考点：
1，面向对象的Java编程，数据结构一般要写一个class
2，国家名不能重复，要考虑HasMap，把国家名作为一个key
3，先国家名输入者，后面不能update，考虑在改写put方法，检验是否存在重复。

import java.util.HashMap;
import java.util.Map.Entry;

public class MyMap {

protected String name;
protected String country;
protected HashMap<String, String> map = new HashMap<String, String>();

public MyMap() {}

public void put(String name, String country) {
if(!map.containsKey(country)) {
map.put(country, name);
}
}

public String toString() {
String s = "";
for(Entry<String, String> e : map.entrySet()) {
s += e.getValue() + " " + e.getKey() + "n";
}
return s;
}


public static void main(String[] args) {

MyMap myMap = new MyMap();
myMap.put("John", "Canada");
myMap.put("Bob", "USA");
myMap.put("Tom", "USA");
myMap.put("Alice", "Canada");
myMap.put("Alan", "Canada");
myMap.put("Peter", "UK");
myMap.put("Peter", "China");

System.out.println(myMap.toString());
}

}

输出：
John Canada
Bob USA
Peter UK
Peter China