[Ljava.lang.String; cannot be cast to java.lang.String

试图通过resquest对象获取前端请求键值对，即request.getParamMap()
本来代码如下：
Map<String,String> map=request.getParamMap();
Map<String,String> map= request.getParameterMap();
Set<String> set=map.keySet();
Iterator<String> it=set.iterator();

while(it.hasNext())
{
String next= it.next();
System.out.println("key:"+next+"->value:"+map.get(next));

}

然而报错，这是为什么呢？

看源码，map来自request对象，查看源码即可知道：
/** Returns a java.util.Map of the parameters of this request.
* Request parameters
* are extra information sent with the request. For HTTP servlets,
* parameters are contained in the query string or posted form data.
*
* @return an immutable java.util.Map containing parameter names as
* keys and parameter values as map values. The keys in the parameter
* map are of type String. The values in the parameter map are of type
* String array.
*
*/返回一个request的请求参数map
重点看划线部分的of type String  ，of type String array
就是说key是String
value是String[]
解决后代码：Map<String,String[]> map= request.getParameterMap();
Set<String> set=map.keySet();
Iterator<String> it=set.iterator();

while(it.hasNext())
{
String next= it.next();
System.out.println("key:"+next+"->");
for(String s:map.get(next))
{
System.out.println("value:"+s);
}
}

运行结果就正常了，
这告诉我们，多
ad6e
看源码，抠单词