java 获取 Map中所有的 key 和 value 值

1、Student.java

package bo;

public class Student {
private String id ;
public Student(String id){
this.id = id;
}

@Override
public String toString() {
return "   学生：" + id ;
}


}

2、ClassRoom.java

package bo;

public class ClassRoom {
private String id;
public ClassRoom(String id ){
this.id = id;
}
@Override
public String toString() {
return "班级：" + id ;
}
}

3、Test.java

package bo;

import java.util.ArrayList;
import java.util.Collection;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Map.Entry;

public class Test {
public static void main(String[] args) {
// 一班
Collection<Student> c1= new ArrayList<Student>(); 
ClassRoom classroom_1 = new ClassRoom("1班");
Student s1 = new Student("小明");
Student s2 = new Student("小红");
Student s3 = new Student("小光");
c1.add(s1);
c1.add(s2);
c1.add(s3);

// 二班
Collection<Student> c2 = new ArrayList<Student>();
ClassRoom classroom_2 = new ClassRoom("2班");
Student t1 = new Student("小黑");
Student t2 = new Student("小黄");
Student t3 = new Student("小米");
c2.add(t1);
c2.add(t2);
c2.add(t3);

Map<ClassRoom, Collection<Student>> m = new HashMap<ClassRoom,Collection<Student>>();
m.put(classroom_1, c1);
m.put(classroom_2, c2);

// 方法 一 ：
// 转化成  Set<Entry<ClassRoom, Collection<Student>>>，然后在转化为迭代器 
Iterator<Entry<ClassRoom, Collection<Student>>>  iterator = m.entrySet().iterator();

while(iterator.hasNext()){
Entry<ClassRoom, Collection<Student>> entry = iterator.next();  
System.out.println(entry.getKey());   // 获取key, 输出 ClassRoom
//System.out.println(entry.getValue());  // 获取value ,输出 Collection<Student>> 
for(Student s : entry.getValue()){         
System.out.println(s);
}
}

System.out.println("===");

// 方法 二 ：
Iterator it = m.keySet().iterator();  // Set类型的key值集合，并转换为迭代器
while(it.hasNext()){                        
ClassRoom key = (ClassRoom) it.next();   // 获取 key 值，ClassRoom
System.out.println(key);                 
for(Student s : m.get(key)){             // 获取value 值，Collection (Student)
System.out.println(s);
}
}
/*
输出结果：
班级：1班
   学生：小明
   学生：小红
   学生：小光
班级：2班
   学生：小黑
   学生：小黄
   学生：小米
*/   
}
}