具有复用和扩展性.

//定义一个可以存放三个数据的集合——班级名称，学生ID,学生名称

import java.util.*;

class MyMap

{

private Map map;

MyMap()

{

map = new HashMap();

}

public void createClass(String className,
String id, String name)

{

Map subMap =
(Map)map.get(className);

if (subMap == null)

{ subMap = new
HashMap();

map.put(className,
subMap);
//已经存在班级，只需要向学生这个Map集合里边存数据，而班级的Map不需要再存了

}

subMap.put(id, name);

}

public String getName(String className, String
id)

{

Map subMap =
(Map)map.get(className);

return
(String)subMap.get(id);

}

public void getStudent(String className)

{

System.out.println(className);

Map subMap =
(Map)map.get(className);

Set mySet =
subMap.keySet();

for (Iterator it =
mySet.iterator(); it.hasNext(); )

{

String id =
(String)it.next();

String name =
(String)subMap.get(id);

System.out.println(id
+ "..." + name);

}

}

public void getAll()

{

Set set1 = map.keySet();

for (Iterator it =
set1.iterator(); it.hasNext(); )

{

String
className = (String)it.next();

System.out.println(className);

Map subMap
= (Map)map.get(className);

Set set2 =
subMap.keySet();

for (Iterator
it1 = set2.iterator(); it1.hasNext(); )

{

String
id = (String)it1.next();

String
name = (String)subMap.get(id);

System.out.println(it
+ "..." + name);

}

}

}

}

class MyMapDemo

{

public static void main(String[] args)

{

MyMap mm = new MyMap();

mm.createClass("班级一", "01",
"小张");

mm.createClass("班级一", "02",
"小王");

mm.createClass("班级一", "03",
"小李");

mm.createClass("班级二", "01",
"小A");

mm.createClass("班级二", "02",
"小B");

mm.createClass("班级二", "03",
"小C");

//System.out.println(mm.getName("班级一",
"02"));

//mm.getStudent("班级一");

//mm.getStudent("班级二");

mm.getAll();

}

}

//前期设计代码，不具备扩展性和复用性.

import java.util.*;

class MapDemo

{

public static void main(String[] args)

{

//初期设计————(代码缺少复用性，冗余)

//定义一个学校，里边可以有多个班级

HashMap <String,
HashMap> school = new HashMap<String,
HashMap>();

//班级一

HashMap<String,
String> class1 = new HashMap<String
,String>();

class1.put("01", "小张");

class1.put("02", "小王");

class1.put("03",
"小李");

//班级二

HashMap<String,
String> class2 = new HashMap<String
,String>();

class2.put("01", "小周");

class2.put("02", "小孙");

class2.put("03", "小赵");

school.put("班级一", class1);

school.put("班级二",
class2);

show(school);

}

public static void show(Map map)

{

Set<String>
s = map.keySet();

for
(Iterator<String> it = s.iterator();
it.hasNext(); )

{

String
className = it.next();

System.out.println(className);

Map hs =
(Map)map.get(className);

Set
<String> s1 = hs.keySet();

for (Iterator
<String> it1 = s1.iterator();
it1.hasNext(); )

{

String
id = it1.next();

String
name = (String)hs.get(id);

System.out.println(id+".."+name);

}

}

}

}