Map练习中的错误，有些不明白 - 黑马程序员技术交流社区

import java.util.*;
class MapTest
{
public static void main(String[] args)
{
String s = myTest("zzfdqavcbsacdfs");
System.out.println(s);
}
public static String myTest(String s)
{
char[] ch = s.toCharArray();
TreeMap<Character,Integer> tm = new TreeMap<Character,Integer>();
for (int i = 0;i<ch.length ; i++ )
{
if(!(ch[i]>='a' && ch[i]<='z' || ch[i]>='A' && ch[i]<='Z'))
continue;
int value = 1;
if(!tm.containsKey(ch[i]))
tm.put(ch[i],value);
else
{
value = tm.get(ch[i]);
value += 1;
tm.put(ch[i],value);
}
}
//System.out.println(tm);
Iterator it = tm.keySet().iterator();
StringBuilder sb = new StringBuilder();
while(it.hasNext())
{
Character key = it.next(); //此会编译时报错，说Object无法转化为Charactor 。要写成Object key 才行。为什么？
Integer v = tm.get(key);
sb.append(key+"("+v+")");
}
return sb.toString();
}
}

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