null pointer exception in setting response header in servlet

response.setHeader(keys[i].toString(),value);

in a servlet is throwing null pointer exception even though none of keys[i] or value 开发者_JAVA技巧are null why is it so?

One of the following is true:

1. response is null
2. keys is null
3. keys[i] is null
4. the exception is not thrown from the line you posted.